Chapter 6. EPITHELIAL TISSUE

Chapter 6. EPITHELIAL TISSUE

Epithelial tissues (from Greek. epi- above and thele- skin) are the oldest histological structures that appear first in phylo- and ontogenesis. They are a system of differentials of polarly differentiated cells, closely located in the form of a layer on the basement membrane (plate), on the border with the external or internal environment, and also form the majority of the glands of the body. There are superficial (integumentary and lining) and glandular epithelia.

6.1. GENERAL MORPHOLOGICAL CHARACTERISTICS AND CLASSIFICATIONS

Surface epithelia- these are border tissues located on the surface of the body (integumentary), mucous membranes of internal organs (stomach, intestines, Bladder etc.) and secondary body cavities (lining). They separate the body and its organs from their environment and participate in the metabolism between them, performing the functions of absorbing substances (absorption) and releasing metabolic products (excretion). For example, through the intestinal epithelium, food digestion products are absorbed into the blood and lymph, which serve as a source of energy and building material for the body, and through the renal epithelium a number of nitrogen metabolism products, which are waste products, are released. In addition to these functions, the integumentary epithelium performs an important protective function, protecting the underlying tissues of the body from various external influences - chemical, mechanical, infectious, etc. For example, the skin epithelium is a powerful barrier to microorganisms and many poisons. Finally, the epithelium covering the internal organs creates conditions for their mobility, for example, for heart contraction, lung excursion, etc.

glandular epithelium, forming many glands, performs a secretory function, i.e. synthesizes and secretes specific products -

Rice. 6.1. The structure of single-layer epithelium (according to E. F. Kotovsky): 1 - core; 2 - mitochondria; 2a- Golgi complex; 3 - tonofibrils; 4 - structures of the apical surface of cells: 4a - microvilli; 4b - microvillous (brush) border; 4v- eyelashes; 5 - structures of the intercellular surface: 5a - tight junctions; 5b - desmosomes; 6 - structures of the basal surface of cells: 6a - invaginations of the plasmalemma; 6b - hemidesmosomes; 7 - basement membrane (plate); 8 - connective tissue; 9 - blood capillaries

secrets that are used in processes occurring in the body. For example, the secretion of the pancreas is involved in the digestion of proteins, fats and carbohydrates in the small intestine, the secretions of the endocrine glands - hormones - regulate many processes (growth, metabolism, etc.).

Epithelia are involved in the construction of many organs, and therefore exhibit a wide variety of morphophysiological properties. Some of them are general, allowing one to distinguish epithelia from other tissues of the body. There are the following main features of epithelia.

Epithelia are layers of cells - epithelial cells(Fig. 6.1), which have different shapes and structures in different types of epithelium. There is little intercellular substance between the cells that make up the epithelial layer, and the cells are closely connected to each other through various contacts - desmosomes, intermediate, gap and tight junctions.

Epithelia are located on basement membranes, which are formed as a result of the activity of both epithelial cells and underlying connective tissue. The basement membrane is about 1 µm thick and consists of a subepithelial, electron-transparent, clear lamina

Rice. 6.2. Structure of the basement membrane (diagram according to E. F. Kotovsky): C - light lamina (lamina lucida); T - dark plate (lamina densa); BM - basement membrane. 1 - cytoplasm of epithelial cells; 2 - core; 3 - attachment plate of hemidesmosome (hemidesmosome); 4 - keratin tonofilaments; 5 - anchor filaments; 6 - plasmalemma of epithelial cells; 7 - anchoring fibrils; 8 - subepithelial loose connective tissue; 9 - blood capillary

(lamina lucida) 20-40 nm thick and dark plate (lamina densa) thickness 20-60 nm (Fig. 6.2). The light plate includes an amorphous substance, relatively poor in proteins, but rich in calcium ions. The dark plate has an amorphous matrix rich in proteins, into which fibrillar structures are soldered, providing mechanical strength to the membrane. Its amorphous substance contains complex proteins - glycoproteins, proteoglycans and carbohydrates (polysaccharides) - glycosaminoglycans. Glycoproteins - fibronectin and laminin - act as an adhesive substrate, with the help of which epithelial cells are attached to the membrane. An important role is played by calcium ions, which provide a connection between the adhesive molecules of glycoproteins of the basement membrane and hemidesmosomes of epithelial cells. In addition, glycoproteins induce proliferation and differentiation of epithelial cells during epithelial regeneration. Proteoglycans and glycosaminoglycans create the elasticity of the membrane and its characteristic negative charge, on which its selective permeability to substances depends, as well as the ability to accumulate many toxic substances (toxins), vasoactive amines and complexes of antigens and antibodies under pathological conditions.

Epithelial cells are especially tightly connected to the basement membrane in the region of hemidesmosomes (hemidesmosomes). Here, from the plasma membrane of the basal epithelial cells through the light plate to the dark plate of the basement membrane, “anchors” pass

ny" filaments. In the same area, but from the side of the underlying connective tissue, bundles of “anchoring” fibrils (containing type VII collagen) are woven into the dark lamina of the basement membrane, ensuring strong attachment of the epithelial layer to the underlying tissue.

Thus, the basement membrane performs a number of functions: mechanical (attachment), trophic and barrier (selective transport of substances), morphogenetic (organizing during regeneration) and limiting the possibility of invasive epithelial growth.

Due to the fact that blood vessels do not penetrate into the layers of epithelial cells, nutrition of the epithelial cells is carried out diffusely through the basement membrane from the underlying connective tissue, with which the epithelium is in close interaction.

The epithelium has polarity, i.e., the basal and apical sections of epithelial cells have different structures. In single-layer epithelia, cell polarity is most clearly expressed, manifested by morphological and functional differences in the apical and basal parts of epitheliocytes. Thus, epithelial cells of the small intestine have many microvilli on their apical surface, which ensure the absorption of digestive products. There are no microvilli in the basal part of the epithelial cell; absorption and release of metabolic products into the blood or lymph occurs through it. In multilayered epithelia, in addition, the polarity of the cell layer is noted - a difference in the structure of the epithelial cells of the basal, intermediate and superficial layers (see Fig. 6.1).

Epithelial tissues are usually classified as renewing tissues. Therefore, they have a high ability to regenerate. Restoration of the epithelium occurs due to mitotic division and differentiation of cambial cells. Depending on the location of cambial cells in epithelial tissues, diffuse and localized cambium are distinguished.

Sources of development and classification of epithelial tissues. Epithelia develop from all three germ layers, starting from the 3rd-4th week of human embryonic development. Depending on the embryonic source, epithelia of ectodermal, mesodermal and endodermal origin are distinguished. Epithelial cells form cell layers and are leading cellular differon in this fabric. During histogenesis, the composition of the epithelium (except for epithelial cells) may include histological elements of differons of a different origin (accompanying differons in polydifferent epithelia). There are also epithelia, where, along with border epithelial cells, as a result of divergent differentiation of the stem cell, cellular differentiates of epithelial cells of secretory and endocrine specialization arise, integrated into the composition of the epithelial layer. Only related types of epithelium, developing from the same germ layer, can be subject to pathological conditions. metaplasia, i.e., transition from one type to another, for example, in the respiratory tract, the ectodermal epithelium in chronic bronchitis from a single-layer ciliated one can turn into a multilayered squamous one,

which is normally characteristic of the oral cavity and is also of ectodermal origin.

The cytochemical marker of epithelial cells is the protein cytokeratin, which forms intermediate filaments. In different types of epithelia it has different molecular forms. More than 20 forms of this protein are known. Immunohistochemical detection of these forms of cytokeratin makes it possible to determine whether the material under study belongs to one or another type of epithelium, which has great importance in the diagnosis of tumors.

Classifications. There are several classifications of epithelia, which are based on various signs: origin, structure, function. When constructing classifications, histological features characterizing the leading cellular differentiation are taken into account. The most widely used morphological classification takes into account mainly the relationship of cells to the basement membrane and their shape (Scheme 6.1).

According to this classification, among the integumentary and lining epithelia that make up the skin, serous and mucous membranes of internal organs (oral cavity, esophagus, digestive tract, respiratory organs, uterus, urinary tract, etc.), two main groups of epithelia are distinguished : single-layer And multilayer. In single-layer epithelia, all cells are connected to the basement membrane, but in multilayer epithelia, only one lower layer of cells is directly connected to it, and the remaining overlying layers do not have such a connection. In accordance with the shape of the cells that make up single-layer epithelia, the latter are divided into flat(squamous), cubic And columnar(prismatic). In the definition of multilayer epithelium, only the shape of the cells of the outer layers is taken into account. For example, the epithelium of the cornea of ​​the eye is multilayered squamous, although its lower layers consist of columnar and winged cells.

Single layer epithelium can be single-row or multi-row. U single row epithelium all cells have the same shape - flat, cubic or columnar, their nuclei are located at the same level, i.e. in one row. Such epithelium is also called isomorphic (from the Greek. isos- equal). Single-layer epithelium, which has cells of various shapes and heights, the nuclei of which lie at different levels, i.e. in several rows, is called multi-row, or pseudo-multilayer(anisomorphic).

Stratified epithelium It can be keratinizing, non-keratinizing and transitional. The epithelium in which keratinization processes occur, associated with the differentiation of cells of the upper layers into flat horny scales, is called multilayer flat keratinizing. In the absence of keratinization, the epithelium is multilayer flat non-keratinizing.

Transitional epithelium lines organs subject to strong stretching - the bladder, ureters, etc. When the volume of an organ changes, the thickness and structure of the epithelium also changes.

Along with morphological classification, it is used ontophylogenetic classification, created by Russian histologist N. G. Khlopin. Depending on the embryonic rudiment, which serves as a source of development

Scheme 6.1. Morphological classification of types of surface epithelium

leading cellular differential, epithelia are divided into types: epidermal (skin), enterodermal (intestinal), coelonephrodermal, ependymoglial and angiodermal types of epithelia.

Epidermal type The epithelium is formed from the ectoderm, has a multilayer or multirow structure, and is adapted to perform primarily a protective function (for example, stratified squamous epithelium of the skin).

Enterodermal type The epithelium develops from the endoderm, is single-layered prismatic in structure, carries out the processes of absorption of substances (for example, single-layered marginal epithelium of the small intestine), and performs a glandular function (for example, single-layer epithelium of the stomach).

Coelonephrodermal type epithelium develops from mesoderm, single-layer, flat, cubic or prismatic in structure; performs mainly a barrier or excretory function (for example, the flat epithelium of the serous membranes - mesothelium, cubic and prismatic epithelium in the urinary tubules of the kidneys).

Ependymoglial type represented by a special epithelium lining, for example, the cavities of the brain. The source of its formation is the neural tube.

TO angiodermal type epithelium refers to the endothelial lining of blood vessels. The structure of the endothelium is similar to single-layer squamous epithelium. Its belonging to epithelial tissues is

Xia controversial. Many researchers classify the endothelium as connective tissue, with which it is connected by a common embryonic source of development - mesenchyme.

6.1.1. Single layer epithelia

Single row epithelia

Single layer squamous epithelium(epithelium simplex squamosum) is represented in the body by mesothelium and, according to some data, by endothelium.

Mesothelium covers serous membranes(leaves of the pleura, visceral and parietal peritoneum, pericardial sac). Mesothelial cells - mesotheliocytes- flat, have a polygonal shape and uneven edges (Fig. 6.3, A). In the part where the nucleus is located in them, the cells are thicker. Some of them contain not one, but two or even three nuclei, i.e. polyploid. There are microvilli on the free surface of the cell. Serous fluid is released and absorbed through the mesothelium. Thanks to its smooth surface, internal organs can glide easily. The mesothelium prevents the formation of connective tissue adhesions between the organs of the abdominal and thoracic cavities, the development of which is possible if its integrity is violated. Among mesotheliocytes there are poorly differentiated (cambial) forms capable of reproduction.

Endothelium lines blood and lymphatic vessels, as well as the chambers of the heart. It is a layer of flat cells - endotheliocytes, lying in one layer on the basement membrane. Endotheliocytes are relatively poor in organelles; pinocytotic vesicles are present in their cytoplasm. The endothelium, located in the vessels at the border with lymph and blood, participates in the exchange of substances and gases (O 2, CO 2) between them and other tissues. Endotheliocytes synthesize a variety of growth factors, vasoactive substances, etc. If the endothelium is damaged, blood flow in the vessels may change and blood clots may form in their lumen - thrombi. In different parts of the vascular system, endothelial cells differ in size, shape and orientation relative to the axis of the vessel. These properties of endothelial cells are designated as heteromorphy, or polymorphy(N. A. Shevchenko). Endotheliocytes capable of reproduction are located diffusely, with a predominance in the dichotomous division zones of the vessel.

Single layer cuboidal epithelium(epithelium simplex cuboideum) lines part of the renal tubules (proximal and distal). Proximal tubule cells have a microvillous (brush) border and basal striations. The brush border consists of a large number of microvilli. The striation is due to the presence in the basal sections of the cells of deep folds of the plasmalemma and mitochondria located between them. The epithelium of the renal tubules performs the function of reverse absorption (reabsorption) of a number of substances from the primary urine flowing through the tubules into the blood of the intertubular vessels. Cambial cells

Rice. 6.3. The structure of single-layer epithelium:

A- flat epithelium (mesothelium); b- columnar microvillous epithelium: 1 - microvilli (edge); 2 - epithelial cell nucleus; 3 - basement membrane; 4 - connective tissue; V- microphotograph: 1 - border; 2 - microvillous epithelial cells; 3 - goblet cell; 4 - connective tissue

located diffusely among epithelial cells. However, the proliferative activity of cells is extremely low.

Single-layer columnar (prismatic) epithelium(epithelium simplex columnare). This type of epithelium is characteristic of the middle section digestive system(see Fig. 6.3, b, c). It lines the inner surface of the stomach, small and large intestines, gallbladder, a number of ducts of the liver and pancreas. Epithelial cells are connected to each other using desmosomes, gap communication junctions, lock-type junctions, and tight junctions (see Chapter 4). Thanks to the latter, the contents of the cavity of the stomach, intestines and other hollow organs cannot penetrate into the intercellular gaps of the epithelium.

In the stomach, in the single-layer columnar epithelium, all cells are glandular (surface mucocytes) that produce mucus. The secretion of mucocytes protects the stomach wall from the harsh influence of food lumps and digestive action gastric juice, which has an acidic reaction, and enzymes that break down proteins. A minority of the epithelial cells located in the gastric pits - small depressions in the wall of the stomach - are cambial epithelial cells capable of dividing and differentiating into glandular epithelial cells. Due to pit cells, every 5 days the gastric epithelium is completely renewed - its physiological regeneration.

In the small intestine, the epithelium is single-layer columnar, actively participating in digestion, that is, in the breakdown of food into final products and their absorption into the blood and lymph. It covers the surface of the villi in the intestine and forms the wall of the intestinal glands - the crypts. The villous epithelium mainly consists of microvillous epithelial cells. The microvilli of the apical surface of the epithelial cell are covered with glycocalyx. Membrane digestion occurs here - the breakdown (hydrolysis) of food substances into final products and their absorption (transport through the membrane and cytoplasm of epithelial cells) into the blood and lymphatic capillaries of the underlying connective tissue. In the part of the epithelium that lines the intestinal crypts, there are borderless columnar epithelial cells, goblet cells, as well as endocrine cells and exocrinocytes with acidophilic granules (Paneth cells). Borderless crypt epithelial cells are cambial cells of the intestinal epithelium, capable of proliferation (reproduction) and divergent differentiation into microvillous, goblet, endocrine and Paneth cells. Thanks to cambial cells, microvillous epithelial cells are completely renewed (regenerated) within 5-6 days. Goblet cells secrete mucus onto the surface of the epithelium. Mucus protects it and the underlying tissues from mechanical, chemical and infectious influences, and also participates in parietal digestion, i.e. in the breakdown of proteins, fats and carbohydrates of food with the help of enzymes adsorbed in it to intermediate products. Endocrine (basal granular) cells of several types (EC, D, S, etc.) secrete hormones into the blood that locally regulate the function of the digestive apparatus. Paneth cells produce lysozyme, a bactericidal substance.

Single-layer epithelia are also represented by derivatives of the neuroectoderm - epithelium of the ependymoglial type. The cell structure varies from flat to columnar. Thus, the ependymal epithelium lining the central canal of the spinal cord and the ventricles of the brain is single-layer columnar. The retinal pigment epithelium is a single-layer epithelium consisting of polygonal cells. The perineural epithelium surrounding the nerve trunks and lining the perineural space is single-layer squamous. As derivatives of neuroectoderm, epithelia have disabilities regeneration, mainly intracellularly.

Multirow epithelia

Multirow (pseudostratified) epithelia (epithelium pseudostrati-ficatum) line the airways - nasal cavity, trachea, bronchi, and a number of other organs. In the airways, the multirow columnar epithelium is ciliated. Diversity of cell types

Rice. 6.4. Structure of multirow columnar ciliated epithelium: A- diagram: 1 - flickering cilia; 2 - goblet cells; 3 - ciliated cells; 4 - intercalary cells; 5 - basal cells; 6 - basement membrane; 7 - connective tissue; b- microphotograph: 1 - cilia; 2 - nuclei of ciliated and intercalary cells; 3 - basal cells; 4 - goblet cells; 5 - connective tissue

the composition of the epithelium (ciliated, intercalated, basal, goblet, Clara cells and endocrine cells) is the result of divergent differentiation of cambial (basal) epithelial cells (Fig. 6.4).

Basal epithelial cells low, located on the basement membrane deep in the epithelial layer, participate in the regeneration of the epithelium. Ciliated (ciliated) epithelial cells tall, columnar (prismatic) shape. These cells constitute the leading cellular differential. Their apical surface is covered with cilia. The movement of cilia ensures the transport of mucus and foreign particles towards the pharynx (mucociliary transport). Goblet epithelial cells secrete mucus (mucins) onto the surface of the epithelium, which protects it from mechanical, infectious and other influences. The epithelium also contains several types endocrinocytes(EC, D, P), hormones of which carry out local regulation of the muscle tissue of the airways. All these types of cells have different shapes and size, therefore their nuclei are located at different levels of the epithelial layer: in the upper row - the nuclei of ciliated cells, in the lower row - the nuclei of basal cells, and in the middle - the nuclei of intercalary, goblet and endocrine cells. In addition to epithelial differentials, the multirow columnar epithelium contains histological elements hematogenous differential(specialized macrophages, lymphocytes).

6.1.2. Stratified epithelia

Stratified squamous non-keratinizing epithelium(epithelium stiatificatum squamosum noncornificatum) covers the outside of the cornea of ​​the eye, lining

Rice. 6.5. The structure of the multilayered squamous non-keratinizing epithelium of the cornea (micrograph): 1 - layer of flat cells; 2 - spinous layer; 3 - basal layer; 4 - basement membrane; 5 - connective tissue

oral cavity and esophagus. There are three layers in it: basal, spinous (intermediate) and superficial (Fig. 6.5). Basal layer consists of columnar epithelial cells located on the basement membrane. Among them there are cambial cells capable of mitotic division. Due to the newly formed cells entering differentiation, the epithelial cells of the overlying layers of the epithelium are replaced. Layer spinosum consists of cells of irregular polygonal shape. In the epithelial cells of the basal and spinous layers, tonofibrils (bundles of tonofilaments made from keratin protein) are well developed, and between epithelial cells there are desmosomes and other types of contacts. Surface layers epithelium is formed by flat cells. Finishing your life cycle, the latter die and disappear.

Stratified squamous keratinizing epithelium(epithelium stratificatum squamosum comificatum)(Fig. 6.6) covers the surface of the skin, forming its epidermis, in which the process of keratinization (keratinization) occurs, associated with the differentiation of epithelial cells - keratinocytes into the horny scales of the outer layer of the epidermis. Differentiation of keratinocytes is manifested by their structural changes in connection with the synthesis and accumulation of specific proteins in the cytoplasm - cytokeratins (acidic and alkaline), filaggrin, keratolinin, etc. There are several layers of cells in the epidermis: basal, spinous, granular, shiny And horny. The last three layers are especially pronounced in the skin of the palms and soles.

The leading cellular differentiation in the epidermis is represented by keratinocytes, which, as they differentiate, move from the basal layer to the overlying layers. In addition to keratinocytes, the epidermis contains histological elements of accompanying cellular differentials - melanocytes(pigment cells), intraepidermal macrophages(Langerhans cells), lymphocytes And Merkel cells.

Basal layer consists of columnar-shaped keratinocytes, in the cytoplasm of which keratin protein is synthesized, forming tonofilaments. The cambial cells of the differon of keratinocytes are also located here. Layer spinosum formed by polygonal keratinocytes, which are tightly connected to each other by numerous desmosomes. In place of desmosomes on the surface of cells there are tiny projections -

Rice. 6.6. Stratified squamous keratinizing epithelium:

A- diagram: 1 - stratum corneum; 2 - shiny layer; 3 - granular layer; 4 - spinous layer; 5 - basal layer; 6 - basement membrane; 7 - connective tissue; 8 - pigmentocyte; b- microphotography

“spines” in adjacent cells directed towards each other. They are clearly visible when the intercellular spaces expand or when cells shrink, as well as during maceration. In the cytoplasm of spinous keratinocytes, tonofilaments form bundles - tonofibrils, and keratinosomes - granules containing lipids appear. These granules are released by exocytosis into the intercellular space, where they form a lipid-rich substance that cements keratinocytes.

Processed forms are also present in the basal and spinous layers melanocytes with granules of black pigment - melanin, Langerhans cells(dendritic cells) and Merkel cells(tactile epithelial cells), which have small granules and are in contact with afferent nerve fibers (Fig. 6.7). Melanocytes use pigment to create a barrier that prevents ultraviolet rays from penetrating the body. Langerhans cells are a type of macrophage that are involved in protective immune reactions and regulate the reproduction (division) of keratinocytes, forming together with them “epidermal-proliferative units”. Merkel cells are sensory (tactile) and endocrine (apudocytes) that influence epidermal regeneration (see Chapter 15).

Granular layer consists of flattened keratinocytes, the cytoplasm of which contains large basophilic granules, called keratohyaline. They include intermediate filaments (keratin) and the protein synthesized in the keratinocytes of this layer - filaggrin, and

Rice. 6.7. Structure and cellular-differential composition of multilayered squamous epithelium (epidermis) (according to E. F. Kotovsky):

I - basal layer; II - spinous layer; III - granular layer; IV, V - shiny and stratum corneum. K - keratinocytes; P - corneocytes (horny scales); M - macrophage (Langerhans cell); L - lymphocyte; O - Merkel cell; P - melanocyte; WITH - stem cell. 1 - mitotically dividing keratinocyte; 2 - keratin tonofilaments; 3 - desmosomes; 4 - keratinosomes; 5 - keratohyaline granules; 6 - keratolinin layer; 7 - core; 8 - intercellular substance; 9, 10 - keratin fibrils; 11 - cementing intercellular substance; 12 - falling scale; 13 - granules in the shape of tennis rackets; 14 - basement membrane; 15 - papillary layer of dermis; 16 - hemocapillary; 17 - nerve fiber

also substances formed as a result of the disintegration of organelles and nuclei that begins here under the influence of hydrolytic enzymes. In addition, another specific protein is synthesized in granular keratinocytes - keratolinin, which strengthens the plasma membrane of the cells.

Shiny layer detected only in heavily keratinized areas of the epidermis (on the palms and soles). It is formed by postcellular structures. They lack nuclei and organelles. Under the plasmalemma there is an electron-dense layer of the protein keratolinin, which gives it strength and protects it from the destructive effects of hydrolytic enzymes. Keratohyaline granules fuse and inner part cells are filled with a light-refracting mass of keratin fibrils glued together by an amorphous matrix containing filaggrin.

Stratum corneum very powerful in the skin of the fingers, palms, soles and relatively thin in other areas of the skin. It consists of flat polygonal-shaped (tetradecahedron) horny scales, which have a thick shell with keratolinin and filled with keratin fibrils located in an amorphous matrix consisting of another type of keratin. In this case, filaggrin breaks down into amino acids, which are part of the keratin fibrils. Between the scales there is a cementing substance - a product of keratinosomes, rich in lipids (ceramides, etc.) and therefore has a waterproofing property. The outermost horny scales lose contact with each other and constantly fall off the surface of the epithelium. They are replaced by new ones - due to the reproduction, differentiation and movement of cells from the underlying layers. Thanks to these processes, which make up physiological regeneration, in the epidermis the composition of keratinocytes is completely renewed every 3-4 weeks. The significance of the process of keratinization (keratinization) in the epidermis lies in the fact that the resulting stratum corneum is resistant to mechanical and chemical influences, has poor thermal conductivity and is impermeable to water and many water-soluble toxic substances.

Transitional epithelium(epithelium transitionale). This type of multilayer epithelium is typical of urinary drainage organs - renal pelvis, ureters, bladder, the walls of which are subject to significant stretching when filled with urine. It contains several layers of cells - basal, intermediate, superficial (Fig. 6.8, a, b).

Rice. 6.8. Structure of the transitional epithelium (diagram):

A- with an unstretched organ wall; b- with a stretched wall of the organ. 1 - transitional epithelium; 2 - connective tissue

Basal layer formed by small, almost round (dark) cambial cells. IN intermediate layer The cells are polygonal in shape. Surface layer consists of very large, often bi- and trinuclear cells, having a dome-shaped or flattened shape, depending on the condition of the organ wall. When the wall is stretched due to the filling of the organ with urine, the epithelium becomes thinner and its surface cells flatten. During contraction of the organ wall, the thickness of the epithelial layer increases sharply. In this case, some cells in the intermediate layer are “squeezed out” upward and take on a pear-shaped shape, while the surface cells located above them take on a dome-shaped shape. Tight junctions are found between superficial cells, which are important for preventing the penetration of fluid through the wall of an organ (for example, the bladder).

Regeneration. The integumentary epithelium, occupying a border position, is constantly influenced by the external environment, so the epithelial cells wear out and die relatively quickly. The source of their restoration is cambial cells epithelium, which provide a cellular form of regeneration, as they retain the ability to divide throughout the life of the organism. As they multiply, some of the newly formed cells begin to differentiate and turn into epithelial cells similar to the lost ones. Cambial cells in multilayer epithelia are located in the basal (primordial) layer; in multilayer epithelia these include basal cells; in single-layer epithelia they are located in certain areas: for example, in the small intestine - in the epithelium of the crypts, in the stomach - in the epithelium of the pits, and also necks of their own glands, in the mesothelium - among mesotheliocytes, etc. The high ability of most epithelia for physiological regeneration serves as the basis for its quick recovery V pathological conditions(reparative regeneration). In contrast, neuroectoderm derivatives are repaired primarily in an intracellular manner.

With age, a weakening of cell renewal processes is observed in the integumentary epithelium.

Innervation. The epithelium is well innervated. It contains numerous sensory nerve endings - receptors.

6.2. Glandular epithelia

These epithelia are characterized by secretory function. Glandular epithelium (epithelium glandulare) consists of glandular, or secretory, epithelial cells (glandulocytes). They carry out the synthesis, as well as the release of specific products - secretions onto the surface of the skin, mucous membranes and in the cavities of a number of internal organs (external - exocrine secretion) or into the blood and lymph (internal - endocrine secretion).

Through secretion, many important functions are performed in the body: the formation of milk, saliva, gastric and intestinal juice, bile, endo-

crine (humoral) regulation, etc. Most cells are distinguished by the presence of secretory inclusions in the cytoplasm, well-developed endoplasmic reticulum and Golgi complex, polar arrangement of organelles and secretory granules.

Secretory epithelial cells lie on the basement membrane. Their shape is very diverse and varies depending on the phase of secretion. The kernels are usually large, often irregular in shape. In the cytoplasm of cells that produce protein secretions (for example, digestive enzymes), the granular endoplasmic reticulum is well developed. In cells that synthesize non-protein secretions (lipids, steroids), an agranular endoplasmic reticulum is expressed. The Golgi complex is extensive. Its shape and location in the cell change depending on the phase of the secretory process. Mitochondria are usually numerous. They accumulate in places of greatest cell activity, i.e. where secretions are formed. The cytoplasm of cells usually contains secretory granules, the size and structure of which depend on the chemical composition of the secretion. Their number fluctuates depending on the phases of the secretory process. In the cytoplasm of some glandulocytes (for example, those involved in the formation of hydrochloric acid in the stomach), intracellular secretory tubules are found - deep invaginations of the plasmalemma, covered with microvilli. The plasmalemma has a different structure on the lateral, basal and apical surfaces of cells. At the first, it forms desmosomes and tight locking junctions. The latter surround the apical (apical) parts of the cells, thus separating the intercellular gaps from the lumen of the gland. On the basal surfaces of cells, the plasmalemma forms a small number of narrow folds that penetrate the cytoplasm. Such folds are especially well developed in the cells of glands that secrete secretions rich in salts, for example in the cells of the excretory ducts of the salivary glands. The apical surface of the cells is covered with microvilli.

Polar differentiation is clearly visible in glandular cells. It is due to the direction of secretory processes, for example, during external secretion from the basal to the apical part of the cell.

Periodic changes in the glandular cell associated with the formation, accumulation, release of secretion and its restoration for further secretion are called secretory cycle.

To form secretions from the blood and lymph, various inorganic compounds, water and low-molecular organic substances enter the glandular cells from the basal surface: amino acids, monosaccharides, fatty acids, etc. Sometimes larger molecules penetrate into the cell by pinocytosis organic matter, such as proteins. Secrets are synthesized from these products in the endoplasmic reticulum. They move through the endoplasmic reticulum to the Golgi complex zone, where they gradually accumulate, undergo chemical rearrangement and form into granules that are released from epithelial cells. An important role in the movement of secretory products in epithelial cells and their secretion is played by cytoskeletal elements - microtubules and microfilaments.

Rice. 6.9. Different types of secretion (diagram):

A- merocrine; b- apocrine; V- holocrine. 1 - poorly differentiated cells; 2 - degenerating cells; 3 - collapsing cells

However, the division of the secretory cycle into phases is essentially arbitrary, since they overlap each other. Thus, the synthesis of secretion and its release proceed almost continuously, but the intensity of secretion may either increase or decrease. In this case, the release of secretion (extrusion) can be different: in the form of granules or by diffusion without forming into granules or by converting the entire cytoplasm into a mass of secretion. For example, in cases of stimulation of the glandular cells of the pancreas, all secretory granules are quickly released from them, and after that, within 2 hours or more, the secretion is synthesized in the cells without forming into granules and is released diffusely.

The secret release mechanism various glands unequal, and therefore three types of secretion are distinguished: merocrine (eccrine), apocrine and holocrine (Fig. 6.9). At merocrine type secretion, glandular cells completely retain their structure (for example, cells of the salivary glands). At apocrine type secretion, partial destruction of glandular cells (for example, mammary gland cells) occurs, i.e., along with secretory products, either the apical part of the cytoplasm of glandular cells (macroapocrine secretion) or the tips of microvilli (microapocrine secretion) are separated.

Holocrine type secretion is accompanied by the accumulation of secretion (fat) in the cytoplasm and complete destruction glandular cells (for example, cells of the sebaceous glands of the skin). Restoration of the structure of glandular cells occurs either through intracellular regeneration (with mero- and apocrine secretion), or with the help of cellular regeneration, i.e., division and differentiation of cambial cells (with holocrine secretion).

Secretion is regulated using neural and humoral mechanisms: the former act through the release of cellular calcium, and the latter primarily through the accumulation of cAMP. At the same time, enzyme systems and metabolism, the assembly of microtubules and the reduction of microfilaments involved in intracellular transport and excretion of secretions are activated in glandular cells.

Glands

Glands are organs that produce specific substances of various chemical nature and releasing them into the excretory ducts or into the blood and lymph. The secretions produced by the glands are important for the processes of digestion, growth, development, interaction with the external environment, etc. Many glands are independent, anatomically designed organs (for example, the pancreas, large salivary glands, thyroid), some are only part of the organs (for example, the stomach glands).

The glands are divided into two groups: endocrine glands, or endocrine, And exocrine glands, or exocrine(Fig. 6.10, a, b).

Endocrine glands produce highly active substances - hormones, entering directly into the blood. Therefore, they consist only of glandular cells and do not have excretory ducts. All of them are part of the body's endocrine system, which, together with the nervous system, performs a regulatory function (see Chapter 15).

Exocrine glands produce secrets, released into the external environment, i.e. on the surface of the skin or in the cavities of organs lined with epithelium. They can be unicellular (for example, goblet cells) or multicellular. Multicellular glands consist of two parts: secretory or terminal sections (portiones terminalae) and excretory ducts (ductus excretorii). The terminal sections are formed secretory epithelial cells, lying on the basement membrane. The excretory ducts are lined with various

Rice. 6.10. The structure of exocrine and endocrine glands (according to E. F. Kotovsky): A- exocrine gland; b- endocrine gland. 1 - end section; 2 - secretory granules; 3 - excretory duct of the exocrine gland; 4 - integumentary epithelium; 5 - connective tissue; 6 - blood vessel

Scheme 6.2. Morphological classification of exocrine glands

types of epithelium depending on the origin of the glands. In glands formed from endodermal type epithelium (for example, in the pancreas), they are lined with single-layer cubic or columnar epithelium, and in glands developing from ectoderm (for example, in the sebaceous glands of the skin), they are lined with stratified epithelium. Exocrine glands are extremely diverse, differing from each other in structure, type of secretion, i.e., the method of secretion and its composition. The listed characteristics form the basis for the classification of glands. According to their structure, exocrine glands are divided into the following types (see Fig. 6.10, a, b; diagram 6.2).

Simple tubular glands have a non-branching excretory duct, complex glands have a branching one. In unbranched glands one at a time, and in branched glands several terminal sections open into it, the shape of which can be in the form of a tube or a sac (alveolus) or an intermediate type between them.

In some glands derived from ectodermal (stratified) epithelium, for example in salivary glands, in addition to secretory cells, there are epithelial cells that have the ability to contract - myoepithelial cells. These cells, which have a process form, cover the terminal sections. Their cytoplasm contains microfilaments containing contractile proteins. Myoepithelial cells, when contracting, compress the end sections and, therefore, facilitate the release of secretions from them.

The chemical composition of the secretion may be different; therefore, the exocrine glands are divided into protein(serous), mucous membranes(mucosal), protein-mucosal(see Fig. 6.11), greasy, salty(sweat, tears, etc.).

Two types of secretory cells may be present in the mixed salivary glands - protein(serocytes) and mucous membranes(mucocytes). They form

There are protein, mucous and mixed (protein-mucous) terminal sections. Most often, the composition of the secretory product includes protein and mucous components with only one of them predominant.

Regeneration. In the glands, in connection with their secretory activity, processes of physiological regeneration constantly occur. In merocrine and apocrine glands, which contain long-lived cells, restoration of the original state of secretory epithelial cells after secretion from them occurs through intracellular regeneration, and sometimes through reproduction. In holocrine glands, restoration is carried out due to the proliferation of cambial cells. The newly formed cells are then transformed into glandular cells through differentiation (cellular regeneration).

Rice. 6.11. Types of exocrine glands:

1 - simple tubular glands with unbranched end sections;

2 - simple alveolar gland with an unbranched end section;

3 - simple tubular glands with branched end sections;

4 - simple alveolar glands with branched terminal sections; 5 - complex alveolar-tubular gland with branched end sections; 6 - complex alveolar gland with branched end sections

In old age, changes in the glands can be manifested by a decrease in the secretory activity of glandular cells and changes in the composition

secretions produced, as well as weakening of regeneration processes and proliferation of connective tissue (gland stroma).

Control questions

1. Sources of development, classification, topography in the body, main morphological properties epithelial tissues.

2. Multilayer epithelia and their derivatives: topography in the body, structure, cellular differential composition, functions, patterns of regeneration.

3. Single-layer epithelia and their derivatives, topography in the body, cellular differential composition, structure, functions, regeneration.

Histology, embryology, cytology: textbook / Yu. I. Afanasyev, N. A. Yurina, E. F. Kotovsky, etc. - 6th ed., revised. and additional - 2012. - 800 p. : ill.

Epithelial tissues, or epithelia (erithelia), cover the surfaces of the body, mucous and serous membranes of internal organs (stomach, intestines, bladder, etc.), and also form most of the glands. In this regard, a distinction is made between the integumentary and glandular epithelia.

Covering epithelium is a border tissue. It separates the body (internal environment) from the external environment, but at the same time participates in the metabolism of the body with environment, performing the functions of absorbing substances (absorption) and releasing metabolic products (excretion). For example, through the intestinal epithelium, products of food digestion are absorbed into the blood and lymph, which serve as a source of energy and building material for the body, and through the renal epithelium, a number of nitrogen metabolism products are released, which are waste products for the body. In addition to these functions, the integumentary epithelium performs an important protective function, protecting the underlying tissues of the body from various external influences - chemical, mechanical, infectious, etc. For example, the skin epithelium is a powerful barrier to microorganisms and many poisons. Finally, the epithelium covering the internal organs located in the body cavities creates conditions for their mobility, for example, for contraction of the heart, excursion of the lungs, etc.

Glandular epithelium carries out a secretory function, that is, it forms and secretes specific products - secretions that are used in processes occurring in the body. For example, the secretion of the pancreas is involved in the digestion of proteins, fats and carbohydrates in the small intestine.

SOURCES OF DEVELOPMENT OF EPITHELIAL TISSUE

Epithelia develop from all three germ layers starting from the 3rd-4th week of human embryonic development. Depending on the embryonic source, epithelia of ectodermal, mesodermal and endodermal origin are distinguished.

Structure. Epithelia are involved in the construction of many organs, and therefore exhibit a wide variety of morphophysiological properties. Some of them are general, allowing one to distinguish epithelia from other tissues of the body.

Epithelia are layers of cells - epithelial cells (Fig. 39), which have different shapes and structures in different types of epithelium. There is no intercellular substance between the cells that make up the epithelial layer and the cells are closely connected to each other through various contacts - desmosomes, tight junctions, etc. Epithelia are located on basement membranes (lamellas). The basement membranes are about 1 µm thick and consist of amorphous substance and fibrillar structures. The basement membrane contains carbohydrate-protein-lipid complexes, on which its selective permeability to substances depends. Epithelial cells can be connected to the basement membrane by hemidesmosomes, similar in structure to the halves of desmosomes.

Epithelia do not contain blood vessels. Nutrition of epithelial cells occurs diffusely through the basement membrane from the side of the underlying connective tissue, with which the epithelium is in close interaction. Epithelia have polarity, that is, the basal and apical sections of the entire epithelial layer and its constituent cells have a different structure. Epithelia have a high ability to regenerate. Epithelial restoration occurs due to mitotic division and differentiation of stem cells.

CLASSIFICATION

There are several classifications of epithelia, which are based on various characteristics: origin, structure, function. Of these, the most widespread is the morphological classification, which takes into account the relationship of cells to the basement membrane and their shape on the free, apical (from the Latin apex - apex) part of the epithelial layer (Scheme 2).

In morphological classification reflects the structure of epithelia, depending on their function.

According to this classification, first of all, single-layer and multilayer epithelia are distinguished. In the first, all epithelial cells are connected to the basement membrane, in the second, only one lower layer of cells is directly connected to the basement membrane, and the remaining layers are deprived of such a connection and are connected to each other. According to the shape of the cells that make up the epithelium, they are divided into flat, cubic and prismatic (cylindrical). In this case, in multilayered epithelium, only the shape of the outer layers of cells is taken into account. For example, the epithelium of the cornea is multilayered squamous, although its lower layers consist of prismatic and winged cells.

Single layer epithelium can be single-row or multi-row. In single-row epithelium, all cells have the same shape - flat, cubic or prismatic and, therefore, their nuclei lie at the same level, i.e. in one row. Such an epithelium is also called isomorphic (from the Greek isos - equal). Single-layer epithelium, which has cells of various shapes and heights, the nuclei of which lie at different levels, i.e., in several rows, is called multi-row, or pseudo-stratified.

Stratified epithelium It can be keratinizing, non-keratinizing and transitional. The epithelium in which keratinization processes occur, associated with the transformation of the cells of the upper layers into horny scales, is called multilayered squamous keratinization. In the absence of keratinization, the epithelium is stratified squamous non-keratinizing.

Transitional epithelium lines organs subject to strong stretching - the bladder, ureters, etc. When the volume of an organ changes, the thickness and structure of the epithelium also changes.

Along with morphological classification, it is used ontophylogenetic classification, created by the Soviet histologist N. G. Khlopin. It is based on the peculiarities of the development of epithelia from tissue primordia. It includes epidermal (cutaneous), enterodermal (intestinal), coelonephrodermal, ependymoglial and angiodermal types of epithelium.

Epidermal type The epithelium is formed from the ectoderm, has a multilayer or multirow structure, and is adapted to perform primarily a protective function (for example, stratified squamous epithelium of the skin).

Enterodermal type The epithelium develops from the endoderm, is single-layer prismatic in structure, carries out the processes of absorption of substances (for example, the single-layer bordered epithelium of the small intestine), and performs a glandular function.

Coelonephrodermal type the epithelium is of mesodermal origin, its structure is single-layer, flat, cubic or prismatic, and performs mainly a barrier or excretory function (for example, the flat epithelium of the serous membranes - mesothelium, cubic and prismatic epithelium in the urinary tubules of the kidneys).

Ependymoglial type represented by a special epithelium lining, for example, the cavities of the brain. The source of its formation is the neural tube.

To angiodermal type include the endothelial lining of blood vessels, which is of mesenchymal origin. The structure of the endothelium is single-layer squamous epithelium.

STRUCTURE OF DIFFERENT TYPES OF COVERING EPITHELIA

Single-layer squamous epithelium (epithelium simplex squamosum).
This type of epithelium is represented in the body by endothelium and mesothelium.

Endothelium (entothelium) lines blood and lymphatic vessels, as well as the chambers of the heart. It is a layer of flat cells - endothelial cells, lying in one layer on the basement membrane. Endotheliocytes are distinguished by a relative paucity of organelles and the presence of pinocytotic vesicles in the cytoplasm.

The endothelium is involved in the exchange of substances and gases (O2, CO2) between the blood and other tissues of the body. If it is damaged, it is possible to change the blood flow in the vessels and form blood clots - thrombi - in their lumens.

Mesothelium covers the serous membranes (leaves of the pleura, visceral and parietal peritoneum, pericardial sac, etc.). Mesothelial cells - mesotheliocytes are flat, have a polygonal shape and uneven edges (Fig. 40, A). At the location of the nuclei, the cells are somewhat thickened. Some of them contain not one, but two or even three cores. There are single microvilli on the free surface of the cell. Serous fluid is released and absorbed through the mesothelium. Thanks to its smooth surface, internal organs can glide easily. The mesothelium prevents the formation of connective tissue adhesions between the organs of the abdominal and thoracic cavities, the development of which is possible if its integrity is violated.

Single-layer cubic epithelium (epithelium simplex cubuideum). It lines part of the renal tubules (proximal and distal). Proximal tubule cells have a brush border and basal striations. The striation is due to the concentration of mitochondria in the basal parts of the cells and the presence here of deep folds of the plasmalemma. The epithelium of the renal tubules performs the function of reverse absorption (reabsorption) of a number of substances from primary urine into the blood.

Single-layer prismatic epithelium (epithelium simplex columnare). This type of epithelium is characteristic of the middle section of the digestive system. It lines the inner surface of the stomach, small and large intestines, gallbladder, a number of ducts of the liver and pancreas.

In the stomach, in a single-layer prismatic epithelium, all cells are glandular, producing mucus, which protects the stomach wall from the harsh influence of food lumps and the digestive action of gastric juice. In addition, water and some salts are absorbed into the blood through the epithelium of the stomach.

In the small intestine, a single-layer prismatic (“bordered”) epithelium actively performs the function of absorption. The epithelium is formed by prismatic epithelial cells, among which goblet cells are located (Fig. 40, B). Epithelial cells have a well-defined striated (brush) suction border, consisting of many microvilli. They participate in the enzymatic breakdown of food (parietal digestion) and the absorption of the resulting products into the blood and lymph. Goblet cells secrete mucus. Covering the epithelium, mucus protects it and the underlying tissues from mechanical and chemical influences.

Along with border and goblet cells, there are basal granular endocrine cells of several types (EC, D, S, J, etc.) and apical granular glandular cells. The hormones of endocrine cells released into the blood take part in regulating the function of the digestive system.

Multi-row (pseudostratified) epithelium (epithelium pseudostratificatum). It lines the airways - the nasal cavity, trachea, bronchi, and a number of other organs. In the airways multirow epithelium is ciliated, or ciliated. There are 4 types of cells in it: ciliated (ciliated) cells, short and long intercalary cells, mucous (goblet) cells (Fig. 41; see Fig. 42, B), as well as basal granular (endocrine) cells. The intercalary cells are likely stem cells capable of dividing and developing into ciliated and mucous cells.

Intercalary cells are attached to the basement membrane by a wide proximal part. In ciliated cells, this part is narrow, and their wide distal part faces the lumen of the organ. Thanks to this, three rows of nuclei can be distinguished in the epithelium: the lower and middle rows are the nuclei of intercalary cells, the upper row are the nuclei of ciliated cells. The apices of the intercalary cells do not reach the surface of the epithelium, so it is formed only by the distal parts of the ciliated cells, covered with numerous cilia. The mucous cells have a goblet or ovoid shape and secrete mucins onto the surface of the layer.

Trapped with air in Airways dust particles settle on the mucous surface of the epithelium and are gradually pushed out by the movement of its ciliated cilia into the nasal cavity and further into the external environment. In addition to ciliated, intercalated and mucous epithelial cells, several types of endocrine, basal granular cells (EC-, P-, D-cells) were found in the epithelium of the airways. These cells secrete biologically active substances into the blood vessels - hormones, with the help of which local regulation of the respiratory system is carried out.

Stratified squamous non-keratinized epithelium (epithelium stratificatum squamosum noncornificatum). Covers the outside of the cornea of ​​the eye, lining the oral cavity and esophagus. There are three layers distinguished in it: basal, spinous (intermediate) and flat (superficial) (Fig. 42, A).

Basal layer consists of prismatic epithelial cells located on the basement membrane. Among them there are stem cells capable of mitotic division. Due to the newly formed cells entering differentiation, the epithelial cells of the overlying layers of the epithelium are replaced.

Layer spinosum consists of cells of irregular polygonal shape. In the basal and spinous layers in epithelial cells, tonofibrils (tonofilament bundles) are well developed, and between epithelial cells there are desmosomes and other types of contacts. The upper layers of the epithelium are formed by flat cells. Having completed their life cycle, they die and fall off the surface of the epithelium.

Stratified squamous keratinizing epithelium (epithelium stratificatum squamosum cornificatum). Covers the surface of the skin, forming its epidermis, in which the process of transformation (transformation) of epithelial cells into horny scales occurs - keratinization. At the same time, specific proteins (keratins) are synthesized in the cells and accumulate more and more, and the cells themselves gradually move from the lower layer to the overlying layers of the epithelium. In the epidermis of the skin of the fingers, palms and soles, 5 main layers are distinguished: basal, spinous, granular, shiny and horny (Fig. 42, B). The skin of the rest of the body has an epidermis in which there is no shiny layer.

Basal layer consists of cylindrical epithelial cells. In their cytoplasm, specific proteins are synthesized that form tonofilaments. This is where stem cells are located. Stem cells divide, after which some of the newly formed cells differentiate and move to the overlying layers. Therefore, the basal layer is called germinal, or germinal (stratum germinativum).

Layer spinosum formed by polygonal-shaped cells that are firmly connected to each other by numerous desmosomes. In place of desmosomes on the surface of cells there are tiny projections - “spines” directed towards each other. They are clearly visible when intercellular spaces expand or when cells shrink. In the cytoplasm of spinous cells, tonofilaments form bundles - tonofibrils.

In addition to epithelial cells, the basal and spinous layers contain process-shaped pigment cells - melanocytes, containing granules of the black pigment - melanin, as well as epidermal macrophages - dendrocytes and lymphocytes, which form a local immune surveillance system in the epidermis.

Granular layer consists of flattened cells, the cytoplasm of which contains tonofibrils and keratohyalin grains. Keratohyalin is a fibrillar protein that can subsequently be converted into eleidin in the cells of the overlying layers, and then into keratin - the horny substance.

Shiny layer formed by flat cells. Their cytoplasm contains highly refractive eleidin, which is a complex of keratohyalin with tonofibrils.

Stratum corneum very powerful in the skin of the fingers, palms, soles and relatively thin in other areas of the skin. As cells move from the stratum lucidum to the stratum corneum, their nuclei and organelles gradually disappear with the participation of lysosomes, and the complex of keratohyalin with tonofibrils turns into keratin fibrils and the cells become horny scales, shaped like flat polyhedra. They are filled with keratin (horny substance), consisting of densely packed keratin fibrils, and air bubbles. The outermost horny scales, under the influence of lysosome enzymes, lose contact with each other and constantly fall off the surface of the epithelium. They are replaced by new ones due to the proliferation, differentiation and movement of cells from the underlying layers. The stratum corneum of the epithelium is characterized by significant elasticity and poor thermal conductivity, which is important for protecting the skin from mechanical influences and for the processes of thermoregulation of the body.

Transitional epithelium (epithelium transitionale). This type of epithelium is typical of urinary drainage organs - renal pelvis, ureters, bladder, the walls of which are subject to significant stretching when filled with urine. It contains several layers of cells - basal, intermediate, superficial (Fig. 43, A, B).

Basal layer formed by small round (dark) cells. The intermediate layer contains cells of various polygonal shapes. The surface layer consists of very large, often bi- and trinuclear cells, having a dome-shaped or flattened shape, depending on the condition of the organ wall. When the wall is stretched due to the filling of the organ with urine, the epithelium becomes thinner and its surface cells flatten. During contraction of the organ wall, the thickness of the epithelial layer increases sharply. In this case, some cells in the intermediate layer are “squeezed out” upward and take on a pear-shaped shape, and the surface cells located above them take on a dome-shaped shape. Tight junctions are found between superficial cells, which are important for preventing the penetration of fluid through the wall of an organ (for example, the bladder).

Regeneration. The integumentary epithelium, occupying a border position, is constantly influenced by the external environment, so the epithelial cells wear out and die relatively quickly.

The source of their restoration is epithelial stem cells. They retain the ability to divide throughout the life of the organism. While multiplying, some of the newly formed cells begin to differentiate and turn into epithelial cells similar to the lost ones. Stem cells in multilayer epithelia are located in the basal (primordial) layer; in multilayer epithelia these include intercalary (short) cells; in single-layer epithelia they are located in certain areas, for example, in the small intestine in the epithelium of the crypts, in the stomach in the epithelium of the necks of the own glands and etc. The high ability of the epithelium for physiological regeneration serves as the basis for its rapid restoration under pathological conditions (reparative regeneration).

Vascularization. Covering epithelia do not have blood vessels, with the exception of the stria vascularis. inner ear. Nutrition to the epithelium comes from vessels located in the underlying connective tissue.

Innervation. The epithelium is well innervated. It contains numerous sensitive nerve endings - receptors.

Age-related changes. With age, a weakening of renewal processes is observed in the integumentary epithelium.

STRUCTURE OF GLONUS EPITHELIA

The glandular epithelium (epithelium glandulare) consists of glandular, or secretory, cells - glandulocytes. They carry out the synthesis, as well as the release of specific products - secretions onto the surface of the skin, mucous membranes and in the cavities of a number of internal organs [external (exocrine) secretion] or into the blood and lymph [internal (endocrine) secretion].

Through secretion, many important functions are performed in the body: the formation of milk, saliva, gastric and intestinal juice, bile, endocrine (humoral) regulation, etc.

Most glandular cells with external secretion (exocrine) are distinguished by the presence of secretory inclusions in the cytoplasm, a developed endoplasmic reticulum, and a polar arrangement of organelles and secretory granules.

Secretion (from Latin secretio - separation) is complex process, including 4 phases:

1. absorption of starting products by glandulocytes,
2. synthesis and accumulation of secretions in them,
3. secretion from glandulocytes - extrusion
4. and restoration of their structure.

These phases can occur in glandulocytes cyclically, that is, one after another, in the form of the so-called secretory cycle. In other cases, they occur simultaneously, which is typical for diffuse or spontaneous secretion.

First phase of secretion lies in the fact that various inorganic compounds, water and low-molecular organic substances enter the glandular cells from the blood and lymph from the basal surface: amino acids, monosaccharides, fatty acids, etc. Sometimes larger molecules of organic substances penetrate into the cell by pinocytosis, for example proteins.

In the second phase From these products, secretions are synthesized in the endoplasmic reticulum, protein secretions with the participation of the granular endoplasmic reticulum, and non-protein secretions with the participation of the agranular endoplasmic reticulum. The synthesized secretion moves through the endoplasmic reticulum to the zone of the Golgi complex, where it gradually accumulates, undergoes chemical restructuring and is formed in the form of granules.

In the third phase the resulting secretory granules are released from the cell. Secretion is released differently, and therefore three types of secretion are distinguished:

• merocrine (eccrine)
• apocrine
• holocrine (Fig. 44, A, B, C).

With the merocrine type of secretion, glandular cells completely retain their structure (for example, cells of the salivary glands).

With the apocrine type of secretion, partial destruction of glandular cells (for example, mammary gland cells) occurs, i.e., along with secretory products, either the apical part of the cytoplasm of glandular cells (macroapocrine secretion) or the tips of microvilli (microapocrine secretion) are separated.

The holocrine type of secretion is accompanied by the accumulation of fat in the cytoplasm and the complete destruction of glandular cells (for example, cells of the sebaceous glands of the skin).

Fourth phase of secretion consists in restoring the original state of glandular cells. Most often, however, the restoration of cells occurs as they are destroyed.

Glandulocytes lie on the basement membrane. Their shape is very diverse and varies depending on the phase of secretion. The kernels are usually large, with a rugged surface, which gives them an irregular shape. In the cytoplasm of glandulocytes, which produce protein secretions (for example, digestive enzymes), a granular endoplasmic reticulum is well developed.

In cells that synthesize non-protein secretions (lipids, steroids), an agranular cytoplasmic reticulum is expressed. The Golgi complex is extensive. Its shape and location in the cell change depending on the phase of the secretory process. Mitochondria are usually numerous. They accumulate in places of greatest cell activity, i.e. where secretions are formed. The cytoplasm of cells usually contains secretory granules, the size and structure of which depend on the chemical composition of the secretion. Their number fluctuates depending on the phases of the secretory process.

In the cytoplasm of some glandulocytes (for example, those involved in the formation of hydrochloric acid in the stomach), intracellular secretory tubules are found - deep invaginations of the cytolemma, the walls of which are covered with microvilli.

The cytolemma has a different structure on the lateral, basal and apical surfaces of cells. On the lateral surfaces it forms desmosomes and tight junctions (terminal bridges). The latter surround the apical (apical) parts of the cells, thus separating the intercellular gaps from the lumen of the gland. On the basal surfaces of cells, the cytolemma forms a small number of narrow folds that penetrate the cytoplasm. Such folds are especially well developed in the cells of the glands that secrete secretions rich in salts, for example in the duct cells of the salivary glands. The apical surface of the cells is covered with microvilli.

Polar differentiation is clearly visible in glandular cells. It is due to the direction of secretory processes, for example, during external secretion from the basal to the apical part of the cells.

GLANDS

Glands (glandulae) perform a secretory function in the body. Most of them are derivatives of glandular epithelium. The secretions produced in the glands are important for the processes of digestion, growth, development, interaction with the external environment, etc. Many glands are independent, anatomically designed organs (for example, the pancreas, large salivary glands, thyroid gland). Other glands are only part of the organs (for example, the glands of the stomach).

Glands are divided into two groups:

1. endocrine glands, or endocrine glands
2. exocrine glands, or exocrine (Fig. 45, A, B, C).

Endocrine glands produce highly active substances - hormones that enter directly into the blood. That is why these glands consist only of glandular cells and do not have excretory ducts. These include the pituitary gland, pineal gland, thyroid and parathyroid gland, adrenal glands, pancreatic islets, etc. All of them are part of the body’s endocrine system, which, together with the nervous system, performs a regulatory function.

Exocrine glands produce secretions that are released into the external environment, i.e., onto the surface of the skin or into organ cavities lined with epithelium. In this regard, they consist of two parts:

1. secretory, or terminal, sections (pirtiones terminalae)
2. excretory ducts (ductus excretorii).

The terminal sections are formed by glandulocytes lying on the basement membrane. Excretory ducts are lined various types epithelium depending on the origin of the glands. In the glands developing from the enterodermal epithelium (for example, in the pancreas), they are lined with single-layer cubic or prismatic epithelium, and in the glands developing from the ectodermal epithelium (for example, in the sebaceous glands of the skin), they are lined with stratified non-keratinizing epithelium. Exocrine glands are extremely diverse, differing from each other in structure, type of secretion, i.e., the method of secretion and its composition.

The listed characteristics form the basis for the classification of glands. Based on their structure, exocrine glands are divided into the following types (Scheme 3).

Simple glands have a non-branching excretory duct, complex glands - branching (see Fig. 45, B). It opens into it in unbranched glands one at a time, and in branched glands into several terminal sections, the shape of which can be in the form of a tube or a sac (alveolus) or an intermediate type between them.

In some glands derived from ectodermal (stratified) epithelium, for example in salivary glands, in addition to secretory cells, there are epithelial cells that have the ability to contract - myoepithelial cells. These cells, which have a process form, cover the terminal sections. Their cytoplasm contains microfilaments containing contractile proteins. Myoepithelial cells, when contracting, compress the end sections and, therefore, facilitate the release of secretions from them.

The chemical composition of the secretion may be different; therefore, the exocrine glands are divided into

• proteinaceous (serous)
• mucous membranes
• protein-mucosal (see Fig. 42, D)
• greasy.

In mixed glands, two types of secretory cells may be present - protein and mucous. They form either separately end sections (purely proteinaceous and purely mucous), or together mixed end sections (proteinaceous and mucous). Most often, the composition of the secretory product includes protein and mucous components with only one of them predominant.

Regeneration. In the glands, in connection with their secretory activity, processes of physiological regeneration constantly occur.

In merocrine and apocrine glands, which contain long-lived cells, restoration of the original state of glandulocytes after secretion from them occurs through intracellular regeneration, and sometimes through reproduction.

In holocrine glands, restoration is carried out through the proliferation of special stem cells. The newly formed cells are then transformed into glandular cells through differentiation (cellular regeneration).

Vascularization. The glands are abundantly supplied with blood vessels. Among them there are arteriole-venular anastomoses and veins equipped with sphincters (closing veins). Closing the anastomoses and sphincters of the closing veins leads to an increase in pressure in the capillaries and ensures the release of substances used by glandulocytes to form secretions.

Innervation. Carried out by the sympathetic and parasympathetic nervous system. Nerve fibers follow in the connective tissue along the blood vessels and excretory ducts of the glands, forming nerve endings on the cells of the terminal sections and excretory ducts, as well as in the walls of blood vessels.

Except nervous system, the secretion of exocrine glands is regulated by humoral factors, i.e., hormones of the endocrine glands.

Age-related changes. In old age, changes in the glands can manifest themselves in a decrease in the secretory activity of glandular cells and changes in the composition of secretions produced, as well as a weakening of regeneration processes and the proliferation of connective tissue (gland stroma).

Single layer epithelia – all cells are located on the basement membrane, with the cell nuclei single row epithelia are at the same level, and cell nuclei multi-row epithelium are at different levels, which creates the effect of multi-row (and the false impression of multi-layering).

1. Single layer squamous epithelium formed by flattened polygonal cells with thickening in the area where the discoid nucleus is located. There are single microvilli on the free surface of the cell. An example of this type is the epithelium (mesothelium) covering the lung (visceral pleura) and the epithelium lining the inside of the chest cavity (parietal pleura), as well as the parietal and visceral layers of the peritoneum, the pericardial sac.

2. Single layer cuboidal epithelium formed by cells containing a spherical nucleus. Such epithelium is found in the follicles of the thyroid gland, in the small ducts of the pancreas and bile ducts, in the renal tubules .

3. Single-layer prismatic (cylindrical) epithelium (Fig. 1) is formed by cells with a pronounced polarity. The ellipsoidal nucleus lies along the long axis of the cell and is shifted to their basal part; well-developed organelles are unevenly distributed throughout the cytoplasm. On the apical surface there are microvilli, brush border. This type of epithelium is characteristic of the middle section of the digestive canal and lines the inner surface of the small and large intestines, stomach, gall bladder, a number of large pancreatic ducts and bile ducts liver. This type of epithelium is characterized by the functions secretion and/or absorption.

There are two main types of differentiated cells found in the epithelium of the small intestine: prismatic edged, providing parietal digestion, and goblet, producing mucus. Such unequal structure and functions of cells in single layer epithelium called horizontal anisomorphic.

4. Multirow ciliated (ciliated) epithelium of the airways (Fig. 2) is formed by several types of cells: 1) low intercalary (basal), 2) high intercalary (intermediate), 3) ciliated (ciliated), 4) goblet. Low intercalary cells are cambial; with their wide base they are adjacent to the basement membrane, and with their narrow apical part they do not reach the lumen. Goblet cells produce mucus that covers the surface of the epithelium, moving along it thanks to the beating of the cilia of the ciliated cells. The apical parts of these cells border the lumen of the organ.

Stratified epithelia– epithelia, in which only the cells forming the basal layer are located on the basement membrane. The cells that make up the remaining layers lose contact with it. Multilayer epithelia are characterized by vertical anisomorphy – unequal morphological properties of cells of different layers of the epithelial layer. The classification of multilayer epithelium is based on the shape of the cells of the surface layer.

Maintaining the integrity of multilayer epithelia is ensured by regeneration. Epithelial cells continuously divide in the deepest basal layer at the expense of stem cells, followed by a shift to the overlying layers. After differentiation, degeneration and exfoliation of cells from the surface of the layer occurs. Processes proliferation And differentiation epithelial cells are regulated by a number of biological active substances, some of which are secreted by the cells of the underlying connective tissue. The most important of them are cytokines, in particular epidermal growth factor; they are influenced by hormones, mediators and other factors. The differentiation of epithelial cells is accompanied by a change in the expression of the cytokeratins they synthesize, which form intermediate filaments.

Stratified squamous epithelia Depending on the presence or absence of the stratum corneum, they are divided into keratinizing and non-keratinizing.

1. Stratified squamous keratinizing epithelium (Fig. 3) forms the outer layer of skin - the epidermis, and covers some areas of the oral mucosa. It consists of five layers:

Basal layer(1) formed by cubic or prismatic cells lying on the basement membrane. They are capable of mitotic division, therefore, due to them, the overlying layers of the epithelium change.

Layer spinosum(2) formed by large, irregularly shaped cells. Dividing cells may be found in the deep layers. In the basal and spinous layers, tonofibrils (bundles of tonofilaments) are well developed, and between the cells there are desmosomal, tight, gap-like contacts.

Granular layer(3) consists of flattened cells, the cytoplasm of which contains grains of keratohyalin - a fibrillar protein, which during the process of keratinization is converted into eleidinkeratin.

Shiny layer(4) expressed only in the epithelium of the thick skin covering the palms and soles. It represents a transition zone from living cells of the granular layer to scales of the stratum corneum, which do not have the characteristics of living cells. On histological preparations it looks like a narrow oxyphilic homogeneous strip and consists of flattened cells. Processes are completed in the shiny layer keratinization , which consists in the transformation of living epithelial cells into horny scales - mechanically strong and chemically stable postcellular structures that together form stratum corneum epithelium, which performs protective functions. Although the actual formation of horny scales occurs in the outer parts of the granular layer or in the stratum lucidum, the synthesis of substances that ensure keratinization occurs already in the spinous layer.

Stratum corneum(5) the most superficial and has the maximum thickness in the epidermis of the skin in the area of ​​​​the palms and soles. It is formed by flat horny scales with a sharply thickened plasmalemma. The cells do not contain a nucleus or organelles and are filled with a network of thick bundles of keratin filaments embedded in a dense matrix. Horny scales retain connections with each other for a certain time and are retained in the layers due to partially preserved desmosomes, as well as the mutual penetration of grooves and ridges that form rows on the surface of adjacent scales. In the outer parts of the stratum corneum, the desmosomes are destroyed, and the horny scales are peeled off from the surface of the epithelium.

Most cells stratified keratinizing epithelium refers to keratinocytes. Keratinocyte differentiation includes cells of all layers of this epithelium: basal, spinous, granular, shiny, horny. In addition to keratinocytes, the layer contains small numbers of melanocytes and macrophages.

2. Stratified squamous non-keratinizing epithelium covers the surface of the cornea of ​​the eye, mucous membrane of the oral cavity, esophagus, and vagina. It is formed by three layers:

1) Basal layer similar in structure and function to the corresponding layer of keratinizing epithelium.

2) Layer spinosum formed by large polygonal cells, which flatten as they approach the surface layer. Their cytoplasm is filled with numerous tonofilaments, which are distributed diffusely. In the outer cells of this layer, keratohyalin accumulates in the form of small round granules.

3) Surface layer vaguely separated from spinous. The content of organelles is reduced compared to that in the cells of the spinous layer, the plasmolemma is thickened, the nucleus has poorly distinguishable chromatin granules (pyknotic). During desquamation, the cells of this layer are constantly removed from the surface of the epithelium.

Due to the availability and ease of obtaining material stratified squamous epithelium The oral mucosa is a convenient object for cytological studies. Cells are obtained by scraping, smearing or imprinting. Next, it is transferred to a glass slide and a permanent or temporary cytological preparation is prepared. The most widely used diagnostic cytological examination this epithelium in order to reveal the genetic sex of the individual; violations normal course the process of epithelial differentiation during the development of inflammatory, precancerous or tumor processes oral cavity. The cells of this epithelium are studied to determine the level of adaptation of the body and the influence of certain biologically active substances. For this, in particular, you can use the method of intravital research with the analysis of microelectrophoresis of cells, improved at the Department of Histology of IGMA.

3. Transitional epithelium (Fig.4) – a special type of stratified epithelium that lines most of the urinary tract. It is formed by three layers:

1) Basal layer formed by small cells that have triangular shape and with their wide base they are adjacent to the basement membrane.

2) Intermediate layer consists of elongated cells, the narrower part directed towards the basal layer and imbricately overlapping each other.

3) Surface layer formed by large mononuclear polyploid or binuclear cells, which change their shape to the greatest extent when the epithelium is stretched (from round to flat). This is facilitated by the formation in the apical part of the cytoplasm of these cells in the resting state of numerous invaginations of the plasmalemma and special disc-shaped vesicles - reserves of the plasmalemma, which are built into it as the organ and cells stretch.

Regeneration of integumentary epithelium . The integumentary epithelium, occupying a border position, is constantly influenced by the external environment, so the epithelial cells quickly wear out and die. Restoration of the epithelium – physiological regeneration - occurs through mitotic cell division. In a single-layer epithelium, most cells are capable of dividing, while in a multilayer epithelium only cells of the basal and partially spinous layers have this ability. The high ability of the epithelium for physiological regeneration serves as the basis for its rapid restoration in pathological conditions - reparative regeneration.

Histogenetic classification of integumentary epithelia ( according to N.G. Khlopin ) identifies 5 main types of epithelium that develop in embryogenesis from various tissue primordia.

Epithelial tissues communicate between the body and the external environment. They perform integumentary and glandular (secretory) functions.

The epithelium is located in the skin, lines the mucous membranes of all internal organs, is part of the serous membranes and lines the cavities.

Epithelial tissues perform various functions - absorption, excretion, perception of irritations, secretion. Most of the body's glands are made of epithelial tissue.

All germ layers take part in the development of epithelial tissues: ectoderm, mesoderm and endoderm. For example, the epithelium of the skin of the anterior and posterior sections of the intestinal tube is a derivative of ectoderm, the epithelium of the middle section of the gastrointestinal tube and respiratory organs is of endodermal origin, and the epithelium of the urinary system and reproductive organs is formed from mesoderm. Epithelial cells are called epithelial cells.

To the main general properties epithelial tissues include the following:

1) Epithelial cells fit tightly to each other and are connected by various contacts (using desmosomes, closure bands, gluing bands, slits).

2) Epithelial cells form layers. There is no intercellular substance between the cells, but there are very thin (10-50 nm) intermembrane gaps. They contain the intermembrane complex. Substances entering and secreted by cells penetrate here.

3) Epithelial cells are located on the basement membrane, which in turn lies on loose connective tissue that nourishes the epithelium. basement membrane up to 1 micron thick, it is a structureless intercellular substance through which nutrients come from blood vessels located in the underlying connective tissue. Both epithelial cells and loose connective underlying tissue participate in the formation of basement membranes.

4) Epithelial cells have morphofunctional polarity or polar differentiation. Polar differentiation is the different structure of the surface (apical) and lower (basal) poles of the cell. For example, at the apical pole of some epithelial cells, the plasmalemma forms an absorptive border of villi or ciliated cilia, and the basal pole contains the nucleus and most organelles.

In multilayer layers, the cells of the superficial layers differ from the basal ones in shape, structure and function.

The polarity indicates that different areas cells are committed various processes. The synthesis of substances occurs at the basal pole, and at the apical pole absorption, movement of cilia, and secretion occur.

5) Epithelia have a well-expressed ability to regenerate. When damaged, they quickly recover through cell division.

6) There are no blood vessels in the epithelium.

Classification of epithelia

There are several classifications of epithelial tissues. Depending on the location and function performed, two types of epithelia are distinguished: integumentary and glandular .

The most common classification of integumentary epithelium is based on the shape of the cells and the number of their layers in the epithelial layer.

According to this (morphological) classification, integumentary epithelia are divided into two groups: I) single-layer and II) multi-layer .

IN single-layer epithelia the lower (basal) poles of the cells are attached to the basement membrane, and the upper (apical) poles border on the external environment. IN stratified epithelia only the lower cells lie on the basement membrane, all the rest are located on the underlying ones.

Depending on the shape of the cells, single-layer epithelia are divided into flat, cubic and prismatic, or cylindrical . In squamous epithelium, the height of the cells is much less than the width. This epithelium lines respiratory departments lungs, middle ear cavity, some parts of the renal tubules, covers all serous membranes of the internal organs. Covering the serous membranes, epithelium (mesothelium) participates in the secretion and absorption of fluid into the abdominal cavity and back, and prevents the fusion of organs with each other and with the walls of the body. By creating a smooth surface of the organs lying in the chest and abdominal cavity, provides the ability to move them. The epithelium of the renal tubules is involved in the formation of urine, the epithelium of the excretory ducts performs a delimiting function.

Due to the active pinocytotic activity of squamous epithelial cells, substances are rapidly transferred from the serous fluid to the lymphatic bed.

The single-layer squamous epithelium covering the mucous membranes of organs and serous membranes is called lining.

Single layer cuboidal epithelium lines the excretory ducts of the glands, kidney tubules, and forms the follicles of the thyroid gland. The height of the cells is approximately equal to the width.

The functions of this epithelium are related to the functions of the organ in which it is located (in the ducts - delimiting, in the kidneys osmoregulatory, and other functions). Microvilli are located on the apical surface of cells in the kidney tubules.

Single-layer prismatic (cylindrical) epithelium has a greater cell height compared to width. It lines the mucous membrane of the stomach, intestines, uterus, oviducts, collecting ducts of the kidneys, excretory ducts of the liver and pancreas. Develops mainly from the endoderm. The oval nuclei are shifted to the basal pole and are located at the same height from the basement membrane. In addition to the delimiting function, this epithelium performs specific functions inherent in a particular organ. For example, the columnar epithelium of the gastric mucosa produces mucus and is called mucous epithelium, the intestinal epithelium is called edged, since at the apical end it has villi in the form of a border, which increase the area of ​​parietal digestion and absorption of nutrients. Each epithelial cell has more than 1000 microvilli. They can only be examined with an electron microscope. Microvilli increase the absorption surface of the cell up to 30 times.

IN epithelia, lining the intestines are goblet cells. These are single-celled glands that produce mucus, which protects the epithelium from the effects of mechanical and chemical factors and promotes better movement of food masses.

Single-layer multirow ciliated epithelium lines the airways of the respiratory organs: the nasal cavity, larynx, trachea, bronchi, as well as some parts of the reproductive system of animals (vas deferens in males, oviducts in females). The epithelium of the airways develops from the endoderm, the epithelium of the reproductive organs from the mesoderm. Single-layer multirow epithelium consists of four types of cells: long ciliated (ciliated), short (basal), intercalated and goblet. Only ciliated (ciliated) and goblet cells reach the free surface, and basal and intercalary cells do not reach the upper edge, although together with others they lie on the basement membrane. Intercalary cells differentiate during growth and become ciliated (ciliated) and goblet-shaped. The nuclei of different types of cells lie at different heights, in the form of several rows, which is why the epithelium is called multirow (pseudo-stratified).

Goblet cells are single-celled glands that secrete mucus that covers the epithelium. This promotes the adhesion of harmful particles, microorganisms, and viruses that enter with the inhaled air.

Ciliated cells on their surface they have up to 300 cilia (thin outgrowths of the cytoplasm with microtubules inside). The cilia are in constant motion, due to which, along with mucus, dust particles trapped in the air are removed from the respiratory tract. In the genitals, the flickering of cilia promotes the advancement of germ cells. Consequently, the ciliated epithelium, in addition to its delimiting function, performs transport and protective functions.

II. Stratified epithelia

1. Stratified non-keratinizing epithelium covers the surface of the cornea of ​​the eye, oral cavity, esophagus, vagina, caudal part of the rectum. This epithelium comes from the ectoderm. It has 3 layers: basal, spinous and flat (superficial). The cells of the basal layer are cylindrical in shape. Oval nuclei are located at the basal pole of the cell. Basal cells divide mitotically, replacing dying cells of the surface layer. Thus, these cells are cambial. With the help of hemidesmosomes, basal cells are attached to the basement membrane.

The cells of the basal layer divide and, moving upward, lose contact with the basement membrane, differentiate and become part of the spinous layer. Layer spinosum formed by several layers of cells of irregular polygonal shape with small processes in the form of spines, which, with the help of desmosomes, firmly connect the cells to each other. Circulates through the gaps between cells tissue fluid With nutrients. Thin filaments-tonofibrils are well developed in the cytoplasm of spinous cells. Each tonofibril contains thinner filaments-microfibrils. They are built from the protein keratin. Tonofibrils, attached to desmosomes, perform a supporting function.

The cells of this layer have not lost mitotic activity, but their division is less intense than that of the cells of the basal layer. The upper cells of the spinous layer gradually flatten and move into the superficial flat layer 2-3 rows of cells thick. The cells of the flat layer seem to spread out over the surface of the epithelium. Their kernels also become flat. Cells lose their ability to undergo mitosis and take the form of plates and then scales. The connections between them weaken and they fall off the surface of the epithelium.

2. Stratified squamous keratinizing epithelium develops from the ectoderm and forms the epidermis, covering the surface of the skin.

The epithelium of hairless skin has 5 layers: basal, spinous, granular, shiny and horny.

In the skin with hair, only three layers are well developed - basal spinous and horny.

The basal layer consists of a single row of prismatic cells, most of which are called keratinocytes. There are other cells - melanocytes and non-pigmented Langerhans cells, which are skin macrophages. Keratinocytes participate in the synthesis of fibrous proteins (keratins), polysaccharides, and lipids. The cells contain tonofibrils and grains of the melanin pigment that come from melanocytes. Keratinocytes have high mitotic activity. After mitosis, some of the daughter cells move to the superior spinous layer, while others remain in reserve in the basal layer.

The main significance of keratinocytes- formation of a dense, protective, non-living horny substance of keratin.

Melanocytes stitched shape. Their cell bodies are located in the basal layer, and processes can reach other layers of the epithelial layer.

Main function of melanocytes- education melanosomes containing skin pigment - melanin. Melanosomes enter neighboring epithelial cells along the processes of the melanocyte. Skin pigment protects the body from excessive ultraviolet radiation. Participating in the synthesis of melanin are: ribosomes, granular endoplasmic reticulum, and Golgi apparatus.

Melanin in the form of dense granules is located in the melanosome between the protein membranes that cover the melanosomes and on the outside. Thus, melanosomes are chemically melanoprodeids. Cells of the spinous layer multifaceted, have uneven boundaries due to cytoplasmic projections (spines), with the help of which they are connected to each other. The stratum spinosum is 4-8 layers of cells wide. In these cells, tonofibrils are formed, which end in desmosomes and firmly connect the cells to each other, forming a supporting-protective frame. The spinous cells retain the ability to reproduce, which is why the basal and spinous layers are collectively called the germinal layer.

Granular layer consists of 2-4 rows of flat-shaped cells with a reduced number of organelles. The tonofibrils are impregnated with keratohealin substance and turned into grains. Keratinocytes of the granular layer are the precursors of the next layer - brilliant.

Shiny layer consists of 1-2 rows of dying cells. In this case, the keratogealin grains merge. Organelles degrade, nuclei disintegrate. Keratohealin transforms into eleidin, which refracts light strongly, giving the layer its name.

The most superficial stratum corneum consists of horny scales arranged in many rows. The scales are filled with the horny substance keratin. On skin covered with hair, the stratum corneum is thin (2-3 rows of cells).

So, the keratinocytes of the surface layer turn into a dense non-living substance - keratin (keratos - horn). It protects underlying living cells from strong mechanical stress and drying out.

The stratum corneum serves as the primary protective barrier, impermeable to microorganisms. The specialization of the cell is expressed in its keratinization and transformation into a horny scale containing chemically stable proteins and lipids. The stratum corneum has poor thermal conductivity and prevents the penetration of water from the outside and its loss by the body. During the process of histogenesis, sweat - hair follicles, sweat, sebaceous and mammary glands are formed from epidermal cells.

Transitional epithelium- originates from mesoderm. It lines the inner surfaces of the renal pelvis, ureters, bladder and urethra, i.e., organs that are subject to significant stretching when filled with urine. The transitional epithelium consists of 3 layers: basal, intermediate and superficial.

The cells of the basal layer are small cubic, have high mitotic activity and perform the function of cambial cells.

Morphological classification cover epithelium takes into account the number of cell layers (single- and multilayer), the rows of single-layer epithelium (single- and multilayer), the shape of cells (for multilayers - the surface layer):

Relationship between the morphological characteristics of epithelia and their functional characteristics serves as a striking example of the inextricable unity of tissue structure and function. Thus, epithelia, which perform a predominantly protective function and are resistant to mechanical, chemical and microbial factors, usually have significant thickness and are multilayer . The higher the load, the thicker the epithelium and the more significant its keratinization. Another strategy for protecting the epithelium from microbes, dust particles or the action of an aggressive environment is the release of a constantly renewed protective agent onto its surface. layer of mucus . Epithelia, which provide the function of active absorption, on the contrary, single-layer , on the apical surface of which there are microvilli that increase the surface area.

Functional classification subdivides the epithelium only according to functional characteristics (absorptive, osmoregulatory, ciliated, glandular epithelium and other types).

The structure of different types of integumentary epithelium.

Single layer epithelia – all cells are located on the basement membrane, with the cell nuclei single row epithelia are at the same level, and cell nuclei multi-row epithelium are at different levels, which creates the effect of multi-row (and the false impression of multi-layering).

1. Single layer squamous epithelium formed by flattened polygonal cells with thickening in the area where the discoid nucleus is located. There are single microvilli on the free surface of the cell. An example of this type is the epithelium (mesothelium) covering the lung (visceral pleura) and the epithelium lining the inside of the chest cavity (parietal pleura), as well as the parietal and visceral layers of the peritoneum, the pericardial sac.

2. Single layer cuboidal epithelium formed by cells containing a spherical nucleus. Such epithelium is found in the follicles of the thyroid gland, in the small ducts of the pancreas and bile ducts, in the renal tubules .

3. Single-layer prismatic (cylindrical) epithelium (Fig.1) formed by cells with a pronounced polarity. The ellipsoidal nucleus lies along the long axis of the cell and is shifted to their basal part; well-developed organelles are unevenly distributed throughout the cytoplasm. On the apical surface there are microvilli, brush border. This type of epithelium is characteristic of the middle section of the digestive canal and lines the inner surface of the small and large intestines, stomach, gallbladder, a number of large pancreatic ducts and bile ducts of the liver. This type of epithelium is characterized by the functions secretion and/or absorption.

There are two main types of differentiated cells found in the epithelium of the small intestine: prismatic edged, providing parietal digestion, and goblet, producing mucus. This unequal structure and function of cells in single-layer epithelium is called horizontal anisomorphic.

4. Multirow ciliated (ciliated) epithelium of the airways (Fig.2) formed by several types of cells: 1) low intercalary (basal), 2) high intercalary (intermediate), 3) ciliated (ciliated), 4) goblet. Low intercalary cells are cambial; with their wide base they are adjacent to the basement membrane, and with their narrow apical part they do not reach the lumen. Goblet cells produce mucus that covers the surface of the epithelium, moving along it thanks to the beating of the cilia of the ciliated cells. The apical parts of these cells border the lumen of the organ.

Stratified epithelia – epithelia, in which only the cells forming the basal layer are located on the basement membrane. The cells that make up the remaining layers lose contact with it. Multilayer epithelia are characterized by vertical anisomorphy – unequal morphological properties of cells of different layers of the epithelial layer. The classification of multilayer epithelium is based on the shape of the cells of the surface layer.

Maintaining the integrity of multilayer epithelia is ensured by regeneration. Epithelial cells continuously divide in the deepest basal layer at the expense of stem cells, followed by a shift to the overlying layers. After differentiation, degeneration and exfoliation of cells from the surface of the layer occurs. Processes proliferation And differentiation epithelial cells are regulated by a number of biologically active substances, some of which are secreted by the cells of the underlying connective tissue. The most important of these are cytokines, in particular epidermal growth factor; hormones, mediators and other factors influence. The differentiation of epithelial cells is accompanied by a change in the expression of the cytokeratins they synthesize, which form intermediate filaments.

Stratified squamous epithelia Depending on the presence or absence of the stratum corneum, they are divided into keratinizing and non-keratinizing.

1. Stratified squamous keratinizing epithelium (Fig. 3) forms the outer layer of skin - the epidermis, and covers some areas of the oral mucosa. It consists of five layers:

Basal layer (1) formed by cubic or prismatic cells lying on the basement membrane. They are capable of mitotic division, therefore, due to them, the overlying layers of the epithelium change.

Layer spinosum (2) formed by large, irregularly shaped cells. Dividing cells may be found in the deep layers. In the basal and spinous layers, tonofibrils (bundles of tonofilaments) are well developed, and between the cells there are desmosomal, tight, gap-like contacts.

Granular layer (3) consists of flattened cells, the cytoplasm of which contains grains of keratohyalin - a fibrillar protein, which during keratinization turns into eleidine And keratin.

Shiny layer (4) expressed only in the epithelium of the thick skin covering the palms and soles. It represents a transition zone from living cells of the granular layer to scales of the stratum corneum, which do not have the characteristics of living cells. On histological preparations it looks like a narrow oxyphilic homogeneous strip and consists of flattened cells. Processes are completed in the shiny layer keratinization , which consists in the transformation of living epithelial cells into horny scales - mechanically strong and chemically stable postcellular structures that together form stratum corneum epithelium, which performs protective functions. Although the actual formation of horny scales occurs in the outer parts of the granular layer or in the stratum lucidum, the synthesis of substances that ensure keratinization occurs already in the spinous layer.

Stratum corneum (5) the most superficial and has the maximum thickness in the epidermis of the skin in the area of ​​​​the palms and soles. It is formed by flat horny scales with a sharply thickened plasmalemma. The cells do not contain a nucleus or organelles and are filled with a network of thick bundles of keratin filaments embedded in a dense matrix. Horny scales retain connections with each other for a certain time and are retained in the layers due to partially preserved desmosomes, as well as the mutual penetration of grooves and ridges that form rows on the surface of adjacent scales. In the outer parts of the stratum corneum, the desmosomes are destroyed, and the horny scales are peeled off from the surface of the epithelium.

Most cells stratified keratinizing epithelium refers to keratinocytes. Differenton of keratinocyte includes cells of all layers of this epithelium: basal, spinous, granular, shiny, horny. In addition to keratinocytes, the layer contains small numbers of melanocytes and macrophages.

2. Stratified squamous non-keratinizing epithelium covers the surface of the cornea of ​​the eye, mucous membrane of the oral cavity, esophagus, and vagina. It is formed by three layers:

1) Basal layer similar in structure and function to the corresponding layer of keratinizing epithelium.

2) Layer spinosum formed by large polygonal cells, which flatten as they approach the surface layer. Their cytoplasm is filled with numerous tonofilaments, which are distributed diffusely. In the outer cells of this layer, keratohyalin accumulates in the form of small round granules.

3) Surface layer vaguely separated from spinous. The content of organelles is reduced compared to that in the cells of the spinous layer, the plasmolemma is thickened, the nucleus has poorly distinguishable chromatin granules (pyknotic). During desquamation, the cells of this layer are constantly removed from the surface of the epithelium.

Due to the availability and ease of obtaining material stratified squamous epithelium The oral mucosa is a convenient object for cytological studies. Cells are obtained by scraping, smearing or imprinting. Next, it is transferred to a glass slide and a permanent or temporary cytological preparation is prepared. The most widely used diagnostic cytological study of this epithelium is to identify the genetic sex of the individual; disruption of the normal course of the epithelial differentiation process during the development of inflammatory, pretumor or tumor processes in the oral cavity. The cells of this epithelium are studied to determine the level of adaptation of the body and the influence of certain biologically active substances. For this, in particular, you can use the method of intravital research with the analysis of microelectrophoresis of cells, improved at the Department of Histology of IGMA.

3. Transitional epithelium (Fig.4) – a special type of stratified epithelium that lines most of the urinary tract. It is formed by three layers:

1) Basal layer formed by small cells that have a triangular shape on a section and, with their wide base, are adjacent to the basement membrane.

2) Intermediate layer consists of elongated cells, the narrower part directed towards the basal layer and imbricately overlapping each other.

3) Surface layer formed by large mononuclear polyploid or binuclear cells, which change their shape to the greatest extent when the epithelium is stretched (from round to flat). This is facilitated by the formation in the apical part of the cytoplasm of these cells in the resting state of numerous invaginations of the plasmalemma and special disc-shaped vesicles - reserves of the plasmalemma, which are built into it as the organ and cells stretch.

Regeneration of integumentary epithelium. The integumentary epithelium, occupying a border position, is constantly influenced by the external environment, so the epithelial cells quickly wear out and die. Restoration of the epithelium – physiological regeneration - occurs through mitotic cell division. In a single-layer epithelium, most cells are capable of dividing, while in a multilayer epithelium only cells of the basal and partially spinous layers have this ability. The high ability of the epithelium for physiological regeneration serves as the basis for its rapid restoration in pathological conditions - reparative regeneration.

Histogenetic classification of integumentary epithelia ( according to N.G. Khlopin ) distinguishes 5 main types of epithelium that develop in embryogenesis from various tissue primordia:

1) Epidermal type The epithelium is formed from the ectoderm, has a multilayer or multirow structure, and is adapted to perform primarily a barrier and protective function.

2) Enterodermal type The epithelium develops from the endoderm, is single-layer cylindrical in structure, and carries out the processes of absorption of substances.

3) Coelonephrodermal type The epithelium is of mesodermal origin; its structure is single-layer, flat or prismatic, and performs mainly a barrier or excretory function.

4) Angiodermal type includes endothelial cells of mesenchymal origin.

5) Ependymoglial type presented special type tissue of neural origin, lining the cavities of the brain and having a structure similar to epithelium.

The given classification is not generally accepted.

Glandular epithelia. Glandular epithelial cells can be arranged in the same way, but more often form glands.

Glands , consisting of glandular epithelium, perform a secretory function, producing and secreting various substances.

Glandular epithelial cells - glandulocytes or glandular cells, the process of secretion in them occurs cyclically, called secretory cycle and includes five stages:

1. Absorption phase of starting substances , serving as substrates for the synthesis of a secretory product, is ensured by the high activity of transport mechanisms associated with the plasmalemma of the basal pole of the cell, through which these substances come from the blood.

2. Secretion synthesis phase associated with the processes of transcription and translation, the activity of grEPS and agrEPS, and the Golgi complex.

3. Secretion maturation phase associated with a decrease in water in the secretion and replenishment of the secretion with new Golgi molecules

4. Accumulation phase of the synthesized product in the cytoplasm of glandular cells is usually manifested by an increase in the content of secretory granules.

5. Secretion phase can be done in several ways (Fig. 5):

merocrine - without compromising the integrity of the cell,

apocrine – with destruction of the apical part of the cytoplasm,

holocrine - with complete disruption of cell integrity.

Structure of glandulocytes . Glandulocytes located on the basement membrane. Their shape is very diverse and varies depending on the phase of secretion. Glandulocytes are characterized by a well-defined polar differentiation, which is due to the direction of secretory processes from basal To apical parts of cells (with external secretion). In this regard, the plasmalemma has a different structure on the apical (microvilli), basal (basal membrane) and lateral (intercellular contacts) cell surfaces.

In the apical parts of the cells there are usually secretory granules, the size and structure of which depend on the chemical composition of the secretion. In cells that produce protein secrets (for example: digestive enzymes), the granular endoplasmic reticulum is well developed. In cells synthesized non-protein secretions (lipids, steroids), agranular endoplasmic reticulum is expressed.

The Golgi complex is well developed and participates in the secretory cycle. Mitochondria are numerous and accumulate in areas of greatest cell activity. In a number of glands in the cytoplasm of glandulocytes there are intracellular secretory tubules– deep invaginations of the cytolemma, the walls of which are covered with microvilli (for example, in the parietal cells of the gastric glands).

The glands are divided into two groups : endocrine glands, or endocrine, and exocrine glands, or exocrine.

Endocrine glands (endocrine glands) produce hormones - substances with high biological activity that are removed from the cell through basal pole. There are no excretory ducts in such glands; the secretion subsequently enters the blood through the capillaries.

Exocrine glands They produce secretions that are released into the external environment and consist of terminal sections and excretory ducts.

1) terminal (secretory) sections ( Fig.6 ) consist of glandular cells that produce secretions. In some glands formed by epidermal type epithelia (for example, sweat, mammary, salivary), the terminal sections, in addition to glandular cells contain myoepithelial cells – modified epithelial cells with a developed contractile apparatus. Myoepithelial cells with their processes cover the glandular cells from the outside and, by contracting, contribute to the release of secretions from the end section.

2) excretory ducts connect the terminal sections with the integumentary epithelium and ensure the release of synthesized substances onto the surface of the body or into the cavity of organs .

Division into terminal sections and excretory ducts difficult in some glands (for example, stomach, uterus), since all areas of these simple glands are capable of secretion.