Single layer single row prismatic epithelium. Covering epithelium

Classification

There are several classifications of epithelia, which are based on various characteristics: origin, structure, functions. Of these, the most widespread is the morphological classification, which takes into account mainly the relationship of cells to the basement membrane and their shape.

Morphological classification

• Single layer epithelium can be single-row or multi-row. In single-row epithelium, all cells have the same shape - flat, cubic or prismatic, their nuclei lie at the same level, that is, in one row. In multirow epithelium, prismatic and intercalary cells are distinguished (here: using the example of the trachea), stained with hematoxylin-eosin, the latter, in turn, are divided according to the principle of the ratio of the nucleus to the basement membrane into high intercalary and low intercalary cells.

• 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 multilayered squamous keratinization. In the absence of keratinization, the epithelium is called 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.

Ontophylogenetic classification

Along with morphological classification, it is used ontophylogenetic classification, created by Russian histologist N. G. Khlopin. It is based on the peculiarities of the development of epithelia from tissue primordia.

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

• Endodermal type The epithelium develops from the endoderm, is single-layer prismatic in structure, carries out the processes of absorption of substances, and performs a glandular function.

• Coelonephrodermal type epithelium develops from mesoderm, single-layer, flat, cubic or prismatic in structure; performs a barrier or excretory function.

• Ependymoglial type represented by a special epithelium lining, for example, the cavities of the brain. The source of its formation is the neural tube.
• Angiodermal type epithelium is formed from mesenchyme and lines the inside of blood vessels.

Types of epithelium

Single layer epithelium

• Single layer squamous epithelium(endothelium and mesothelium). The endothelium lines the inside of blood vessels, lymphatic vessels, and the cavities of the heart. Endothelial cells are flat, poor in organelles and form the endothelial layer. The metabolic function is well developed. They create conditions for blood flow. When the epithelium is damaged, blood clots form. The endothelium develops from mesenchyme. The second type - mesothelium - develops from mesoderm. Lines all serous membranes. Consists of flat polygonal cells connected to each other by uneven edges. Cells have one, rarely two, flattened nuclei. There are short microvilli on the apical surface. They have absorptive, excretory and delimiting functions. The mesothelium ensures the free sliding of internal organs relative to each other. The mesothelium secretes a mucous secretion onto its surface. The mesothelium prevents the formation of connective tissue adhesions. They regenerate quite well due to mitosis.

• Single layer cuboidal epithelium develops from endoderm and mesoderm. On the apical surface there are microvilli that increase the working surface, and in the basal part the cytolemma forms deep folds, between which mitochondria are located in the cytoplasm, so the basal part of the cells looks striated. Lines the small excretory ducts of the pancreas, bile ducts and renal tubules.

• Single layer columnar epithelium found in the organs of the middle part of the digestive canal, digestive glands, kidneys, gonads and genital tract. In this case, the structure and function are determined by its localization. Develops from endoderm and mesoderm. The gastric mucosa is lined with single-layer glandular epithelium. It produces and secretes a mucous secretion that spreads over the surface of the epithelium and protects the mucous membrane from damage. The cytolemma of the basal part also has small folds. The epithelium has high regeneration.

• The kidney tubules and intestinal mucosa are lined bordered epithelium. In the bordered epithelium of the intestine, border cells - enterocytes - predominate. At their top there are numerous microvilli. In this zone, parietal digestion and intensive absorption of food occur. Mucous goblet cells produce mucus on the surface of the epithelium, and small endocrine cells are located between the cells. They secrete hormones that provide local regulation.

• Single layer multirow ciliated epithelium. It lines the airways and is of endodermal origin. In it, cells are of different heights, and the nuclei are located at different levels. The cells are arranged in a layer. Under the basement membrane lies loose connective tissue with blood vessels, and the epithelial layer is dominated by highly differentiated ciliated cells. They have a narrow base and a wide top. At the top there are flickering cilia. They are completely immersed in mucus. Between the ciliated cells are goblet cells - these are single-celled mucous glands. They produce a mucous secretion onto the surface of the epithelium. There are endocrine cells. Between them there are short and long intercalary cells; these are stem cells, poorly differentiated, due to which cell proliferation occurs. The ciliated cilia perform oscillatory movements and move the mucous film along the airways to the external environment.

Stratified epithelium

• Stratified squamous non-keratinizing epithelium. It develops from the ectoderm, lining the cornea, anterior section the digestive canal and the anal section of the digestive canal, vagina. The cells are arranged in several layers. On the basement membrane lies a layer of basal or columnar cells. Some of them are stem cells. They proliferate, separate from the basement membrane, transform into polygonal cells with projections, spines, and the combination of these cells forms a layer of spinous cells arranged in several floors. They gradually flatten and form a surface layer of flat ones, which are torn away from the surface in external environment.

• Stratified squamous keratinizing epithelium- epidermis, it lines the skin. In thick skin ( palmar surfaces), which is constantly under stress, the epidermis contains 5 layers:
  • 1 - basal layer - contains stem cells, differentiated cylindrical and pigment cells (pigmentocytes).
  • 2 - spinous layer - polygonal cells, they contain tonofibrils.
  • 3 - granular layer - the cells acquire a rhomboid shape, the tonofibrils disintegrate and inside these cells the protein keratohyalin is formed in the form of grains, this is where the process of keratinization begins.
  • 4 - stratum lucidum - a narrow layer, in which the cells become flat, they gradually lose their intracellular structure, and keratohyalin turns into eleidin.
  • 5 - stratum corneum - contains horny scales that have completely lost their cell structure and contain the protein keratin. With mechanical stress and deterioration of blood supply, the process of keratinization intensifies.

• Multilayer cubic and columnar epithelium are extremely rare - in the area of ​​the conjunctiva of the eye and the area of ​​​​the junction of the rectum between single-layer and multilayer epithelium.

• Transitional epithelium(uroepithelium) lines the urinary tract and allantois. Contains a basal layer of cells, some of the cells gradually separate from the basement membrane and form an intermediate layer of pyriform cells. On the surface there is a layer of integumentary cells - large cells, sometimes double-rowed, covered with mucus. The thickness of this epithelium varies depending on the degree of stretching of the wall of the urinary organs. The epithelium is capable of secreting a secretion that protects its cells from the effects of urine.

• Glandular epithelium- a type of epithelial tissue, which consists of epithelial glandular cells, which in the process of evolution acquired the leading property of producing and secreting secretions. Such cells are called secretory (glandular) - glandulocytes. They have exactly the same general characteristics as covering epithelium. Located in the glands of the skin, intestines, salivary glands ah, endocrine glands, etc. Among the epithelial cells there are secretory cells, there are 2 types of them.
  • exocrine - release their secretion into the external environment or the lumen of the organ.
  • endocrine - release their secretions directly into the bloodstream.

Characteristics

There are five main features of epithelia:

Epithelia are layers (less often strands) of cells - epithelial cells. There is almost no intercellular substance between them, and the cells are closely connected to each other through various contacts. Epithelia are located on basement membranes that separate epithelial cells from the underlying connective tissue. The epithelium has polarity. The two cell sections - basal (lying at the base) and apical (apical) - have different structures. The epithelium does not contain blood vessels. Epithelial cells are nourished diffusely through the basement membrane from the side of the underlying connective tissue. Epithelia have a high ability to regenerate. Epithelial restoration occurs due to mitotic division and differentiation of stem cells

see also

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See what “Epithelium” is in other dictionaries:

Epithelium... Spelling dictionary-reference book

- (Greek). Upper skin of mucous membranes. Dictionary foreign words, included in the Russian language. Chudinov A.N., 1910. EPITHELIA Greek. The delicate outer skin on the mucous membrane of the lips, nipples, etc. Explanation of 25,000 foreign words,... ... Dictionary of foreign words of the Russian language

EPITHELIA, a layer of cells densely packed so that they form the surface or line the inside of the canals and cavities of the body. Epithelium covers not only the SKIN, but also various internal organs and surfaces, e.g. nasal cavity, mouth and... ... Scientific and technical encyclopedic dictionary

- (from epi... and Greek thele nipple), epithelial tissue, in multicellular animals the tissue covering the body and lining its cavities in the form of a layer, also makes up the main. functional component of most glands. In embryogenesis, E. is formed earlier than others... ... Biological encyclopedic dictionary

EPITHELIUM- (from the Greek epi on and thele nipple), a term introduced by Reish (Ruysch, 1703) and originally denoting the outer cover of the nipple. Then the term "E." very diverse hists began to be designated. structures consisting of cells, b. h... ... Great Medical Encyclopedia

- (from epi... and Greek thele nipple) in animals and humans (epithelial tissue) a layer of closely spaced cells covering the surface of the body (for example, skin), lining all its cavities and performing mainly protective, excretory and... ... Big encyclopedic Dictionary

- [te], epithelium, pl. no, husband (Greek epi above and thele nipple) (anat.). Tissue of one or more layers of cells that lines the surface of the animal's body and its cavities. (originally about the layer covering the breast nipple.) Ushakov’s Explanatory Dictionary. D.N... Ushakov's Explanatory Dictionary

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Simple squamous (flat) epithelium covers all serous membranes of internal organs, forms some sections of the renal tubules, excretory ducts of small-diameter glands. The epithelium of the serous membranes, or mesothelium, is involved in the secretion and absorption of fluid into the abdominal cavity and back. By creating a smooth surface for the organs lying in the chest and abdominal cavities, it provides the opportunity for their movement. The epithelium of the renal tubules is involved in the formation of urine, the epithelium of the excretory ducts of the glands performs the integumentary function.

All cells of this epithelium are located on the basement membrane and have the appearance of thin plates (Fig. 79), since their height is much less than their width. This form facilitates the transport of substances. Adjacent to each other, the cells form an epithelial layer in which the boundaries between cells are very poorly stained. They can be detected with a weak solution of silver nitrate. Under the influence of light, it is reduced to metallic silver, deposited between the cells. Under these conditions, the border between cells turns black and has sinuous contours (Fig. 80).

Epithelial cells contain one, two or many nuclei. Multinucleation is a consequence of amitosis, which occurs intensively during inflammation or irritation of the mesothelium.

Simple cuboidal epithelium found in kidney tubules, follicles thyroid gland, in the excretory ducts of the glands. It develops from all three germ layers - ectoderm, mesoderm, endoderm. Epithelial cells of this type of epithelium are of the same type in shape, their height corresponds to their width, rounded nuclei occupy a central position in the cell. All epithelial cells are located on the basement membrane and form a single epithelial layer in morpho-functional terms.

Varieties of simple cubic epithelium differ not only genetically, but also in their fine structure and function. Thus, on the apical surface of epithelial cells in the kidney tubules there is a brush border - microvilli formed by protrusion of the plasmalemma. The membrane of the basal pole of the cells, invaginating into the cytoplasm, forms the basal striation. The presence of these structures is associated with the participation of epithelial cells in the synthesis of urine; therefore, these structures are absent in the cubic epithelial cells of the thyroid follicles or in the excretory glands of other glands.

Simple columnar epithelium lines the inner surface of the mucous membrane of the stomach, intestines, uterus, oviducts, as well as the excretory ducts of the liver and pancreas. This epithelium develops mainly from the endoderm. The epithelial layer consists of cells whose height significantly exceeds their width. Neighboring cells are connected by their lateral surfaces to each other using desmosomes, locking zones, zones

Rice. 79. Integumentary epithelium (according to Aleksandrovskaya) (diagram): I - single-layer (simple) epithelium; II - multilayered epithelia; a - single-layer flat (squamous);

b- single-layer cubic; V- single-layer cylindrical (columnar); G- single-layer multi-row cylindrical ciliated (pseudo-multilayer); g - 1 - ciliated cell; G - 2 - flickering eyelashes: g - 3 - intercalary (replacement) cells; d- multilayer flat (squamous) non-keratinizing; d - 1 - cells of the basal layer; d - 2 - cells of the spinous layer; d - 3 - cells of the surface layer; e- multilayered squamous (squamous) keratinizing epithelium; e - A- basal layer; e - b- spinous layer; e - V- granular layer; e - G- shiny layer; e - d- stratum corneum; and- transitional epithelium; g - a - cells of the basal layer; and- b - cells of the intermediate layer; and - V- cells of the integumentary layer; 3 And- goblet cell.

Rice. 80. Single-layer squamous epithelium (top view):

1 - core; 2 - cytoplasm; 3 - boundary between cells.

adhesion, finger-like joints. The oval nuclei of epithelial cells are usually shifted to the basal pole and located at the same height from the basement membrane.

Modifications of simple columnar epithelium are the bordered epithelium of the intestine (Fig. 81) and the glandular epithelium of the stomach (see Chapter 11). Covering the inner surface of the intestinal mucosa, the bordered epithelium is involved in absorption nutrients. All the cells of this epithelium, called microvillous epithelial cells, are located on the basement membrane. In this epithelium, polar differentiation is well expressed, which is determined by the structure and function of its epithelial cells. The cell pole facing the intestinal lumen (apical pole) is covered with a striated border. Below it in the cytoplasm is a centrosome. The nucleus of the epithelial cell lies in the basal pole. The Golgi complex is adjacent to the nucleus, ribosomes, mitochondria and lysosomes are dispersed throughout the cytoplasm.

Thus, in the apical and basal poles of the microvillous epithelial cell there are different intracellular structures, this is called polar differentiation.

Intestinal epithelial cells are called microvilli, since at their apical pole there is a striated border - a layer of microvilli formed by outgrowths of the plasmalemma of the apical surface of the epithelial cell. Microvilli clearly

Rice. 81. Single-layer (simple) columnar epithelium:

1 - epithelial cell; 2 - basement membrane; 3 - basal pole; 4 - apical pole; 5 - striated border; 6 - loose connective tissue; 7 - blood vessel; 8 - leukocyte.

visible only in an electron microscope (Fig. 82, 83). Each epithelial cell has on average more than a thousand microvilli. They increase the absorption surface of the cell, and therefore the intestine, up to 30 times.

In the epithelial layer of this epithelium there are goblet cells (Fig. 84). These are single-celled glands that produce mucus, which protects cells from the harmful effects of mechanical and chemical factors.

Simple columnar glandular epithelium covers the inner surface of the gastric mucosa. All cells of the epithelial layer are located on the basement membrane, their height is greater than their width. Polar differentiation is clearly represented in the cells: the oval nucleus and organelles are located at the basal pole, droplets of secretion lie at the apical pole, and there are no organelles (see Chapter 10).

Single layer, single row cylindrical ciliated epithelium(pseudostratified ciliated epithelium)(Fig. 85) lines the airways of the respiratory organs - the nasal cavity, larynx, trachea, bronchi, as well as the tubules of the epididymis, the inner surface of the mucous membrane of the oviduct. The epithelium of the airways develops from the endoderm, the epithelium of the reproductive organs - from the mesoderm.

Rice. 82.

A- microvilli of the striated border and the adjacent section of the cytoplasm of the epithelial cell (magnitude 21800, longitudinal section); B- cross-section of microvilli (magnitude 21800); IN- cross-section of microvilli (magnitude 150,000). Electron micrograph.

Rice. 83. Epithelial cells of the small intestine of a newborn calf:

1 - apical pole of the epithelial cell; 2 - suction rim; 3 - plasmalemma of the epithelial cell. Electron micrograph.

Rice. 84. Goblet cells:

1 - epithelial cells; 2 - goblet cells in the initial stage of secretion formation; 3 - goblet cells that secrete secretions; 4 - core; 5 - secret.

All cells of the epithelial layer lie on the basement membrane and differ in shape, structure, and function. The epithelium of the airways also contains goblet cells; Only ciliated cylindrical and goblet cells reach the free surface. Stem (replacement) epithelial cells are wedged between them. The height and width of these cells vary: some of them are columnar in shape, their oval nuclei are located in the center of the cell; others are lower with widened basal and narrowed apical poles. The round nuclei are located closer to the basement membrane. All types of intercalary epithelial cells do not have ciliated cilia. Consequently, the nuclei of cylindrical ciliated, replacement and low replacement cells are located in rows at different heights from the basement membrane, and therefore the epithelium is called multirow. It is called pseudo-multilayer (false multilayer) because all epithelial cells are located on the basement membrane.

Between the ciliated and intercalary (replacement) cells lie unicellular glands - goblet cells that produce mucus. It accumulates in the apical pole, pushing the endoplasmic reticulum, Golgi complex, mitochondria and nucleus to the base of the cell. The latter takes on the shape of a crescent, is very rich in chromatin and is intensely stained. The secretion of goblet cells covers the epithelial layer and promotes the adhesion of harmful particles, microorganisms, viruses that enter the airways along with the inhaled air.

Ciliated epithelial cells are highly differentiated cells and are therefore mitotically inactive. On its surface, a ciliated cell has about three hundred cilia, each of which is formed by a thin outgrowth of the cytoplasm, covered with a plasmalemma. The cilium contains one central pair and nine pairs of peripheral microtubules. At the base of the cilium, the peripheral microtubules disappear, and the central one passes deeper, forming the basal body.

Rice. 85.

A- single-layer multirow cylindrical ciliated epithelium (pseudostratified):
1 - ciliated cells; 2 - intercalary cells; 3 - goblet cells; 4 - basement membrane; 5 - loose connective tissue; B - isolated ciliated epithelial cell.

The basal bodies of all epithelial cells are located at the same level (Fig. 86). Eyelashes are in constant motion. Their direction of movement will be perpendicular to the plane of occurrence of the central pair of microtubules. Thanks to the movement of the cilia, dust particles trapped in the air and excess mucus accumulation are removed from the respiratory system. In the genitals, the flickering of cilia promotes the advancement of eggs.

Non-keratinizing stratified squamous epithelium(see Fig. 79, d). The epithelium covers the surface of the cornea of ​​the eye, the oral cavity, the esophagus, the vagina, and the caudal part of the rectum. Develops from the ectoderm. The epithelial layer consists of cells of different structure and shape, and therefore it is divided into basal, spinous and superficial (flat) layers. All cells of the basal layer (d 1) located on the basement membrane, they are cylindrical (columnar) in shape. The oval nuclei are located in the basal pole. The epithelial cells of this layer divide mitotically, replacing the dying cells of the surface layer. Therefore, the cells of the basal layer are cambial or stem. Basal cells are attached to the basement membrane by hemidesmosomes. Epithelial cells of other layers do not have contact with the basement membrane.

Rice. 86. Diagram of the ciliary apparatus of the epithelium:

A- incision in a plane perpendicular to the plane of movement of the cilium; b- incision in the plane of movement of the cilium; With - h- cross-section of cilia at different levels; i- cross section of eyelashes (dotted line shows a plane perpendicular to the direction of movement).

Rice. 87. Multilayered squamous (squamous) keratinizing epithelium:

1 - germ layer; A- basal cells; b- spinous cells; 2 - granular layer; 3 - stratum corneum; 4 - loose connective tissue; 5 - dense connective tissue.

In the spinous layer (D 2) cell height decreases. They first acquire an irregular polygonal shape, then gradually flatten.

The shape of the nuclei also changes accordingly: first rounded and then flattened. Epithelial cells are connected to neighboring cells using cytoplasmic projections - “bridges”. This connection causes the formation of gaps between the cells, through which tissue fluid circulates with nutrients dissolved in it.

Thin filaments - tonofibrils - are well developed in the cytoplasm of spinous cells. Each tonofibril consists of thinner filaments - tonofilaments (microfibrils). They are built from the protein keratin. Tonofibrils, attached to desmosomes, perform a supporting function in the cell. The cells of this layer have not lost mitotic activity, but their division is less intense. The surface cells of the spinous layer gradually become flattened, and their nuclei also become flat in shape.

Surface layer ( d 3) consists of flat cells that have lost the ability to mitosis. The structure of epithelial cells also changes: flat nuclei become lighter, organelles are reduced. The cells take the form of plates, then scales and fall off.

Keratinizing stratified squamous (squamous) epithelium(f) develops from the ectoderm and covers the outside of the skin. The epithelium of hairless skin contains germinal, granular, lucid and stratum corneum. In the skin with hair, only two layers are well developed - the germ layer and the horny layer (Fig. 87).

The germ layer consists of living cells that have not lost the ability to undergo mitosis. In terms of the structure and arrangement of cells, the germ layer is similar to multilayered non-keratinizing squamous epithelium. It also distinguishes basal, spinous, flat layers of cells.

All cells of the basal layer (see Fig. 79, e - A) located on the basement membrane. Most of the cells in this layer are called keratinocytes. There are other cells - melanocytes and pigmentless granular dendrocytes (Langerhans cells). Keratinocytes participate in the synthesis of fibrous proteins, polysaccharides, and lipids. They are columnar in shape, their nuclei are rich in DNA, and their cytoplasm is rich in RNA. The cells also contain thin filaments - tonofibrils, grains of the melanin pigment.

Keratinocytes of the basal layer have maximum mitotic activity. After mitosis, some of the daughter cells move to the above spinous layer, while others remain in the basal layer as a “reserve”, performing the function of cambial (stem) epithelial cells. The main significance of keratinocytes is the formation of a dense, protective, non-living, horny substance - keratin, which determined the name of the cells.

Processed melaninocytes. Their cell bodies are located in the basal layer, and processes can reach other layers of the epithelial layer. The main function of melanocytes is the formation of melanosomes and the skin pigment melanin. The latter can be transmitted along the processes of the melanocyte to other epithelial cells. Skin pigment protects the body from excessive ultraviolet radiation, which negatively affects the body. Melanocyte nuclei occupy most of the cell, are irregular in shape, and rich in chromatin. The cytoplasm is lighter than that of keratinocytes, it contains many ribosomes, and a granular endoplasmic reticulum and Golgi apparatus are developed. These organelles take part in the synthesis of melanosomes, which are oval in shape and consist of several dense granules covered with a membrane.

Pigmentless (light) granular dendrocytes have 2 - 5 processes. Their cytoplasm contains special granules similar in shape to a tennis racket (Fig. 88). The significance of these cells is not clear. It is believed that their function is associated with the control of the proliferative activity of keratinocytes.

The cells of the stratum spinosum are not connected to the basement membrane. They are multifaceted in shape; moving towards the surface, they gradually flatten. The border between cells is usually uneven, since cytoplasmic projections (“spines”) are formed on the surface of keratinocytes, with the help of which they connect to each other. This leads to the formation of cellular bridges (Fig. 89) and intercellular gaps. Tissue fluid flows through the intercellular gaps, containing nutrients and unnecessary metabolic products intended for removal. Tonofibrils are very well developed in the cells of this layer. Their diameter is 7 - 10 nm. Arranged in bundles, they end in zones of desmosomes that firmly connect cells to each other during the formation of the epithelial layer. Tonofibrils perform the function of a supporting and protective frame.

Rice. 88. A - Langerhans cell; B - specific granules “tennis rackets with an ampullary end extension and longitudinal lamellae in the handle area.” Electron micrograph.

Granular layer (see Fig. 79, e - V) consists of 2 - 4 rows of flat-shaped cells lying parallel to the surface of the epithelial layer. Epithelial cells are characterized by round, oval or elongated nuclei; reduction in the number of organelles; accumulation of keratinohyaline substance impregnating tonofibrils. Keratohyalin is stained with basic dyes and therefore has the appearance of basophilic granules. Keratinocytes

Rice. 89. Cellular bridges in the epidermis of the bovine nasal planum:

1 - core; 2 - cellular bridges.

"The granular layer is the precursor of the cells of the next layer, the stratum lucidum. (e - G). Its cells are devoid of nuclei and organelles, and the tonofibrillary-keratinohyaline complexes merge into a homogeneous mass that strongly refracts light and is stained with acidic dyes. This layer was not detected by electron microscopy, since it has no ultrastructural differences.

Stratum corneum (e - d) consists of horny scales. They are formed from the stratum lucidum and are constructed of keratin fibrils and amorphous electron-dense material; the stratum corneum is externally covered with a single-layer membrane. In the surface zones, the fibrils lie more densely. Horny scales are connected to each other using keratinized desmosomes and other cell contact structures. The loss of horny scales is compensated by the new formation of cells in the basal layer.

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 influences and drying. Keratin prevents leakage tissue fluid from intercellular spaces.

The stratum corneum serves as the primary protective barrier, as it is impenetrable to microorganisms. Keratinizing squamous and multilayered epithelium can reach significant thickness, which leads to disruption of the nutrition of its cells. “This is eliminated by the formation of connective tissue outgrowths - papillae, which increase the contact surface of the cells of the basal layer and loose connective tissue that performs a trophic function.

Transitional epithelium(and) develops from the mesoderm and undermines the inner surface of the renal pelvis, ureters, and bladder. When these organs function, the volume of their cavities changes, and therefore the thickness of the epithelial layer either sharply decreases or increases.

The epithelial layer consists of basal, intermediate, and superficial layers (and- A, b, c).

The basal layer is built of basal cells associated with the basement membrane, different in shape and size: small cubic and large pear-shaped cells. The first of them have round nuclei and basophilic cytoplasm. In the epithelial layer, the nuclei of these cells form the lowest row of nuclei. Small cubic cells are characterized by high mitotic activity and perform the function of stem cells. The second ones are attached to the basement membrane with their narrow part. Their expanded body is located above the cubic cells; the cytoplasm is light, as basophilia is weakly expressed. If the organ is not filled with urine, large pear-shaped cells pile up on top of each other, forming a kind of intermediate layer.

Cover cells are flattened. Often multinucleate or their nuclei are polyploid (contain a larger number of chromosomes according to

Rice. 90. Transitional epithelium of the sheep renal pelvis:

A - A"- mucous cell of the integumentary zone with a weak reaction to mucus; b- intermediate zone; V - mitosis; G- basal zone: d - connective tissue.

Rice. 91. Transitional epithelium of the rabbit bladder:

1 - asleep; 2 - in slightly stretched; 3 - in a highly distended bladder.

compared to the diploid set of chromosomes). Surface cells may become slimy. This ability is especially well developed in herbivores (Fig. 90). Mucus protects epithelial cells from the harmful effects of urine.

Thus, the degree of filling of the organ with urine plays a role in the restructuring of the epithelial layer of this type of epithelium (Fig. 91).

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 organism ( internal environment) from the external environment, but at the same time participates in the metabolism of the body with the 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 embryonic development person. 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 an 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 lumen.

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 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 the 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, the 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.

Dust particles that enter the respiratory tract along with the air 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 are secreted into the blood vessels biologically active substances- 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 quick recovery it 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 the Latin secretio - separation) is a complex process that includes 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 weight organic substances: amino acids, monosaccharides, fatty acid etc. Sometimes larger molecules enter the cell by pinocytosis organic matter, such as 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 complete destruction 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), the 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). 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 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).

Morphological classification of the integumentary 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.

The 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 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) – special kind stratified epithelium, which 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 ) 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 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.

Epithelial tissues, or epithelia(from Greek epi– above and thele- nipple) - border tissues covering the surface of the body and the cavities lining it, the mucous membranes of the internal organs. Epithelia also form glands (glandular epithelium) and receptor cells in the sense organs (sensory epithelium).

1. Lecture: EPITHELIAL TISSUE. COVERING EPITHELIA 1.

2. Lecture: EPITHELIAL TISSUE. COVERING EPITHELIA 2.

3. Lecture: EPITHELIAL TISSUE. Glandular epithelia

Types of epithelial tissue: 1. Covering epithelium, 2. Glandular epithelium (form glands) and 3) Sensory epithelium can be distinguished.

General morphological characteristics of epithelium as tissue:

1) Epithelial cells are located tightly to each other, forming layers of cells;

2) Epithelia are characterized by the presence of a basement membrane - a special non-cellular formation that creates the basis for the epithelium and provides barrier and trophic functions;

3) There is practically no intercellular substance;

4) There are intercellular contacts between cells;

5) Epithelial cells are characterized by polarity - the presence of functionally unequal cell surfaces: apical surface (pole), basal (facing the basement membrane) and lateral surface.

6) Vertical anisomorphy - unequal morphological properties of cells of different layers of the epithelial layer in multilayered epithelia. Horizontal anisomorphy is the unequal morphological properties of cells in single-layer epithelia.

7) There are no vessels in the epithelium; nutrition is carried out by diffusion of substances through the basement membrane from connective tissue vessels;

8) Most epithelia are characterized by a high ability to regenerate – physiological and reparative, which is carried out thanks to cambial cells.

The surfaces of the epithelial cell (basal, lateral, apical) have a distinct structural and functional specialization, which is especially evident in single-layer epithelium, including glandular epithelium.

Lateral surface of epithelial cells ensures the interaction of cells due to intercellular connections, which determine the mechanical connection of epithelial cells with each other - these are tight junctions, desmosomes, interdigitation, and gap junctions ensure the exchange of chemicals (metabolic, ionic and electrical communication).

Basal surface of epithelial cells adjacent to the basement membrane, to which it is connected via hemidesmosomes. The basal and lateral surfaces of the plasmalemma of the epithelial cell together form a single complex, the membrane proteins of which are: a) receptors that perceive various signal molecules, b) carriers of nutrients coming from the vessels of the underlying connective tissue, c) ion pumps, etc.

basement membrane(BM) connects epithelial cells and underlying loose fibrous connective tissue. At the light-optical level on histological preparations, the BM has the appearance of a thin strip and is poorly stained with hematoxylin and eosin. At the ultrastructural level, three layers are distinguished in the basement membrane (in the direction from the epithelium): 1) the light lamina, which connects to the hemidesmosomes of epithelial cells, contains glycoproteins (laminin) and proteoglycans (heparan sulfate), 2) the dense lamina contains collagen types IV, V, VII , has a fibrillar structure. Thin anchor filaments cross the light and dense plates, passing into 3) the reticular plate, where the anchor filaments bind to collagen (types I and II collagen) fibrils of connective tissue.

Under physiological conditions, BM prevents the growth of the epithelium towards the connective tissue, which is disrupted during malignant growth, when cancer cells grow through the basement membrane into the underlying connective tissue (invasive tumor growth).

Apical surface of epithelial cells may be relatively smooth or form protrusions. Some epithelial cells have special organelles on it - microvilli or cilia. Microvilli are maximally developed in epithelial cells involved in absorption processes (for example, in the small intestine or tubules of the proximal nephron), where their totality is called a brush (striated) border.

Microcilia are motile structures containing complexes of microtubules inside.

Sources of epithelial development. Epithelial tissues develop from three germ layers, starting from 3–4 weeks of human embryonic development. Depending on the embryonic source, the epithelium is of ectodermal, mesodermal and endodermal origin.

Morphofunctional classification of epithelial tissue

I. Covering epithelia

1. Single-layer epithelia - all cells lie on the basement membrane:

1.1. Single-row epithelia (cell nuclei at the same level): flat, cubic, prismatic;

1.2. Multirow epithelium (cell nuclei at different levels due to horizontal anisomorphy): prismatic ciliated;

2. Multilayer epithelia - only the lower layer of cells is connected to the basement membrane, the overlying layers are located on the underlying layers:

2.1. Flat – keratinizing, non-keratinizing

3. Transitional epithelium - occupies an intermediate position between single-layer multirow and stratified epithelium

II. Glandular epithelia:

1. With exocrine secretion

2. With endocrine secretion

SINGLE LAYER EPITHELIAS

Single layer single row squamous epithelium formed by flattened polygonal cells. Examples of localization: mesothelium covering the lung (visceral pleura); 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. This epithelium allows organs to come into contact with each other in cavities.

Single layer single row cuboidal epithelium formed by cells containing a spherical nucleus. Examples of localization: thyroid follicles, small pancreatic ducts and bile ducts, renal tubules.

Single-layer single-row prismatic (cylindrical) epithelium formed by cells with pronounced polarity. The ellipsoidal nucleus lies along the long axis of the cell and is shifted to their basal part; the organelles are unevenly distributed throughout the cytoplasm. On the apical surface there are microvilli and a brush border. Examples of localization: lining inner surface small and large intestines, stomach, gall bladder, a number of large pancreatic ducts and bile ducts of the liver. This type of epithelium is characterized by the functions of secretion and (or) absorption.

Single-layer multirow ciliated (ciliated) epithelium The airways are 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 coats the surface of the epithelium, moving along the surface due to the beating of the cilia of the ciliated cells. The apical parts of these cells border the lumen of the organ.

MULTILAYERED EPITHELIA

Stratified squamous keratinizing epithelium(MPOE) forms the outer layer of skin - the epidermis, and covers some areas of the oral mucosa. MPOE consists of five layers: basal, spinous, granular, lucid (not present everywhere) and stratum corneum.

Basal layer formed by cubic or prismatic cells lying on the basement membrane. Cells divide by mitosis - this is the cambial layer, from which all overlying layers are formed.

Layer spinosum formed by large cells of irregular shape. 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 consists of flattened cells - keratinocytes, the cytoplasm of which contains grains of keratohyalin - a fibrillar protein, which during the process of keratinization is converted into eleidin and keratin.

Shiny layer expressed only in the epithelium of thick skin covering the palms and soles. The stratum pellucida is the zone of transition from the living cells of the granular layer to the scales of the stratum corneum. On histological preparations it looks like a narrow oxyphilic homogeneous strip and consists of flattened cells.

Stratum corneum consists of horny scales - postcellular structures. The keratinization processes begin in the stratum spinosum. The stratum corneum has its maximum thickness in the epidermis of the skin of the palms and soles. The essence of keratinization is to provide a protective function skin from external influences.

Differenton of keratinocyte includes cells of all layers of this epithelium: basal, spinous, granular, shiny, horny. In addition to keratinocytes, the stratified keratinizing epithelium contains small numbers of melanocytes, macrophages (Langerhans cells) and Merkel cells (see topic “Skin”).

The epidermis is dominated by keratinocytes, organized according to the columnar principle: cells on different stages differentiations are located on top of each other. At the base of the column are cambial poorly differentiated cells of the basal layer, the top of the column is the stratum corneum. The keratinocyte column includes keratinocyte differon cells. The columnar principle of epidermal organization plays a role in tissue regeneration.

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: basal, spinous and superficial. The basal layer is similar in structure and function to the corresponding layer of the keratinizing epithelium. The stratum spinosum is formed by large polygonal cells, which flatten as they approach the surface layer. Their cytoplasm is filled with numerous tonofilaments, which are distributed diffusely. The surface layer consists of polygonal flat cells. Nucleus with poorly visible 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 the material, the stratified squamous epithelium of 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.

3. Transitional epithelium – special kind multilayered epithelium that lines most of the urinary tract. It is formed by three layers: basal, intermediate and superficial. The basal layer is formed by small cells that have a triangular shape on a section and, with their wide base, are adjacent to the basement membrane. The intermediate layer consists of elongated cells, the narrower part adjacent to the basement membrane. The surface layer is 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. 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. Integumentary epithelia are characterized high degree ability to regenerate, and also in connection with this, up to 90% of all tumors in the body develop from this tissue.

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

1) Epidermal - formed from the ectoderm, has a multilayer or multirow structure, performs barrier and protective functions. For example, the epithelium of the skin.

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

3) Coelonephrodermal - has a mesodermal origin (coelomic lining, nephrotome), its structure is single-layer, flat or prismatic, and performs mainly a barrier or excretory function. For example, the epithelium of the kidneys.

4) Angiodermal - includes endothelial cells of mesenchymal origin (angioblast).

5) The ependymoglial type is represented by a special type of tissue of neural origin (neural tube), lining the cavities of the brain and having a structure similar to epithelium. For example, ependymal gliocytes.

Glandular epithelia

Glandular epithelial cells can be located singly, but more often form glands. The cells of the glandular epithelium are glandulocytes or glandular cells; the process of secretion in them occurs cyclically, called the secretory cycle and includes five stages:

1. The phase of absorption of initial substances (from blood or intercellular fluid), from which the final product (secret) is formed;

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

3. The secretion maturation phase occurs in the Golgi apparatus: dehydration and addition of additional molecules occur.

4. The phase of accumulation of the synthesized product in the cytoplasm of glandular cells is usually manifested by an increase in the content of secretory granules, which can be enclosed in membranes.

5. The phase of secretion excretion can be carried out in several ways: 1) without violating the integrity of the cell (merocrine type of secretion), 2) with the destruction of the apical part of the cytoplasm (apocrine type of secretion), with a complete violation of the integrity of the cell (holocrine type of secretion).

Glands are divided into two groups: 1) endocrine glands, which produce hormones - substances with high biological activity. There are no excretory ducts, the secretion enters the blood through the capillaries;

and 2) external secretion glands, or exocrine, in which the secretion is released into the external environment. Exocrine glands consist of terminal (secretory sections) and excretory ducts.

The structure of the exocrine glands

The terminal (secretory) sections consist of glandular cells (glandulocytes) that produce secretions. The cells are located on the basement membrane and are characterized by pronounced polarity: the plasmalemma has a different structure on the apical (microvilli), basal (interaction with the basement membrane) and lateral (intercellular contacts) cell surfaces. Secretory granules are present in the apical part of the cells. In cells that produce protein secretions (for example: digestive enzymes), grEPS is well developed. In cells that synthesize non-protein secretions (lipids, steroids), aEPS is expressed.

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 cells of the terminal section.

Excretory ducts connect the secretory sections with the integumentary epithelium and ensure the release of synthesized substances onto the surface of the body or into the cavity of organs.

The division into terminal sections and excretory ducts is difficult in some glands (for example, the stomach, uterus), since all parts of these simple glands are capable of secretion.

Classification of exocrine glands

I. Morphological classification exocrine glands is based on a structural analysis of their terminal sections and excretory ducts.

Depending on the shape of the secretory (terminal) section, alveolar, tubular and mixed (alveolar-tubular) glands are distinguished;

Depending on the branching of the secretory department, branched and unbranched glands are distinguished.

The branching of the excretory ducts determines the division of glands into simple (the duct does not branch) and complex (the duct does branch).

II. By chemical composition The secretion produced is divided into serous (proteinaceous), mucous, mixed (proteinaceous-mucosal), lipid and other glands.

III. According to the mechanism (method) of excretion secretion, exocrine glands are divided into apocrine (mammary gland), holocrine ( sebaceous gland) and merocrine (most glands).

Examples of gland classification. Classification characteristics sebaceous gland skin: 1) simple alveolar gland with branched terminal sections, 2) lipid - according to the chemical composition of the secretion, 3) holocrine - according to the method of excretion of the secretion.

Characteristic lactating (secreting) mammary gland: 1) complex branched alveolar-tubular gland, 2) with mixed secretion, 3) apocrine.

Gland regeneration. Secretory cells of merocrine and apocrine glands belong to stable (long-lived) cell populations, and therefore they are characterized by intracellular regeneration. In holocrine glands, restoration is carried out due to the proliferation of cambial (stem) cells, i.e. characterized by cellular regeneration: newly formed cells differentiate into mature cells.