Single-layered cuboidal epithelium of the renal tubules. Cells of the prismatic epithelium of the renal tubules

Chapter 6. EPITHELIAL TISSUES

Chapter 6. EPITHELIAL TISSUES

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

6.1. GENERAL MORPHOLOGICAL CHARACTERISTICS AND CLASSIFICATIONS

Surface epithelium- these are border tissues located on the surface of the body (integumentary), mucous membranes of internal organs (stomach, intestines, Bladder etc.) and secondary body cavities (lining). They separate the body and its organs from their environment and participate in the metabolism between them, carrying out the functions of absorption of substances (absorption) and excretion of metabolic products (excretion). For example, through the intestinal epithelium, the 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 products of nitrogen metabolism, which are slags, are excreted. In addition to these functions, the integumentary epithelium performs an important protective function, protecting the underlying tissues of the body from various external influences - chemical, mechanical, infectious, etc. For example, the skin epithelium is a powerful barrier to microorganisms and many poisons. Finally, the epithelium covering the internal organs creates conditions for their mobility, for example, for heart contraction, lung excursion, etc.

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

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

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

Epithelia are involved in the construction of many organs, and therefore they show a wide variety of morphophysiological properties. Some of them are common, allowing to distinguish epithelium from other tissues of the body. There are the following main features of the epithelium.

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

The epithelium is located on basement membranes, which are formed as a result of the activity of both epithelial cells and the underlying connective tissue. The basement membrane has a thickness of about 1 µm and consists of a subepithelial electron-transparent light plate

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

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

The epithelial cells are especially strongly associated with the basement membrane in the region of hemidesmosomes (hemidesmosomes). Here, from the plasmolemma of the basal epithelial cells through the light plate to the dark plate of the basal

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

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

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

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

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

Sources of development and classification of epithelial tissues. Epithelium develops from all three germ layers, starting from the 3-4th week embryonic development person. Depending on the embryonic source, epithelia of ectodermal, mesodermal and endodermal origin are distinguished. Epithelial cells form cell layers and are leading cellular differon in this fabric. In histogenesis, the composition of the epithelium (except for epitheliocytes) may include histological elements of differons of a different origin (associated differons in polydifferential epithelium). There are also epithelia, where, along with borderline epitheliocytes, as a result of divergent differentiation of the stem cell, cell differons of epithelial cells of secretory and endocrine specialization appear, integrated into the composition of the epithelial layer. Only related types of epithelium, developing from the same germ layer, under conditions of pathology can be subjected to metaplasia, i.e., move from one type to another, for example, in the respiratory tract, the ectodermal epithelium in chronic bronchitis can turn from a single-layer ciliated epithelium into a multi-layered squamous one,

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

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

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

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

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

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

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

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

Scheme 6.1. Morphological classification of types of surface epithelium

the leading cellular differon, the epithelium is divided into types: epidermal (skin), enterodermal (intestinal), whole nephrodermal, ependymoglial and angiodermal types of epithelium.

epidermal type The epithelium is formed from the ectoderm, has a multi-layer or multi-row structure, is adapted to perform primarily a protective function (for example, keratinized stratified squamous epithelium of the skin).

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

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

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

To angiodermal type epithelium is referred to as the endothelial lining blood vessels. In structure, the endothelium is similar to single-layer squamous epithelium. Its belonging to epithelial tissues is

is controversial. Many researchers attribute the endothelium to the connective tissue, with which it is associated with a common embryonic source of development - the mesenchyme.

6.1.1. Single layer epithelium

Single row epithelium

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

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

Endothelium (endothelium) lines the 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 relatively poor in organelles; pinocytic vesicles are present in their cytoplasm. The endothelium, located in the vessels on the border with lymph, blood, is involved in the metabolism and gases (O 2 , CO 2) between them and other tissues. Endotheliocytes synthesize a variety of growth factors, vasoactive substances, etc. If the endothelium is damaged, blood flow in the vessels may change and blood clots, or blood clots, may form in their lumen. In various areas vascular system endotheliocytes differ in size, shape and orientation relative to the axis of the vessel. These properties of endothelial cells are referred to as heteromorphy, or polymorphy(N. A. Shevchenko). Endotheliocytes capable of reproduction are located diffusely, with a predominance in the zones of dichotomous division of the vessel.

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

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

a- flat epithelium (mesothelium); b- columnar microvillous epithelium: 1 - microvilli (border); 2 - the nucleus of the epitheliocyte; 3 - basement membrane; 4 - connective tissue; in- micrograph: 1 - border; 2 - microvillous epitheliocytes; 3 - goblet cell; 4 - connective tissue

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

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

In the stomach, in a single-layer columnar epithelium, all cells are glandular (surface mucocytes) that produce mucus. The secret of mucocytes protects the stomach wall from the rough influence of food lumps and the digestive action of gastric juice, which has an acidic reaction, and enzymes that break down proteins. A smaller part of the epithelial cells located in the gastric pits - small depressions in the wall of the stomach, are cambial epitheliocytes that can divide and differentiate into glandular epitheliocytes. Due to pit cells, every 5 days there is a complete renewal of the epithelium of the stomach - its physiological regeneration.

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

Monolayer epitheliums are also represented by derivatives of the neuroectoderm - epithelium of the ependymoglial type. According to the structure of cells, it varies from flat to columnar. So, the ependymal epithelium lining the central canal of the spinal cord and the ventricles of the brain is a single-layer columnar. The retinal pigment epithelium is a single-layer epithelium consisting of polygonal cells. The perineural epithelium, surrounding the nerve trunks and lining the perineural space, is single-layer flat. As derivatives of the neuroectoderm, epithelia have limited regeneration capabilities, predominantly by intracellular means.

Stratified epithelium

Multi-row (pseudostratified) epithelium (epithelium pseudostratificatum) line the airways nasal cavity, trachea, bronchi, and a number of other organs. In the airways, the stratified columnar epithelium is ciliated. Diversity of cell types

Rice. 6.4. The structure of the multi-row columnar ciliated epithelium: a- scheme: 1 - shimmering cilia; 2 - goblet cells; 3 - ciliated cells; 4 - insert cells; 5 - basal cells; 6 - basement membrane; 7 - connective tissue; b- micrograph: 1 - cilia; 2 - nuclei of ciliated and intercalary cells; 3 - basal cells; 4 - goblet cells; 5 - connective tissue

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

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

6.1.2. Stratified epithelium

Stratified squamous nonkeratinized epithelium(epithelium stiatificatum squamosum noncornificatum) covers the outside of the cornea of ​​the eye

Rice. 6.5. The structure of the stratified squamous non-keratinized epithelium of the cornea of ​​the eye (micrograph): 1 - layer of squamous cells; 2 - prickly layer; 3 - basal layer; 4 - basement membrane; 5 - connective tissue

oral cavity and esophagus. Three layers are distinguished in it: basal, spiny (intermediate) and superficial (Fig. 6.5). Basal layer consists of columnar epithelial cells located on the basement membrane. Among them there are cambial cells capable of mitotic division. Due again formed cells, entering into differentiation, there is a change of epithelial cells of the overlying layers of the epithelium. Spiny layer consists of cells of irregular polygonal shape. In the epitheliocytes of the basal and spiny layers, tonofibrils (bundles of tono-filaments from keratin protein) are well developed, and between epitheliocytes there are desmosomes and other types of contacts. Surface layers The epithelium is made up of squamous cells. Finishing my life cycle, the latter die off and disappear.

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

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

Basal layer consists of columnar-shaped keratinocytes, in the cytoplasm of which keratin protein is synthesized, which forms tonofilaments. The cambial cells of keratinocytes differon are also located here. Spiny layer It is formed by polygonal-shaped keratinocytes, which are firmly interconnected by numerous desmosomes. In place of desmosomes on the surface of cells there are tiny outgrowths -

Rice. 6.6. Stratified squamous keratinized epithelium:

a- scheme: 1 - stratum corneum; 2 - shiny layer; 3 - granular layer; 4 - prickly layer; 5 - basal layer; 6 - basement membrane; 7 - connective tissue; 8 - pigmentocyte; b- micrograph

"Spikes" in adjacent cells directed towards each other. They are clearly visible with the expansion of intercellular spaces or with wrinkling of cells, as well as during maceration. In the cytoplasm of spiny keratinocytes, tonofilaments form bundles - tonofibrils and keratinosomes appear - granules containing lipids. These granules are released by exocytosis into the intercellular space, where they form a lipid-rich substance that cements keratinocytes.

In the basal and spinous layers, there are also process-shaped melanocytes with granules of black pigment - melanin, Langerhans cells(dendritic cells) and Merkel cells(tactile epithelial cells), having small granules and in contact with afferent nerve fibers(Fig. 6.7). Melanocytes with the help of pigment create a barrier that prevents the penetration of ultraviolet rays into the body. Langerhans cells are a type of macrophage, participate in protective immune reactions and regulate the reproduction (division) of keratinocytes, forming together with them "epidermal proliferative units". Merkel cells are sensitive (tactile) and endocrine (apudocytes), affecting the regeneration of the epidermis (see Chapter 15).

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

Rice. 6.7. The structure and cell-differential composition of the stratified squamous keratinized epithelium (epidermis) (according to E. F. Kotovsky):

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

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

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

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

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

Rice. 6.8. The structure of the transitional epithelium (scheme):

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

Basal layer formed by small, almost rounded (dark) cambial cells. AT intermediate layer polygonal cells are located. Surface layer consists of very large, often two- and three-nuclear cells, having a dome-shaped or flattened shape, depending on the state 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 the contraction of the wall of the organ, the thickness of the epithelial layer increases sharply. At the same time, some cells in the intermediate layer are “squeezed out” upwards and take on a pear-shaped shape, while the superficial cells located above them are dome-shaped. Tight junctions were found between the surface 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 borderline position, is constantly under the influence of the external environment, therefore epithelial cells wear out and die relatively quickly. The source of their recovery is cambial cells epithelium, which provide a cellular form of regeneration, as they retain the ability to divide throughout the life of the organism. Reproducing, part of the newly formed cells enter into differentiation and turn into epithelial cells, similar to the lost ones. Cambial cells in stratified epithelium are located in the basal (rudimentary) layer, in stratified epithelia they include basal cells, in single-layer epithelium 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 dimples, as well as necks of their own glands, in the mesothelium - among mesotheliocytes, etc. The high ability of most epithelia to physiological regeneration serves as the basis for its quick recovery under pathological conditions (reparative regeneration). On the contrary, derivatives of neuroectoderm are restored predominantly by intracellular means.

With age, integumentary epithelium weakens the processes of cell renewal.

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

6.2. glandular epithelium

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

By secretion in the body, many important features: formation of milk, saliva, gastric and intestinal juice, bile, endo-

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

secretory epitheliocytes lie on the basement membrane. Their form is very diverse and varies depending on the phase of secretion. The nuclei are usually large, often irregular in shape. In the cytoplasm of cells that produce secrets of a protein nature (for example, digestive enzymes), the granular endoplasmic reticulum is well developed. In cells synthesizing non-protein secrets (lipids, steroids), an agranular endoplasmic reticulum is expressed. The Golgi complex is extensive. Its shape and location in the cell change depending on the phase of the secretory process. Mitochondria are usually numerous. They accumulate in places of greatest cell activity, i.e., where a secret is formed. In the cytoplasm of cells, secretory granules are usually present, the size and structure of which depend on the chemical composition of the secret. Their number fluctuates in connection with 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 plasmolemma, covered with microvilli. The plasma membrane has different structure on the lateral, basal and apical surfaces of cells. At first, it forms desmosomes and tight locking junctions. The latter surround the apical (apical) parts of the cells, thus separating the intercellular gaps from the lumen of the gland. On the basal surfaces of cells, the plasmolemma forms a small number of narrow folds penetrating into the cytoplasm. Such folds are especially well developed in the cells of the glands that secrete a secret rich in salts, for example, in the cells of the excretory ducts of the salivary glands. The apical surface of the cells is covered with microvilli.

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

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

For the formation of a secret from the blood and lymph, various inorganic compounds, water and low molecular weight organic substances: amino acids, monosaccharides, fatty acids, etc. Sometimes larger molecules of organic substances, such as proteins, enter the cell through pinocytosis. Secrets are synthesized from these products in the endoplasmic reticulum. They move through the endoplasmic reticulum to the zone of the Golgi complex, where they gradually accumulate, undergo chemical restructuring and take the form of granules that are released from epithelial cells. An important role in the movement of secretory products in epithelial cells and their release is played by elements of the cytoskeleton - microtubules and microfilaments.

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

a- merocrine; b- apocrine; in- holocrine. 1 - poorly differentiated cells; 2 - regenerating cells; 3 - collapsing cells

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

The secretion mechanism in different glands is not the same, and therefore there are three types of secretion: merocrine (eccrine), apocrine and holocrine (Fig. 6.9). At merocrine type secretion, glandular cells completely retain their structure (for example, cells of the salivary glands). At apocrine type secretion, partial destruction of glandular cells (for example, cells of the mammary glands) occurs, i.e., together with secretory products, either the apical part of the cytoplasm of glandular cells (macroapocrine secretion) or the tops of microvilli (microapocrine secretion) are separated.

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

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

glands

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

The glands are divided into two groups: endocrine glands, or endocrine, and glands of external secretion, or exocrine(Fig. 6.10, a, b).

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

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

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

Scheme 6.2. Morphological classification of exocrine glands

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

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

In some glands, derivatives of the 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, having a process shape, cover the terminal sections. Their cytoplasm contains microfilaments containing contractile proteins. Myoepithelial cells, when contracted, compress the terminal sections and, therefore, facilitate the secretion of secretions from them.

The chemical composition of the secret may be different, in connection with this, the exocrine glands are divided into protein(serous), mucous(mucosal), protein-mucous(see fig. 6.11), sebaceous, saline(sweat, lacrimal, etc.).

In mixed salivary glands two types of secretory cells may be present - protein(serocytes) and mucous(mucocytes). They form

yut protein, mucous and mixed (protein-mucous) end sections. Most often, the composition of the secretory product includes protein and mucous components with only one of them predominating.

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

Rice. 6.11. Types of exocrine glands:

1 - simple tubular glands with unbranched terminal sections;

2 - a simple alveolar gland with an unbranched terminal section;

3 - simple tubular glands with branched terminal sections;

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

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

produced secrets, as well as the weakening of the regeneration processes and the growth of connective tissue (glandular stroma).

test questions

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

2. Stratified epithelium and their derivatives: topography in the body, structure, cellular differential composition, functions, regularities of regeneration.

3. Monolayer epithelium and their derivatives, topography in the body, cellular differential composition, structure, functions, regeneration.

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

Exercise 1. Consider and draw preparations 1,2,3,4,5.

Drug number 2. High prismatic (cylindrical) epithelium. Rabbit kidney. Hematoxylin-eosin
At low magnification, the tubules of the kidneys cut in different directions are clearly visible. Depending on how they were cut, the tubules can be in the form of circles or ovals and have a gap of various sizes. Connective tissue fibers and blood vessels are visible between the tubules. Under high magnification, a cross section of the renal tubule should be found, where a row of tall cylindrical cells is clearly visible, adjacent closely to each other. Cells are located on a thin basement membrane. In cells, the basal and apical edges are distinguished. The nucleus lies closer to the basal part of the cell. Sketch a section of one tubule labeling the listed structures.	Single-layer cylindrical epithelium of the collecting ducts of the kidney: 1-cylindrical cells; 2- basement membrane; 3- connective tissue and vessels surrounding the tubes
Drug number 3. Low prismatic epithelium. Rabbit kidney. Hematoxylin-eosin.
Locate the transverse section of the renal tubules on the preparation at low magnification. The size of the gap may vary. Epithelial cells are arranged in one row and adjoin very tightly to each other, forming a continuous layer. Determine the shape of the epithelial cells by comparing their width and height. End plates can be seen between cells in the apical part. The nuclei are rounded, large, and lie closer to the basal part and practically at the same level. The basement membrane separates the epithelial cells from the underlying connective tissue. In the connective tissue in large numbers there are blood capillaries. Examine the preparation under high magnification, examine the basement membrane,	Low prismatic epithelium of the renal tubules of the rabbit: 1-lumen of the tubule; 2 - prismatic cells; 3 - basement membrane; 4 - connective tissue and vessels surrounding the tubules. having the appearance of a thin oxyphilic border outside the tubule, consider the cytoplasm and nuclei of epithelial cells. Sketch a section of one tubule labeling the listed structures.

Drug number 4. Single layered squamous epithelium (mesothelium). Impregnation with silver nitrate + hematoxylin. Total drug

A total film preparation of the mesentery of the intestine, in which the lateral borders of tightly fitting epithelial cells of irregular shape were revealed by impregnation with silver nitrate. The thinnest parts of the preparation are stained in light yellow color, and the convoluted borders of the cell (1) are colored black. The cell contains one or two nuclei. This is due to the fact that the mesentery consists of two layers of epithelium, and between them there is a thin layer of connective tissue. Nuclei (2) were stained with hematoxylin. Examine the preparation under high magnification and draw 5-6 cells, marking the tortuous cell borders, nuclei and cytoplasm

Single-layer squamous epithelium (mesothelium) of the omentum: 1-epithelial cells; a-cytoplasm; b-core;

Drug number 5. transitional epithelium. Rabbit bladder. Hematoxylin-eosin.

The drug is a transverse section of the bladder wall. From the inside, the wall is lined with transitional epithelium. The epithelial layer forms folds. view the preparation at low magnification. The epithelial layer is represented by several layers of cells: the basal layer, the intermediate layer and surface layer. Cells of the intermediate layer of various shapes (rounded, cubic and irregular polygonal, and on the surface - elongated if the layer is not stretched), some of them are binuclear. The lowest layer of the epithelial layer is separated from the connective tissue by a thin basement membrane.

Transitional epithelium of the bladder (epithelium with an unstretched wall of the organ): 1- superficial cells with a cuticle on the surface; 2- cells of the intermediate layers of the epithelium; 3-cells of the basal layer of the epithelium; 4- loose connective tissue A blood vessel can be seen located in the loose connective tissue (4).

INDEPENDENT WORK.

Exercise 1. Draw a diagram of the structure of the desmosome, hemidesmosome and its relationship with the basement membrane, noting the main chemical components of these structures.

Task 2. Make a diagram of the morphological classification of epithelia, giving appropriate examples.

Recommended Further Reading.

1. Shubnikova E.A. Epithelial tissues.-M.: Publishing house of Moscow State University, 1996.-256 p.

2. Ham A., Cormac D. Histology.-M., Mir, 1983.-T.2.-S.5-34.

Laboratory work №2

Topic: Epithelial tissues. glandular epithelium. exocrine glands

The purpose of the lesson.

After independent study of theoretical material and work on practical lesson the student must know:

1. Characteristics of glandular epitheliocytes, features of their structure.

2. Classifications and typical examples various kinds glands.

3. Secretory cycle of glandular epithelial cells, its morphofunctional characteristics and structure various types secretory cells.

Topic study plan

glandular epithelium

Definitions and classification

Secretion types

Merocrine

Apocrine

Holocrine

Genetic classification of epithelia (examples)

• Skin type epithelium (ectodermal) Stratified squamous keratinized and non-keratinized epithelium .; epithelium of salivary, sebaceous, milk and sweat glands; transitional epithelium of the urethra; multi-row ciliated epithelium of the airways; alveolar epithelium of the lungs; thyroid epithelium and parathyroid gland, thymus and adenohypophysis.
• Epithelium of the intestinal type (enterodermal) Single layer prismatic epithelium of the intestinal tract; epithelium of the liver and pancreas.
• Epithelium of the renal type (nephrodermal) Epithelium of the nephron.
• Epithelium of the coelomic type (coelodermal) Single-layer squamous epithelium of the serous integuments (peritoneum, pleura, pericardial sac); epithelium of the gonads; epithelium of the adrenal cortex.
• Epithelium of the neuroglial type Epidymal epithelium of the cerebral ventricles; epithelium meninges; retinal pigment epithelium; olfactory epithelium; glial epithelium of the organ of hearing; taste epithelium; epithelium of the anterior chamber of the eye; chromophobic epithelium of the adrenal medulla; perineural epithelium.

Topography, sources of development, structure, regeneration.

Single layer epithelium

The sources of embryonic development of the epithelium are the ectoderm, endoderm, intermediate and lateral (splanchnotome) parts of the mesoderm, as well as the mesenchyme (endothelium of blood vessels, heart chambers). Development begins from 3-4 weeks of embryonic development. Epithelia do not have a single source of origin.

The endothelium develops from the mesenchyme. The single-layer squamous epithelium of the serous integument is from splanchnotomes (ventral part of the mechoderm).

Morphological classification

All cells of a single-layer epithelium are located on the basement membrane. single layer flat epithelium (vascular and cardiac endothelium and mesothelium)

• single layer cubic epithelium (lines the proximal and distal parts of the renal tubules, has a brush border and basal striation)
• single layer prismatic(columnar) epithelium
  • Bandless (gall bladder)
  • Kamenchaty (small intestine)
  • glandular (stomach)
• multi-row (pseudo-layered) epithelium
  • Ciliated, or ciliated (airways)

The structure of various types of single-layer epithelium

Single layered squamous epithelium formed by flattened cells with some thickening in the region of the discoid nucleus. These cells are characterized by diplomatic differentiation of the cytoplasm: it is divided into the inner part (endoplasm), which is located around the nucleus and contains most of the relatively few organelles, and outer part(ectoplasm), relatively free of organelles. Examples of such epithelium are the lining of blood vessels - endothelium, body cavities - mesothelium(part of the serous membranes), some renal tubules ( thin part loops of Henle), lung alveoli(type I cells).

Single layered cuboidal epithelium formed by cells containing a spherical nucleus and a set of organelles that are better developed than in squamous epithelial cells. This epithelium is found in renal tubules, in follicles of the thyroid gland, in small ducts of the pancreas, bile ducts of the liver, small collecting ducts of the kidney.

Single-layer prismatic (cylindrical, or columnar) epithelium formed by cells with a pronounced polarity. The ellipsoidal nucleus lies along the long axis of the cells and is usually somewhat displaced towards their basal part, and well-developed organelles are unevenly distributed over the cytoplasm. This epithelium covers the surface stomach, guts, forms a lining large pancreatic ducts, large bile ducts, gallbladder, fallopian tube , wall large collecting ducts of the kidney. in the gut and gallbladder this epithelium bordered.

Single-layer multi-row (pseudostratified) prismatic epithelium formed by cells of several types, having different sizes. In these cells, the nuclei are located at different levels, which creates a false impression of multi-layering (causing the second name of the epithelium).

Single layer multi-row prismatic ciliated (ciliated) epithelium airways- the most typical representative of multi-row epithelium. It also lines the cavity of the fallopian tubes.

Single layer double row prismatic epithelium found in the duct of the epididymis, vas deferens, terminal parts of the prostate, seminal vesicles.

Localization of single-layer epithelium in the body

1) Mesothelium - covers the serous membranes: pleura, epi-, pericardium, peritoneum

2) Endothelium - lining the inside of the walls of the heart, blood, lymphatic vessels

3) the epithelium of some tubules of the kidneys, the outer sheet of the capsule of the renal tubules, etc.

Stratified epithelium

Sources of development

The sources of embryonic development of the epithelium are the ectoderm, endoderm, intermediate and lateral (splanchnotome) parts of the mesoderm, as well as the mesenchyme (endothelium of blood vessels, heart chambers). Development begins from 3-4 weeks of embryonic development. Epithelium does not have a single source of origin.

Localization in the body

Stratified squamous epithelium is the most common type of epithelium in the body.

Stratified squamous keratinized epithelium

• Epidermis skin
• Some plots oral mucosa

Stratified squamous nonkeratinized epithelium

• Cornea eyes
• conjunctiva
• Mucous membranes of the pharynx, esophagus, vagina, vaginal part of the cervix, part of the urethra, oral cavity

Stratified cuboidal epithelium is rare in the human body. It is similar in structure to stratified squamous epithelium, but the cells of the surface layer have a cubic shape.

• Wall of large ovarian follicles
• Sweat ducts and sebaceous glands skin.

Stratified prismatic epithelium is also rare.

• Some parts of the urethra
• Large excretory ducts of the salivary and mammary glands(partially)
• Zones sharp transition between multilayer flat and single-layer multi-row epithelium

transitional epithelium

• Most of urinary tract

Structure, cellular composition of layers

Layered flat keratinizing epithelium is the epithelium of the skin. It develops from the ectoderm. Layers:

• Basal layer- in many ways similar to a similar layer of stratified non-keratinized epithelium; additionally: contains up to 10% of melanocytes - outgrowth cells with melanin inclusions in the cytoplasm - provide protection from UV radiation; there is a small amount Merkel cells (part of mechanoreceptors); dendritic cells With protective function by phagocytosis; in epitheliocytes contains tonofibrils (special-purpose organoid - provides strength).
• Spiny layer- from epitheliocytes with spiny outgrowths; meet dendrocytes and lymphocytes blood; epitheliocytes are still dividing.
• Granular layer- from several rows elongated flattened oval cells with basophilic granules of keratohyalin (the precursor of the horny substance - keratin) in the cytoplasm; cells do not divide.
• glitter layer- the cells are completely filled with elaidin (formed from keratin and tonofibril decay products), which reflects and strongly refracts light; under a microscope, the boundaries of cells and nuclei are not visible.
• Layer of stratum corneum (stratum corneum)- comprises horny plates from keratin containing vesicles with fat and air, keratosomes (corresponding to lysosomes). The scales peel off from the surface.

Layered flat non-keratinizing epithelium. Layers:

• Basal layer — cylindrical shape epitheliocytes with weakly basophilic cytoplasm, often with a mitotic figure; in a small amount stem cells for regeneration;
• Spiny layer- consists of a significant number of layers spiny shaped cells , cells actively share.
• integumentary cells — flat, aging cells do not share, are gradually peeled off from the surface.

Transition epithelium. Layers:

• Basal layer- from small dark low-prismatic or cubic cells - undifferentiated and stem cells , provide regeneration;
• Intermediate layer- from large pear-shaped cells , a narrow basal part in contact with the basement membrane (the wall is not stretched, so the epithelium is thickened); when the wall of the organ is stretched, pear-shaped cells decrease in height and are located among the basal cells.
• integumentary cells — large domed cells ; with a stretched wall of an organ, the cells flatten; cells do not share, gradually slough off.

Single layered squamous epithelium represented in the body by endothelium and mesothelium. Mesothelium covers the serous membranes (pleura, peritoneum and pericardium). Its cells - mesotheliocytes - lie in one layer on the basement membrane, they are flat, have a polygonal shape and jagged edges. Through the mesothelium, serous fluid is secreted and absorbed, which facilitates the movement, sliding of organs (heart, lungs, organs abdominal cavity).Endothelium lines blood vessels, lymphatic vessels and the heart. It is a layer of flat cells - endotheliocytes, lying in one layer on the basement membrane. Only they are in contact with the blood and through them in the blood capillaries there is an exchange of substances between the blood and tissues.

Single layered cuboidal epithelium lines part of the renal tubules. It is a layer of cubic cells lying in one layer on the basement membrane. The epithelium of the renal tubules performs the function of reabsorption of a number of substances from the primary urine into the blood.

Single layer prismatic epithelium is a layer of prismatic (cylindrical) cells lying in one layer on the basement membrane. Such an epithelium lines the inner surface of the stomach, intestines, gallbladder, a number of ducts of the liver and pancreas, and some tubules of the kidney. In the single-layered prismatic epithelium lining the stomach, all cells are glandular, producing mucus that protects the stomach wall from damage and the digestive action of gastric juice. The intestine is lined with a single layer of prismatic bordered epithelium, which provides absorption of nutrients. To do this, on the apical surface of its epitheliocytes, numerous outgrowths are formed - microvilli, which together form a brush border.

Single-layer multi-row (pseudostratified) epithelium lines the airways: nasal cavity, trachea, bronchi. This epithelium is ciliated, or flickering ( his cilia can move quickly in one plane - flicker). It consists of cells of different sizes, the nuclei of which lie at different levels and form several rows - therefore it is called multi-row. It only seems that it is multi-layered (pseudo-layered). But it is single-layered, since all its cells are connected to the basement membrane. It distinguishes several types of cells:

a) ciliated(ciliated) cells; the movement of their cilia removes dust particles that have entered the respiratory tract along with air;

b) mucous(goblet) cells secrete mucus on the surface of the epithelium, performing a protective function;

in) endocrine, these cells secrete hormones into the blood vessels;

G) basal(short intercalary) cells are stem and cambial, capable of dividing and turning into ciliated, mucous and endocrine cells;

e) long insertion, lie between the ciliated and goblet, performing supporting and supporting functions.

Stratified squamous nonkeratinized epithelium covers the outside of the cornea of ​​​​the eye, lines the oral cavity, esophagus, vagina. It has three layers:

a) basal the layer consists of prismatic epithelial cells located on the basement membrane. Among them there are stem and cambial cells capable of mitotic division (due to newly formed cells, epitheliocytes are replaced above the underlying layers of the epithelium);

b) prickly(intermediate) layer consists of cells of irregular polygonal shape, interconnected by desmosomes;

in) flat(superficial) layer - ending their life cycle, these cells die and fall off the surface of the epithelium.

Stratified squamous keratinized epithelium(epidermis) covers the surface of the skin. The epidermis of the skin of the palms and soles has a significant thickness and 5 main layers are distinguished in it:

a) basal the layer consists of epithelial cells prismatic in shape, containing keratin intermediate filaments in the cytoplasm, there are also stem and cambial cells, after the division of which, some of the newly formed cells move to the overlying layers;

b) prickly layer - formed by polygonal cells, which are firmly interconnected by numerous desmosomes; the tonofilaments of these cells form bundles - tonofibrils, granules with lipids - keratinosomes appear;

in) grainy the layer consists of flattened cells, the cytoplasm of which contains grains of the protein filaggrin and keratolinin;

G) brilliant the layer is formed by flat cells in which there are no nuclei and organelles, and the cytoplasm is filled with the protein keratolinin;

e) horny the layer consists of postcellular structures - horny scales; they are filled with keratin (horny substance) and air bubbles; the outermost horny scales lose their connection with each other and fall off the surface of the epithelium, and they are replaced by new cells from the basal layer.

Stratified transitional epithelium lines urinary tract(calyces and pelvises of the kidneys, ureters, bladder), which are subject to significant stretching when filled with urine. It distinguishes the following layers of cells: a) basal; b) intermediate; c) superficial. When stretched, the cells of the surface layer are flattened, and the cells of the intermediate layer are embedded between the basal ones; while the number of layers is reduced.

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

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

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

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

The main general properties of epithelial tissues include the following:

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

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

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

4) Epithelial cells have morphofunctional polarity or polar differentiation. Polar differentiation is a different structure of the superficial (apical) and lower (basal) poles of the cell. For example, at the apical pole of the cells of some epithelia, the plasmolemma forms a suction border of villi or ciliated cilia, and the nucleus and most organelles are located at the basal pole.

In multilayer layers, the cells of the surface layers differ from the basal layers in form, structure, and functions.

Polarity indicates that in different parts of the cell, various processes. The synthesis of substances occurs at the basal pole, and at the apical pole, absorption, movement of cilia, secretion occurs.

5) The epithelium has a well-defined ability to regenerate. When damaged, they quickly recover by cell division.

6) There are no blood vessels in the epithelium.

Classification of epithelia

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

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

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

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

Depending on the shape of the cells, single-layer epithelium is divided into flat, cubic and prismatic, or cylindrical . In squamous epithelium, the height of the cells is much less than the width. Such an epithelium lines the respiratory sections of the lungs, the middle ear cavity, some sections of the renal tubules, and covers all the serous membranes of the internal organs. Covering the serous membranes, the epithelium (mesothelium) participates in the release and absorption of fluid into the abdominal cavity and back, prevents the organs from merging with each other and with the walls of the body. By creating a smooth surface of the organs lying in the chest and abdominal cavity, it provides the possibility of their movement. The epithelium of the renal tubules is involved in the formation of urine, the epithelium of the excretory ducts performs a delimiting function.

Due to the active pinocytotic activity of squamous epithelial cells, there is a rapid transfer of substances from the serous fluid to the lymphatic channel.

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

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

The functions of this epithelium are associated with the functions of the organ in which it is located (in the ducts - delimiting, in the kidneys osmoregulatory, and other functions). On the apical surface of the cells in the tubules of the kidney are microvilli.

Single layer prismatic (cylindrical) epithelium has a greater height of the cells compared to the width. It lines the mucous membrane of the stomach, intestines, uterus, oviducts, collecting ducts of the kidneys, excretory ducts of the liver and pancreas. It develops mainly from the endoderm. The oval nuclei are shifted to the basal pole and are located at the same height from the basement membrane. In addition to the delimiting function, this epithelium performs specific functions inherent in a particular organ. For example, the columnar epithelium of the gastric mucosa produces mucus and is called mucous epithelium the intestinal epithelium is called bordered, since at the apical end it has villi in the form of a border, which increase the area of ​​\u200b\u200bparietal digestion and absorption of nutrients. Each epithelial cell has more than 1000 microvilli. They can only be seen with an electron microscope. Microvilli increase the absorptive surface of the cell up to 30 times.

AT epithelium, lining the intestines are goblet cells. These are unicellular glands that produce mucus, which protects the epithelium from the effects of mechanical and chemical factors and contributes to a better promotion of food masses.

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

goblet cells are unicellular glands that secrete mucus covering the epithelium. This contributes to the adhesion of harmful particles, microorganisms, viruses that have entered along with the inhaled air.

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

II. Stratified epithelium

1. Stratified non-keratinized epithelium covers the surface of the cornea of ​​the eye, oral cavity, esophagus, vagina, caudal part of the rectum. This epithelium originates from the ectoderm. It distinguishes 3 layers: basal, spiny and flat (superficial). The cells of the basal layer are cylindrical. The oval nuclei are located in the basal pole of the cell. Basal cells divide in a mitotic way, compensating for the dying cells of the surface layer. Thus, these cells are cambial. With the help of hemidesmosomes, basal cells are attached to the basement membrane.

The cells of the basal layer divide and, moving up, lose contact with the basal membrane, differentiate and become part of the spiny layer. Spiny layer It is formed by several layers of cells of an irregular polygonal shape with small processes in the form of spikes, which, with the help of desmosomes, firmly connect the cells to each other. Tissue fluid with nutrients circulates through the gaps between the cells. Thin filaments-tonofibrils are well developed in the cytoplasm of spiny cells. Each tonofibril contains thinner filaments called microfibrils. They are built from the protein keratin. Tonofibrils, attached to desmosomes, perform a supporting function.

The cells of this layer have not lost their mitotic activity, but their division proceeds less intensively than the cells of the basal layer. top cells spinous layer gradually flatten and move into a superficial flat layer with a thickness of 2-3 rows of cells. The cells of the flat layer, as it were, spread out over the surface of the epithelium. Their nuclei also become flat. Cells lose the ability to mitosis, take the form of plates, then scales. The bonds between them weaken and they fall off the surface of the epithelium.

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

In the epithelium of hairless areas of the skin there are 5 layers: basal, spiny, granular, lustrous, and horny.

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

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

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

melanocytes stringed form. Their cell bodies are located in the basal layer, and processes can reach other layers of the epithelial layer.

The main function of melanocytes- education melanosome containing skin pigment - melanin. Melanosomes travel along the melanocyte processes to neighboring epithelial cells. Skin pigment protects the body from excessive ultraviolet radiation. In the synthesis of melanin involved: ribosomes, granular endoplasmic reticulum, Golgi apparatus.

Melanin in the form of dense granules is located in the melanosome between the protein membranes that cover the melanosomes and outside. Thus, melanosomes chemical composition are melanoprodeids. Spiny layer cells are multifaceted, have uneven boundaries due to cytoplasmic outgrowths (spikes), with the help of which they are connected to each other. The spiny layer has a width of 4-8 layers of cells. In these cells, tonofibrils are formed, which end in desmosomes and firmly connect the cells to each other, forming a supporting-protective frame. Spiny cells retain the ability to reproduce, which is why the basal and spiny layers are collectively called germ cells.

Granular layer consists of 2-4 rows of cells flat shape with fewer organelles. Tonofibrils are impregnated with keratohealin substance and turned into grains. Keratinocytes of the granular layer are the precursors of the next layer - brilliant.

glitter layer consists of 1-2 rows of dying cells. At the same time, keratohealin grains merge. Organelles degrade, nuclei disintegrate. Keratogealin is converted to eleidin, which strongly refracts light, giving the layer its name.

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

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

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

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

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