Epithelial tissueor epithelium,- border tissues, which are located on the border with the external environment, cover the surface of the body and mucous membranes of internal organs, line its cavities and form most of the glands.

The most important properties of epithelial tissues:closed cell arrangement (epithelial cells),forming layers, the presence of well-developed intercellular connections, location on basement membrane(a special structural formation that is located between the epithelium and the underlying loose fibrous connective tissue), the minimum amount of intercellular substance,

borderline position in the body, polarity, high ability to regenerate.

The main functions of epithelial tissues:barrier, protective, secretory, receptor.

The morphological features of epithelial cells are closely related to the function of cells and their position in the epithelial layer. By shape, epithelial cells are divided into flat, cubicand columnar(prismatic, or cylindrical). The nucleus of epithelial cells in most cells is relatively light (euchromatin predominates) and large, in shape it corresponds to the shape of the cell. The cytoplasm of epithelial cells, as a rule, contains well

1 Absent in international histological terminology.

2 In the foreign literature, the term “syncytium” usually denotes symplastic structures, and the term “symplast” is practically not used.

developed organelles. The cells of the glandular epithelium have an active synthetic apparatus. The basal surface of epithelial cells is adjacent to the basement membrane, to which it is attached with semi-desmos- compounds similar in structure to halves of desmosomes.

Basement membraneconnects the epithelium and the underlying connective tissue; at the light-optical level on the preparations, it looks like a structureless strip, is not stained with hematoxylin-eosin, but is detected by silver salts and gives an intense PIC reaction. At the ultrastructural level, two layers are found in it: (1) light plate (lamina lucida,or lamina rara),adjacent to the plasmolemma of the basal surface of epithelial cells, (2) dense plate (lamina densa),facing towards the connective tissue. These layers differ in the content of proteins, glycoproteins and proteoglycans. The third layer is often described - reticular plate (lamina reticularis),containing reticular fibrils, but many authors consider it as a component of connective tissue, not referring to the basement membrane itself. The basement membrane contributes to the maintenance of normal architectonics, differentiation and polarization of the epithelium, ensures its strong connection with the underlying connective tissue, and selectively filters nutrients entering the epithelium.

Intercellular connections,or contacts,epithelial cells (Fig. 30) - specialized areas on their lateral surface, which provide communication of cells with each other and contribute to the formation of layers, which is the most important distinctive feature of the organization of epithelial tissues.

(1)Tight (closing) connection (zonula occludens)is an area of \u200b\u200bpartial fusion of the outer layers of the plasmolemma of two neighboring cells, which blocks the spread of substances through the intercellular space. It has the form of a belt surrounding the cell along the perimeter (at its apical pole) and consisting of anastomosing strands intramembrane particles.

(2)Shingles desmosome, or adhesive band (zonula adherens),localized on the lateral surface of the epithelial cell, covering the cell along the perimeter in the form of a belt. Elements of the cytoskeleton are attached to the plasmolemma sheets, thickened from the inside in the junction area - actin microfilaments.The widened intercellular gap contains adhesive protein molecules (cadherins).

(3)Desmosome, or adhesion spot (macula adherens),consists of thickened disc-shaped areas of plasmolemmas of two adjacent cells (intracellular desmosome seals,or desmosome plates),which serve as attachment sites

laziness to the plasmolemma intermediate filaments (tonofilaments)and are separated by a widened intercellular gap containing adhesive protein molecules (desmocollins and desmogleins).

(4)Finger-shaped intercellular junction (interdigitation) is formed by protrusions of the cytoplasm of one cell, protruding into the cytoplasm of another, as a result of which the strength of the connection of cells with each other increases and the surface area through which intercellular metabolic processes can be carried out increases.

(5)Slotted connection, or nexus (nexus),formed by a set of tubular transmembrane structures (connexons),piercing the plasmolemma of neighboring cells and joining each other in the region of a narrow intercellular gap. Each connexon consists of subunits formed by the connexin protein and is penetrated by a narrow channel, which determines the free exchange of low molecular weight compounds between cells, ensuring their ionic and metabolic conjugation. That is why gap joints are referred to as communication connections,providing a chemical (metabolic, ionic and electrical) connection between epithelial cells, in contrast to dense and intermediate compounds, desmosomes and interdigitations, which condition the mechanical connection of epithelial cells with each other and therefore are called mechanical intercellular connections.

The apical surface of epithelial cells may be smooth, folded, or contain cilia,and / or microvilli.

Types of epithelial tissues:1) integumentary epithelium(form a variety of pavements); 2) glandular epithelium(form glands); 3) sensory epithelium(they perform receptor functions, are part of the sense organs).

Epithelium classificationsare based on two features: (1) structure, which is determined by the function (morphological classification),and (2) sources of development in embryogenesis (histogenetic classification).

Morphological classification of epithelia separates them depending on the number of layers in the epithelial layer and the shape of the cells (Fig. 31). By number of layersepithelium are subdivided into single layer(if all cells are located on the basement membrane) and multilayer(if only one layer of cells is located on the basement membrane). If all epithelial cells are associated with the basement membrane, but have a different shape, and their nuclei are located in several rows, then such an epithelium is called multi-row (pseudo-layered).By cell shapeepithelium are subdivided into flat, cubicand columnar(prismatic, cylindrical). In stratified epitheliums, their shape refers to the shape of the cells of the surface layer. This classification

also takes into account some additional features, in particular, the presence of special organelles (microvillous, or brush, border and cilia) on the apical surface of cells, their ability to keratinize (the last feature refers only to stratified squamous epithelium). A special type of stratified epithelium, which changes its structure depending on stretching, is found in the urinary tract and is called transitional epithelium (urothelium).

Histogenetic classification of epithelia developed by Acad. N.G. Khlopin and distinguishes five main types of epithelium developing in embryogenesis from various tissue primordia.

1.Epidermal typedevelops from the ectoderm and the prechordal plate.

2.Enterodermal typedevelops from the intestinal endoderm.

3.Celonephrodermal typedevelops from the coelomic lining and nephrotome.

4.Angiodermal typedevelops from the angioblast (a site of the mesenchyme that forms the vascular endothelium).

5.Ependymoglial typedevelops from the neural tube.

Integumentary epithelium

Monolayer squamous epithelium formed by flattened cells with some thickening in the area of \u200b\u200bthe discoid nucleus (Fig. 32 and 33). These cells are characterized by diplasmic differentiation of the cytoplasm,in which the denser part located around the nucleus stands out (endoplasm),containing most of the organelles, and the lighter outer part (ectoplasm)low in organelles. Due to the small thickness of the epithelial layer, gases easily diffuse through it and various metabolites are quickly transported. Examples of monolayer squamous epithelium are the lining of body cavities - mesothelium(see Fig. 32), blood vessels and heart - endothelium(fig. 147, 148); it forms the wall of some renal tubules (see Fig. 33), lung alveoli (Fig. 237, 238). The thinned cytoplasm of the cells of this epithelium on transverse histological sections is usually traced with difficulty, only flattened nuclei are clearly identified; a more complete picture of the structure of epithelial cells can be obtained on planar (film) preparations (see Fig. 32 and 147).

Unilamellar cubic epithelium formed by cells containing a spherical nucleus and a set of organelles that are better developed than in squamous epithelial cells. Such an epithelium is found in the small collecting ducts of the medulla of the kidney (see Fig. 33), renal

naltsah (Fig. 250), in the follicles of the thyroid gland (Fig. 171), in the small ducts of the pancreas, bile ducts of the liver.

Monolayer columnar epithelium (prismatic, or cylindrical) is formed by cells with a pronounced polarity. The nucleus of a spherical, more often ellipsoidal shape is usually displaced to their basal part, and well-developed organelles are unevenly distributed over the cytoplasm. This epithelium forms the wall of the large collecting ducts of the kidney (see Fig. 33), covers the surface of the gastric mucosa

(Fig. 204-206), intestines (Fig. 34, 209-211, 213-215),

forms the lining of the gallbladder (Fig. 227), large bile ducts and pancreatic ducts, fallopian tube (Fig. 271) and uterus (Fig. 273). For most of these epitheliums, the functions of secretion and (or) absorption are characteristic. So, in the epithelium of the small intestine (see Fig. 34), there are two main types of differentiated cells - columnar edged cells,or enterocytes(provide parietal digestion and absorption), and goblet cells,or goblet exocrinocytes(produce mucus that has a protective function). Absorption is provided by numerous microvilli on the apical surface of enterocytes, the aggregate of which forms striated (microvillous) border(see fig. 35). Microvilli are covered with a plasmolemma, on top of which a layer of glycocalyx is located; their base is formed by a bundle of actin microfilaments, woven into the cortical network of microfilaments.

Unilamellar, multilayered columnar ciliated epithelium most typical for the airways (Fig. 36). It contains cells (epithelial cells) of four main types: (1) basal, (2) intercalated, (3) ciliated and (4) goblet.

Basal cellssmall in size with their wide base adjacent to the basement membrane, and narrow apical part do not reach the lumen. They are cambial elements of the tissue that ensure its renewal, and, differentiating, gradually turn into intercalation cells,which then give rise to ciliatedand goblet cells.The latter produce mucus that covers the surface of the epithelium, moving along it due to the beating of the cilia of the ciliated cells. The ciliate and goblet cells with their narrow basal part contact the basement membrane and attach to the intercalary and basal cells, and the apical ones border on the lumen of the organ.

Cilia- organelles participating in the processes of movement, on histological preparations, look like thin transparent outgrowths on the apical

the surface of the cytoplasm of epithelial cells (see Fig. 36). Electron microscopy reveals that they are based on a framework of microtubules (axoneme,or axial filament), which is formed by nine peripheral doublets (pairs) of partially fused microtubules and one centrally located pair (Fig. 37). Axoneme is associated with basal body,which lies at the base of the cilium, is identical in structure to the centriole and continues into striated spine.The central pair of microtubules is surrounded by central shell,from which to peripheral doublets diverge radial knitting needles.Peripheral doublets are linked to each other nexin bridgesand interact with each other using dynein pens.In this case, adjacent doublets in the axoneme slide relative to each other, causing the beating of the cilium.

Stratified squamous keratinizing epithelium consists of five layers: (1) basal, (2) prickly, (3) granular, (4) shiny, and (5) horny (Fig. 38).

Basal layerformed by cubic or columnar cells with basophilic cytoplasm, lying on the basement membrane. This layer contains cambial elements of the epithelium and ensures the attachment of the epithelium to the underlying connective tissue.

Spiny layerformed by large cells of irregular shape, connected to each other by numerous processes - "thorns". Electron microscopy reveals desmosomes and associated tonofilament bundles in the area of \u200b\u200bthe thorns. As one approaches the granular layer, the polygonal cells gradually become flattened.

Granular layer- relatively thin, formed by flattened (fusiform in section) cells with a flat nucleus and cytoplasm with large basophilic keratohyalin granules,containing one of the precursors of the horny substance - profilaggrin.

Shiny layerexpressed only in the epithelium of the thick skin (epidermis), covering the palms and soles. It looks like a narrow homogeneous oxyphilic strip and consists of flattened living epithelial cells that turn into horny scales.

Stratum corneum(the most superficial) has a maximum thickness in the epithelium of the skin (epidermis) in the area of \u200b\u200bthe palms and soles. It is formed by flat horny scales with a sharply thickened plasmolemma (shell), which do not contain nuclei and organelles, dehydrated and filled with horny matter. At the ultrastructural level, the latter is represented by a network of thick bundles of keratin filaments immersed in a dense matrix. Horny scales retain bonds with each

another and are retained in the stratum corneum due to partially preserved desmosomes; as desmosomes are destroyed in the outer parts of the layer, the scales slough off (desquamate) from the surface of the epithelium. Stratified squamous keratinizing epithelium forms epidermis- the outer layer of the skin (see Fig. 38, 177), covers the surface of some areas of the oral mucosa (Fig. 182).

Stratified squamous non-keratinizing epithelium formed by three layers of cells: (1) basal, (2) intermediate and (3) surface (Fig. 39). The deep part of the intermediate layer is sometimes distinguished as a parabasal layer.

Basal layerhas the same structure and performs the same functions as the layer of the same name in the stratified squamous keratinizing epithelium.

Intermediate layerformed by large polygonal cells, which flatten as they approach the surface layer.

Surface layerit is not sharply separated from the intermediate and is formed by flattened cells, which are constantly removed from the surface of the epithelium by the desquamation mechanism. Stratified squamous non-keratinized epithelium covers the surface of the cornea of \u200b\u200bthe eye (see Fig. 39, 135), conjunctiva, mucous membranes of the oral cavity - partially (see Fig. 182, 183, 185, 187), pharynx, esophagus (Fig. 201, 202) , vagina and vaginal part of the cervix (Fig. 274), part of the urethra.

Transitional epithelium (urothelium) - a special type of stratified epithelium that lines most of the urinary tract - the calyx, pelvis, ureters and bladder (Fig. 40, 252, 253), part of the urethra. The shape of the cells of this epithelium and its thickness depend on the functional state (degree of stretching) of the organ. The transitional epithelium is formed by three layers of cells: (1) basal, (2) intermediate and (3) superficial (see Fig. 40).

Basal layerrepresented by small cells, which with their wide base adjoin the basement membrane.

Intermediate layerconsists of elongated cells, with a narrower part directed to the basal layer and overlapping in tiles.

Surface layerformed by large mononuclear polyploid or binuclear superficial (umbrella) cells, which change their shape to the greatest extent (from rounded to flat) when the epithelium is stretched.

Glandular epithelium

The glandular epithelium forms the majority glands- structures that perform a secretory function, producing and allocating a variety of

ny products (secrets) that provide various functions of the body.

Classification of glandsbased on the consideration of various features.

By the number of cells, the glands are divided into unicellular (eg goblet cells, diffuse endocrine system cells) and multicellular (most glands).

By location (relative to the epithelial layer), endoepithelial (lying within the epithelial layer) and exoepithelial (located outside the epithelial layer) glands. Most of the glands are exoepithelial.

According to the place (direction) of excretion of the secretion, the gland is divided into endocrine (secreting secretory products called hormones,into the blood) and exocrine (secreting secretions on the surface of the body or into the lumen of internal organs).

The exocrine glands contain (1) end (secretory) departments,which are composed of secretion-producing glandular cells, and (2) excretory ducts,providing the release of synthesized products to the surface of the body or into the cavity of organs.

Morphological classification of exocrine glandsbased on the structural features of their end sections and excretory ducts.

According to the shape of the end sections, the glands are divided into tubular and alveolar (spherical). The latter are sometimes also described as acini. If there are two types of end sections, the glands are called tubular alveolar or tubular acinar.

According to the branching of the end sections, unbranched and branched glands, along the branching of the excretory ducts - simple (with an unbranched duct) and complex (with branched ducts).

According to the chemical composition of the secretion produced, the glands are divided into protein (serous), mucous, mixed (protein-mucous) , lipid, etc.

According to the mechanism (method) of secretion elimination (Fig. 41-46), merocrine glands (secretion without disrupting the structure of the cell), apocrine (with secretion of a part of the apical cytoplasm of cells) and holocrine (with complete destruction of cells and the release of their fragments into a secret).

Merocrine glands prevail in the human body; this type of secretion is well demonstrated by the example of the acinar cells of the pancreas - pancreatocytes(see fig. 41 and 42). The synthesis of the protein secretion of acinar cells occurs

in the granular endoplasmic reticulum located in the basal part of the cytoplasm (see Fig. 42), which is why this part is basophilic stained on histological preparations (see Fig. 41). The synthesis ends in the Golgi complex, where secretory granules are formed, which accumulate in the apical part of the cell (see Fig. 42), causing its oxyphilic staining on histological preparations (see Fig. 41).

Apocrine glands in the human body are few; these include, for example, part of the sweat glands and mammary glands (see Fig. 43, 44, 279).

In the lactating mammary gland, the end sections (alveoli) are formed by glandular cells (galactocytes),in the apical part of which large lipid droplets accumulate, separating into the lumen together with small areas of the cytoplasm. This process is clearly seen in electron microscopy (see Fig. 44), as well as at the light-optical level when using histochemical methods for the detection of lipids (see Fig. 43).

Holocrine glands in the human body are represented by a single species - the sebaceous glands of the skin (see Fig. 45 and 46, as well as Fig. 181). In the end section of such a gland, which looks like glandular sac,you can trace the division of small peripheral basal(cambial) cells,their displacement to the center of the bag with filling with lipid inclusions and transformation into sebocytes.Sebocytes take the form vacuolated degenerating cells:their nucleus shrinks (undergoes pycnosis), the cytoplasm is overfilled with lipids, and the plasmolemma in the final stages is destroyed with the release of cellular contents that form the secretion of the gland - sebum.

Secretory cycle.The secretion process in glandular cells proceeds cyclically and includes successive phases, which may partially overlap. The most typical is the secretory cycle of an exocrine glandular cell that produces a protein secret, which includes (1) absorption phasestarting materials, (2) synthesis phasesecret, (3) accumulation phasesynthesized product and (4) secretion phase(fig. 47). In the endocrine glandular cell, synthesizing and secreting steroid hormones, the secretory cycle has some features (Fig. 48): after absorption phasesstarting materials should deposit phasein the cytoplasm of lipid droplets containing a substrate for the synthesis of steroid hormones, and after synthesis phasethe accumulation of secretion in the form of granules does not occur, the synthesized molecules are immediately released from the cell by diffusion mechanisms.

EPITELIAL FABRICS

Integumentary epithelium

Figure: 30. Scheme of intercellular connections in the epithelium:

A - the area of \u200b\u200bthe location of the complex of intercellular connections (highlighted in a frame):

1- epithelial cells: 1.1 - apical surface, 1.2 - lateral surface, 1.2.1 - complex of intercellular connections, 1.2.2 - finger-like connections (interdigitation), 1.3 - basal surface;

2- basement membrane.

B - type of intercellular connections on ultrathin sections (reconstruction):

1 - tight (closing) connection; 2 - encircling desmosome (adhesive band); 3 - desmosome; 4 - slotted connection (nexus).

B - three-dimensional diagram of the structure of intercellular connections:

1 - tight connection: 1.1 - intramembrane particles; 2 - encircling desmosome (adhesive band): 2.1 - microfilaments, 2.2 - intercellular adhesive proteins; 3 - desmosome: 3.1 - desmosome plate (intracellular desmosomal compaction), 3.2 - tonofilaments, 3.3 - intercellular adhesive proteins; 4 - gap junction (nexus): 4.1 - connexons

Figure: 31. Morphological classification of epithelia:

1 - unilamellar squamous epithelium; 2 - single-layer cubic epithelium; 3 - single-layer (single-row) columnar (prismatic) epithelium; 4, 5 - single-layer multilayer (pseudo-stratified) columnar epithelium; 6 - stratified squamous non-keratinizing epithelium; 7 - stratified cubic epithelium; 8 - stratified columnar epithelium; 9 - stratified squamous keratinizing epithelium; 10 - transitional epithelium (urothelium)

The arrow shows the basement membrane

Figure: 32. Unilamellar squamous epithelium (peritoneal mesothelium):

A - planar preparation

Coloring: silver nitrate-hematoxylin

1 - boundaries of epithelial cells; 2 - the cytoplasm of the epithelial cell: 2.1 - endoplasm, 2.2 - ectoplasm; 3 - the nucleus of the epithelial cell; 4 - binucleated cell

B - diagram of the structure on the cut:

1 - epithelial cell; 2 - basement membrane

Figure: 33. Monolayer flat, cubic and columnar (prismatic) epithelium (renal medulla)

Coloring: hematoxylin-eosin

1 - unilamellar squamous epithelium; 2 - single-layer cubic epithelium; 3 - unilamellar columnar epithelium; 4 - connective tissue; 5 - blood vessel

Figure: 34. Single-layer columnar limb (microvillous) epithelium (small intestine)

Coloring: iron hematoxylin-mucycarmine

1 - epithelium: 1.1 - columnar edged (microvillous) epithelial cell (enterocyte), 1.1.1 - striated (microvillous) border, 1.2 - goblet exocrinocyte; 2 - basement membrane; 3 - loose fibrous connective tissue

Figure: 35. Microvilli of intestinal epithelial cells (ultrastructure diagram):

A - longitudinal sections of microvilli; B - cross sections of microvilli:

1 - plasmolemma; 2 - glycocalyx; 3 - a bunch of actin microfilaments; 4 - cortical network of microfilaments

Figure: 36. Single-layer, multi-row columnar ciliated (ciliated) epithelium (trachea)

Coloring: hematoxylin-eosin-mucycarmine

1 - epithelium: 1.1 - ciliated epithelial cell, 1.1.1 - cilia, 1.2 - goblet exocrinocyte, 1.3 - basal epithelial cell, 1.4 - intercalary epithelial cell; 2 - basement membrane; 3 - loose fibrous connective tissue

Figure: 37. Cilium (ultrastructure diagram):

A - longitudinal section:

1 - cilium: 1.1 - plasmolemma, 1.2 - microtubules; 2 - basal body: 2.1 - satellite (center of microtubule organization); 3 - basal root

B - cross section:

1 - plasmolemma; 2 - doublets of microtubules; 3 - central pair of microtubules; 4 - dynein handles; 5 - nexin bridges; 6 - radial spokes; 7 - central shell

Figure: 38. Stratified squamous keratinizing epithelium (thick skin epidermis)

Coloring: hematoxylin-eosin

1 - epithelium: 1.1 - basal layer, 1.2 - thorny layer, 1.3 - granular layer, 1.4 - shiny layer, 1.5 - stratum corneum; 2 - basement membrane; 3 - loose fibrous connective tissue

Figure: 39. Stratified squamous non-keratinizing epithelium (cornea)

Coloring: hematoxylin-eosin

Figure: 40. Transitional epithelium - urothelium (bladder, ureter)

Coloring: hematoxylin-eosin

1 - epithelium: 1.1 - basal layer, 1.2 - intermediate layer, 1.3 - surface layer; 2 - basement membrane; 3 - loose fibrous connective tissue

Glandular epithelium

Figure: 41. Merocrine type of secretion

(terminal section of the pancreas - acinus)

Coloring: hematoxylin-eosin

1 - secretory (acinar) cells - pancreatocytes: 1.1 - nucleus, 1.2 - basophilic zone of the cytoplasm, 1.3 - oxyphilic zone of the cytoplasm with secretion granules; 2 - basement membrane

Figure: 42. Ultrastructural organization of glandular cells in the merocrine type of secretion (area of \u200b\u200bthe terminal section of the pancreas - acinus)

Drawing with EMF

1 - secretory (acinar) cells - pancreatocytes: 1.1 - nucleus, 1.2 - granular endoplasmic reticulum, 1.3 - Golgi complex, 1.4 - secretion granules; 2 - basement membrane

Figure: 43. Apocrine type of secretion (alveolus of the lactating mammary gland)

Coloring: Sudan Black-Hematoxylin

1 - secretory cells (galactocytes): 1.1 - nucleus, 1.2 - lipid drops; 1.3 - apical part with a section of cytoplasm separating from it; 2 - basement membrane

Figure: 44. Ultrastructural organization of glandular cells with apocrine type of secretion (alveoli area of \u200b\u200bthe lactating mammary gland)

Drawing with EMF

1 - secretory cells (galactocytes): 1.1 - nucleus; 1.2 - lipid drops; 1.3 - apical part with a section of cytoplasm separating from it; 2 - basement membrane

Figure: 45. Holocrine type of secretion (sebaceous gland of the skin)

Coloring: hematoxylin-eosin

1 - gland cells (sebocytes): 1.1 - basal (cambial) cells, 1.2 - gland cells at different stages of transformation into a secret, 2 - gland secret; 3 - basement membrane

Figure: 46. \u200b\u200bUltrastructural organization of glandular cells in holocrine type of secretion (area of \u200b\u200bthe sebaceous gland of the skin)

Drawing with EMF

1- gland cells (sebocytes): 1.1 - basal (cambial) cell, 1.2 - gland cells at different stages of transformation into a secret, 1.2.1 - lipid drops in the cytoplasm, 1.2.2 - nuclei undergoing pycnosis;

2- the secret of the gland; 3 - basement membrane

Figure: 47. Structural and functional organization of the exocrine glandular cell in the process of synthesis and release of protein secretion

EMF scheme

AND - absorption phase secret synthesis phaseprovided by the granular endoplasmic reticulum (2) and the Golgi complex (3); AT - secret accumulation phasein the form of secretory granules (4); G - secretion phasethrough the apical surface of the cell (5) into the lumen of the end section (6). The energy required to support all of these processes is produced by numerous mitochondria (7)

Figure: 48. Structural and functional organization of the endocrine glandular cell in the process of synthesis and release of steroid hormones

EMF scheme

AND - absorption phasethe cell of initial substances that are brought in by the blood and transported through the basement membrane (1); B - deposit phasein the cytoplasm of lipid droplets (2) containing a substrate (cholesterol) for the synthesis of steroid hormones; AT - synthesis phasesteroid hormone is provided by a smooth endoplasmic reticulum (3) and mitochondria with tubular-vesicular cristae (4); G - secretion phasethrough the basal surface of the cell and the wall of the blood vessel (5) into the blood. The energy required to support all these processes is produced by numerous mitochondria (4)

The sequence of processes (phases) is shown with red arrows

Epithelial tissues are divided into superficial, including the integumentary and lining, and glandular epithelium. Covering- this is the epidermis of the skin, lining - This is the epithelium that covers the cavities of various organs (stomach, bladder, etc.), glandular - is part of the glands.

Superficial epithelium is on the border between the internal and external environment and performs the following function: protective, barrier, receptor and metabolic, since through the epithelium (intestinal) nutrients are absorbed into the body and through the epithelium (renal) metabolic products are excreted from the body.

Glandular epithelium is a part of the glands that produce secrets and hormones necessary for the body, that is, it performs a secretory function.

Superficial epithelium differs from other tissues in six main features:

1) is located in layers;

2) lies on the basement membrane, consisting of an amorphous substance, including proteins, lipids and carbohydrates, fibronectins, laminins, as well as thin fibrils containing type IV collagen; the basement membrane consists of light and dark layers and performs functions: barrier, trophic, metabolic, anti-invasive, morphogenetic; attaches to itself a layer of epithelium; connective tissue is always located under the basement membrane;

3) there is no intercellular substance in it, therefore the epithelial cells are tightly adjacent to each other and are connected using intercellular contacts:

a) dense (zonula accludens),

b) dentate or finger-like (junctio intercellularis denticulatae),

c) desmosoma (desmosoma) and others;

4) the absence of blood vessels, since the epithelium is nourished from the connective tissue through the basement membrane;

5) epithelial cells have polar differentiation, that is, each cell has a basal end facing the basement membrane and an apical end facing in the opposite direction, which is explained by the borderline position of the tissue; in the cytolemma of the basal part of the cell, there is sometimes a basal striation, on the lateral surface there are intercellular contacts, on the apical surface there are microvilli, in some cases forming a suction border;

6) the integumentary epithelial tissue has a high ability to regenerate.

Classification of epithelial superficial tissues. Epithelial superficial tissues are classified according to 2 characteristics:

1) depending on the structure of the epithelial tissue and the relationship to the basement membrane;

2) depending on the origin (phylogenetic classification according to N.G. Khlopin).

Morphological classification. The superficial epithelium is divided into monolayer and stratified.

Monolayer epithelium in turn, they are divided into single-row and multi-row, or pseudo-layered. Uniform epithelium is divided into flat, cubic and prismatic, or columnar. Layered epitheliumalways prismatic.

Stratified epithelium subdivided into multilayer flat keratinizing, multilayer flat non-keratinizing, multilayer cubic (multilayer prismatic always non-keratinizing) and, finally, transitional. The name flat, cubic or prismatic depends on the shape of the cells in the surface layer. If the surface layer of cells has a flattened shape, then the epithelium is called flat, and all underlying layers can have different shapes: cubic, prismatic, irregular, etc. A single-layer epithelium differs from a multilayer one in that all its cells are located on the basement membrane, whereas while in the stratified epithelium, only one basement layer of cells is associated with the basement membrane, and the remaining layers are located one on top of the other.

Phylogenetic classification according to N.G. Khlopin.According to this classification, 5 types of epithelial tissues are distinguished:

1) epidermal epithelium - develops from ectoderm (for example, skin epithelium);

2) enterodermal epithelium - develops from the endoderm and lines the middle section of the gastrointestinal tract (stomach, small and large intestines);

3) celonephrodermal epithelium - develops from the mesoderm and lines the pleura, peritoneum, pericardium, renal tubules;

4) ependymoglial epithelium - develops from the neural tube, lines the ventricles of the brain and the central canal of the spinal cord;

5) angiodermal epithelium - develops from the mesenchyme, lines the chambers of the heart, blood and lymphatic vessels.

Monolayer squamous epithelium (epithelium squamosum simplex) is divided into endothelium (endothelium) and mesothelium (mesothelium).

Endothelium develops from the mesenchyme, lines the chambers of the heart, blood and lymph vessels. Endothelial cells - endotheliocytes have an irregular flattened shape, the edges of the cells are cut, contain one or more flattened nuclei, the cytoplasm is poor in organelles of general importance, contains many pinocytic vesicles. There are short microvilli on the luminal surface of endothelial cells. What luminal surface? This is the surface facing the lumen of an organ, in this case, a blood vessel or a chamber of the heart.

Endothelial function - metabolism between blood and surrounding tissue. When the endothelium is damaged, blood clots form in the vessels, blocking their lumen.

Mesothelium (mesothelium) develops from sheets of splanchnotome, lines the peritoneum, pleura, pericardium. The cells of mesotheliocytes have a flattened irregular shape, the edges of the cells are cut; the cells contain one, sometimes several flattened nuclei, the cytoplasm is poor in organelles of general importance, it contains pinocytic vesicles, indicating the function of exchange; on the luminal surface there are microvilli that increase the cell surface. The function of the mesothelium is to provide a smooth surface to the serous membranes. This facilitates the sliding of organs in the abdominal, chest and other cavities; through the mesothelium, there is an exchange of substances between the serous cavities and the underlying connective tissue of their walls. The mesothelium secretes the fluid contained in these cavities. If the mesothelium is damaged, adhesions can form between the serous membranes, which impede the movement of organs.

Unilamellar cubic epithelium (epithelium cuboideum simplex) is present in the renal tubules, excretory ducts of the liver. The shape of the cells is cubic, the nuclei are round, organelles of general importance are developed: mitochondria, EPS, lysosomes. The apical surface contains numerous microvilli forming a striated border (limbus striatus), rich in alkaline phosphatase (ALP). On the basal surface there is a basal striation (stria basalis), which is the folds of the cytolemma, between which the mitochondria are located. The presence of a striated border on the surface of epithelial cells indicates the absorption function of these cells, the presence of a basal striation - about the reabsorption (reabsorption) of water. The source of development of the renal epithelium is the mesoderm, or rather, nephrogenic tissue.

Columnar epithelium (epithelium columnare) is located in the small and large intestine and stomach. Columnar (prismatic) epithelium of the stomach lines the mucous membrane of this organ, develops from the intestinal endoderm. The epithelial cells of the gastric mucosa have a prismatic shape, an oval nucleus; in their light cytoplasm, smooth EPS, Golgi complex and mitochondria are well developed, in the apical part there are secretory granules containing mucous secretions. Thus, the superficial epithelium of the gastric mucosa is glandular. Therefore, its functions:

1) secretory, that is, the production of mucous secretions that envelop the gastric mucosa;

2) protective - mucus secreted by the glandular epithelium protects the mucous membrane from chemical and physical influences;

3) absorption - through the integumentary (aka glandular) epithelium of the stomach water, glucose, alcohol are absorbed.

Columnar (edged) epithelium of the small and large intestine (epithelium columnare cum limbus striatus) lines the mucous membrane of the small and large intestines, develops from the intestinal endoderm; characterized by the fact that it has a prismatic shape. The cells of this epithelium are connected to each other using tight contacts, or endplates, that is, the contacts close the intercellular gaps. The cells have well-developed organelles of general importance, as well as tonofilaments that form the cortical layer. In the area of \u200b\u200bthe lateral surfaces of these cells, closer to their base, there are desmosomes, finger-like, or serrated, contacts. On the apical surface of columnar epithelialitis, there are microvilli (up to 1 µm in height and up to 0.1 µm in diameter), the distance between which is 0.01 µm or less. These microvilli form the suction, or striated, border (limbus striatus). Functions of the limb epithelium: 1) parietal digestion; 2) absorption of cleavage products. Thus, a sign confirming the absorption function of this epithelium is: 1) the presence of an absorption border and 2) monolayer.

The epithelium of the small and large intestine includes not only columnar epithelial cells. Between these epithelial cells there are also goblet epithelial cells (epitheliocytus caliciformis), which perform the function of secreting mucous secretions; endocrine cells (endocrinocyti) that produce hormones; poorly differentiated cells (stem), devoid of a border, which perform a regenerative function and due to which the intestinal epithelium is renewed within 6 days; in the epithelium of the gastrointestinal tract, cambial (stem) cells are located compactly; finally, there are cells with acidophilic granularity.

Pseudo-stratified (multi-row) epithelium(epithelium pseudostratificatum) is unilamellar, since all its cells lie on the basement membrane. Why, then, is this epithelium called multi-row? Because its cells have different shapes and sizes, and, therefore, their nuclei are located at different levels and form rows. The nuclei of the smallest cells (basal, or short intercalated) are located closer to the basement membrane, the nuclei of medium-sized cells (long intercalated) are localized higher, the nuclei of the tallest cells (ciliated) are farthest from the basement membrane. The multi-row epithelium is located in the trachea and bronchi, the nasal cavity (develops from the prechordal plate), in the male vas deferens (develops from the mesoderm).

In the multi-row epithelium, 4 types of cells are distinguished:

1) ciliated epithelial cells (epitheliocytus ciliatus);

2) small and large intercalated cells (epitheliocytus intercalatus parvus et epitheliocytus intercalatus magnus);

3) goblet cells (exocrinocytus caliciformis);

4) endocrine cells (endocrinocytus).

Ciliated epithelial cells - these are the highest cells of the pseudostratified epithelium of the respiratory tract mucosa. The nuclei of these cells are oval in shape and, as already mentioned, are most distant from the basement membrane. In their cytoplasm, there are organelles of general importance. The basal narrow end of these cells is connected to the basement membrane; at the wide apical end there are cilia (cilii) 5-10 μm long. At the base of each cilium there is an axial filament (filamenta axialis), which consists of 9 pairs of peripheral and 1 pair of central microtubules. The axial filament connects to the basal corpuscle (modified centriole). The cilia, carrying out oscillatory movements directed against the inhaled air, remove dust particles that have settled on the surface of the mucous membranes of the trachea and bronchi.

Ciliated epithelial cells are also part of the epithelium of the mucous membrane of the fallopian tubes and uterus, although this epithelium is not multi-row.

Small intercalary cells airways - the smallest, have a triangular shape, with a wide basal end they lie on the basement membrane. The function of these cells - regenerative; they are cambial or stem cells. In the trachea, bronchi, nasal cavity and epidermis of the skin, cambial cells are located diffusely.

Large insertion cages above the small intercalary, but their apical part does not reach the surface of the epithelium.

Goblet cells (exocrinocytus caliciformis) are glandular cells (unicellular glands). Until the moment when these cells did not have time to accumulate a secret, they have a prismatic shape. Their cytoplasm has a flattened nucleus, well-developed smooth EPS, Hlgi complex and mitochondria. Granules of mucous secretions accumulate in their apical part. As these granules accumulate, the apical part of the cell expands and the cell at the same time takes the form of a goblet, which is why it is called goblet. The function of goblet cells is the secretion of mucous secretions, which, enveloping the mucous membrane of the trachea and bronchi, protect it from chemical and physical influences.

Endocrinocytes as part of the multi-row epithelium of the respiratory tract, otherwise called basal-granular or chromaffin cells, they perform a hormonal function, that is, they secrete the hormones norepinephrine and serotonin, which regulate the contractility of the smooth muscles of the bronchi and trachea.

Characterization of epithelial tissue

Characteristics of the main types of fabrics

LECTURE No. 2

There are four main types of tissues in the human body: epithelial, muscular, nervous and connective.

Epithelial tissue - consists of individual cells, and covers the surface of the body (for example, skin) or the walls of internal cavities, and also lines the inside of hollow organs (blood vessels and airways). There are two large groups of epithelial tissues (integumentary and glandular), each of which, in turn, consists of several types.

According to the peculiarities of the location of cells relative to each other, two types of epithelial tissue are distinguished - single-layer and multilayer epithelium. All epithelial cells monolayer epithelium are located on the basement membrane, a structure homogeneous in structure that connects them to each other.

Unilamellar epithelium forms only one layer of cells, and has three varieties:

Squamous unilamellar epithelium (consists of flat cells, lines the alveoli of the lungs, the inner surface of blood and lymphatic vessels - called endothelium).

The single-layer prismatic (cylindrical) epithelium consists of one layer of cells (it lines the inside of the ducts of most glands, the gallbladder, almost the entire digestive tract, where it includes goblet cells, as well as individual sections of the genital tract).

Ciliated epithelium - lines the walls of the airways and paranasal sinuses (frontal, maxillary), the ventricles of the brain. The cells are prismatic. At their free end are thin hair-like processes - cilia. They are in constant motion towards the outer opening of the organs. In the respiratory tract, they prevent dust, mucus and other foreign bodies from entering the lungs.

Stratified epithelium - consists of several layers of cells (some of the cells do not have contact with the basement membrane). Consists of two zones: a) keratinization zone (several layers of flat cells); b) primordial (basal zone) - consists of cylindrical cells.

Protective function - protects the tissues located underneath from damage and loss of fluid, and also prevents it from entering the body.

Secretory function - most of the glands and their ducts are formed by cylindrical (prismatic) epithelium.

Endocrine glands are also made up of epithelial cells that adhere tightly to each other or enclose hollow vesicles (as in the thyroid gland).

Shells - consist of specialized cells and line the backs of hollow organs and body cavities. There are three types:

Mucous; They all release fluid for lubrication or

Synovial; wetting the surface of the cavities that they

Serous; cover.

Mucous– lining the walls of the digestive and urogenital organs, as well as the airways from the inside. Consists of goblet cells filled with mucous secretions (consists of water, salts and mucin protein).

Synovial membrane - lines the cavities of the joints. It consists of delicate connective tissue covered with a single layer of flat endothelial cells. This membrane secretes synovial fluid, which moisturizes and lubricates the articular surfaces, eliminating friction between them.

Serous membranes - cover the walls of the abdominal and chest cavities, as well as the internal organs located there. The lungs and walls of the chest cavity are covered pleura.

Pericardiumcovers the heart with a double leaf.

Peritoneum lines the organs and walls of the abdominal cavity. The pleura, pericardium and peritoneum are serous membranes and have a number of properties in common. Each of them consists of two smooth, shiny leaves that delimit the cavity into which the fluid secreted by them enters. In composition, this serous fluid is very similar to blood plasma or lymph. It reduces friction between the organs and the walls of the cavities surrounding them, contains antibodies, and also promotes the excretion of metabolic products hazardous to the body into the lymph flow.

2.2 Muscle

Muscle - intended for contractions, due to which a variety of movements of the human body are performed. It consists of a cylindrical shape of muscle fibers corresponding to the cells of other tissues. With the help of connective tissue, these fibers are combined into small bundles.

Histology.

Cell: structure, properties. Fabrics: definition, properties. Epithelial, connective, muscle tissue: position, types, structure, meaning. Nervous tissue: position, structure, meaning.

The human body is a complex, holistic, self-regulating and self-renewing system, which is characterized by a certain organization of its structure. The basis of the structure and development of man is cell - an elementary structural, functional and genetic unit of a living organism, capable of division and exchange with the environment.

The human body is built of cells and non-cellular structures, united in the process of development into tissues, organs, organ systems and the whole organism. The human body has a huge number of cells (10 14), while their size ranges from 5-7 to 200 microns. The largest are the ovum and nerve cells (up to 1.5 m together with processes), and the smallest are blood lymphocytes. The science that studies the development, structure and function of cells is called cytology. The shape of the cells, as well as their size, is very diverse: flat, cubic, round, elongated, stellate, spherical, fusiform, which is due to the function they perform and the conditions of their life.

All cells are characterized by a common structural principle. The main parts of the cell are: the nucleus, the cytoplasm with the organelles in it, and the cytolemma (plasmalemma, or cell membrane).

Cell membrane is a universal biological membrane that ensures the constancy of the internal environment of the cell by regulating the metabolism between the cell and the external environment - it is the transport (transport of necessary substances into and out of the cell) and the barrier-receptor system of the cell. With the help of the plasmalemma, special structures of the cell surface are formed in the form of microvilli, synapses, etc.

Inside the cell is core- the control center of the cell and the regulator of its vital functions. Usually, a cell has one nucleus, but there are also multinucleated cells (in the epithelium, vascular endothelium) and non-nuclear cells (erythrocytes and platelets). The nucleus has a nuclear envelope, chromatin, nucleolus and nuclear juice (nucleoplasm). The nuclear membrane separates the nucleus from the cytoplasm and is actively involved in the exchange of substances between them. Chromatin contains proteins and nucleic acids (chromosomes are formed during cell division). The nucleolus takes part in the synthesis of cellular proteins.

Cytoplasmis the content of the cell and makes up 1-99% of its mass. It contains the nucleus and organelles, the products of intracellular metabolism. The cytoplasm unites all cellular structures and ensures their chemical interaction with each other. It consists of proteins (of which cellular structures are built), fats and carbohydrates (energy source), water and salts (determine the physicochemical properties of the cell, create osmotic pressure and its electric charge) and nucleic acids (participation in protein biosynthesis).

Cytoplasmic organelles... Organelles are called cytoplasmic microstructures that are present in almost all cells and perform vital functions.

Endoplasmic reticulum -system of tubules, vesicles, the walls of which are formed by cytoplasmic membranes. Distinguish between granular and agranular (smooth) endoplasmic reticulum. The agranular endoplasmic reticulum is involved in the synthesis of carbohydrates and lipids, the granular - in protein synthesis, because ribosomes are located on the membranes of the granular endoplasmic reticulum, which can also be located on the membrane of the nucleus or freely in the cytoplasm. Ribosomes carry out protein synthesis, while in an hour they synthesize more protein than their total mass.

Mitochondria - power energy stations of the cell. In mitochondria, glucose, amino acids, fatty acids are broken down and ATP, a universal cellular fuel, is formed.

Golgi complex- has a mesh structure. Its function consists in the transport of substances, their chemical processing and removal of the products of its vital activity outside the cell.

Lysosomes - contain a large amount of hydrolytic enzymes involved in the process of intracellular digestion of nutrients entering the cell, destroyed parts of the cell, foreign particles that have entered the cell. Therefore, there are especially many lysosomes in cells that take part in phagocytosis: leukocytes, monocytes, liver cells, and small intestine.

Cell center represented by two centrioles located right in the geometric center of the cell. During mitosis, microtubules of the mitotic spindle diverge from the centrioles, providing orientation and movement of chromosomes, and a radiant zone is formed, as well as centrioles form cilia and flagella.

Flagella and cilia - organelles for special purposes - are designed to move specialized cells (spermatozoa) or cause the movement of fluid around the cell (epithelial cells of the bronchi, trachea).

Cell properties:

1. Metabolism (metabolism) - a set of chemical reactions that form the basis of cell life.

2. Irritability - the ability of cells to respond to changes in environmental factors (temperature, light, etc.). The reaction of a cell is movement, increased metabolism, secretion, muscle contraction, etc.

3. Growth - increase in size, development - acquisition of specific functions

4. Reproduction - the ability to reproduce itself. The basis for the preservation and development of cells, replacement of aging and dead cells, regeneration (restoration) of tissues and growth of the body (many cells performing complex functions have lost the ability to divide, but the emergence of new cells occurs only through the division of cells that are capable of dividing). Physiological regeneration - the process of death in tissues of old cells and the appearance of new ones.

There are two main forms of cell division: mitosis (the most common, provides a uniform distribution of hereditary material between daughter cells) and meiosis (reduction division, observed in the development of only sex cells).

The period from one cell division to another is its life cycle.

In the human body, in addition to cells, there are also non-cellular structures: symplast and intercellular substance. Symplast, unlike cells, contains many nuclei (striated muscle fibers). The intercellular substance is secreted by cells, located in the intervals between them.

Intercellular (intercellular) fluid - is replenished by the liquid part of the blood released from the bloodstream, the composition of which changes.

Cells and their derivatives combine into tissues. the cloth Is a system of cells and non-cellular structures, united by the unity of origin, structure and functions. Histology- a science that studies the human structure at the tissue level.

In the process of evolution, with the increasing complexity of the needs of the organism, specialized cells appeared, capable of performing certain functions. The ultrastructure of these cells changed accordingly. The process of tissue formation is long-lasting, it begins in the prenatal period and continues throughout a person's life. The interaction of the organism with the external environment that developed in the process of evolution and the need to adapt to the conditions of existence led to the emergence of 4 types of tissues with certain functional properties:

1.epithelial,

2.connecting,

3.muscular and

4. nervous.

All types of tissues of the human body develop from three germ layers - mesoderm, ectoderm, endoderm.

In the body, tissues are interconnected morphologically and functionally. The morphological relationship is due to the fact that different tissues are part of the same organs. The functional connection is manifested in the fact that the activities of different tissues that make up the organs are coordinated. This consistency is due to the regulatory influence of the nervous and endocrine systems on all organs and tissues - the neurohumoral mechanism of regulation.

Epithelial tissue

Epithelial tissue (epithelium) covers:

1. The entire outer surface of the body of humans and animals

2. All body cavities, lining the mucous membranes of hollow internal organs (stomach, intestines, urinary tract, pleura, pericardium, peritoneum)

3. Is a part of the endocrine glands.

Functions:

1.metabolic function - participates in the exchange of substances between the body and the external environment, absorption (intestinal epithelium) and excretion (renal epithelium, gas exchange (lung epithelium);

2. protective function (skin epithelium) - protection of underlying structures from mechanical, chemical effects and from infections;

3. delimiting;

4. secretory - glands.

Features:

1. Located on the border of the external and internal environments of the body

2. Consists of epithelial cells forming continuous sheets. The cells are closely related to each other.

3. Characteristically weak development of the intercellular substance.

4.there is a basement membrane (carbohydrate-protein-lipid complex with the finest fibrils, separates the epithelial tissue from the underlying loose connective tissue)

5. the cells have polarity (the apical and basal parts differ in structure and function; in the stratified epithelium there are differences in the structure and function of the layers). Epithelial cells can have special-purpose organelles:

Ø cilia (airway epithelium)

Ø microvilli (intestinal and kidney epithelium)

Ø tonofibrils (skin epithelium)

6. There are no blood vessels in the epithelial layers. Cell nutrition is carried out by the diffusion of nutrients through the basement membrane, which separates the epithelial tissue from the loose connective tissue underlying it and serves as a support for the epithelium.

7. Has a great regenerative capacity (has a high ability to recover).

Epithelial tissue classification:

By function distinguish :

1. integumentary;

2. glandular epithelium.

AT cover the epithelium is distinguished by monolayer and stratified epithelium.

1. In a monolayer epithelium, all cells are located on the basement membrane in one row,

2. in a multilayer - several layers are formed, while the upper layers lose their connection with the basement membrane (lining the outer surface of the skin, the esophageal mucosa, the inner surface of the cheeks, the vagina).

The stratified epithelium is:

Ø keratinizing (skin epithelium)

Ø non-keratinizing (epithelium of the cornea of \u200b\u200bthe eye) - in the surface layer, keratinization is not observed, in contrast to the keratinizing epithelium.

A special form of stratified epithelium - transition epithelium, which is located in organs that can change their volume (undergo stretching) - in the bladder, ureters, renal pelvis. The thickness of the epithelial layer changes depending on the functional state of the organ

Monolayer epithelium can be single or multi-row.

By the shape of the cells are distinguished:

Ø unilamellar squamous epithelium (mesothelium) - consists of one layer of sharply flattened polygonal cells (polygonal); the base (width) of the cells is greater than the height (thickness). Covers serous membranes (pleura, peritoneum, pericardium), walls of capillaries and blood vessels, alveoli of the lungs. Carries out the diffusion of various substances and reduces the friction of flowing fluids;

Ø monolayer cubic epithelium - on a cut in cells, the width is equal to the height lines the ducts of many glands, forms tubules of the kidneys, small bronchi, performs a secretory function;

Ø unilamellar columnar epithelium - on a cut, the width of the cells is less than the height lines the stomach, intestines, gallbladder, renal tubules, is part of the thyroid gland.

Depending on the features of the structure and function, they are distinguished:

Ø single-layer prismatic glandular - present in the stomach, in the cervical canal, specialized in the continuous production of mucus;

Ø single-layer prismatic edged - lining the intestine, on the apical surface of the cells there are a large number of microvilli, specialized for absorption;

Ø unilamellar epithelium - more often prismatic multi-row, the cells of which have outgrowths at the upper, apical, end - cilia that move in a certain direction, creating a mucus flow. Lines the respiratory tract, fallopian tubes, cerebral ventricles, spinal canal. Provides the transport of various substances. It contains a variety of cells:

1.short and long intercalated cells (poorly differentiated and among them stem cells; provide regeneration);

2. goblet cells - poorly perceive dyes (in the preparation - white), produce mucus;

3. ciliated cells - on the apical surface have ciliated cilia; purify and humidify the passing air.

Glandular epithelium makes up the bulk of the glands, the epithelial cells of which are involved in the formation and secretion of substances necessary for the life of the body. The glands are subdivided into exocrine and endocrine. Exocrine glands secrete a secret in the cavity of internal organs (stomach, intestines, respiratory tract) or on the surface of the body - sweat, salivary, mammary, etc., the endocrine glands have no ducts and secrete a secret (hormone) into the blood or lymph - pituitary, thyroid and parathyroid glands, adrenal glands.

In structure, exocrine glands can be tubular, alveolar and combined - tubular-alveolar.

Epithelial tissues link 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 cavities.

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

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

The main general properties of epithelial tissues include the following:

1) Epithelial cells adhere tightly to each other and are connected by various contacts (using desmosomes, closure belts, adhesion belts, gaps).

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 that enter the cells and are secreted by them penetrate here.

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

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

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

Polarity indicates that different processes occur in different parts of the cell. The synthesis of substances occurs at the basal pole, while at the apical one there is absorption, movement of cilia, secretion.

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

6) There are no blood vessels in the epithelium.

Epithelium classification

There are several classifications of epithelial tissues. Depending on the location and function performed, there are two types of epithelium: 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 epithelia are divided into two groups: I ) single-layer and II ) multilayer .

AT monolayer epithelium the lower (basal) poles of the cells are attached to the basement membrane, and the upper (apical) poles are adjacent to 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, monolayer epithelium is subdivided into flat, cubic and prismatic, or cylindrical ... In squamous epithelium, the height of the cells is much less than the width. This epithelium lines the respiratory parts of the lungs, the middle ear cavity, some parts 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 fusing with each other and with the body walls. By creating a smooth surface for the organs lying in the chest and abdominal cavity, it allows them to move. 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 pinocytic activity of squamous epithelial cells, there is a rapid transfer of substances from the serous fluid to the lymphatic bed.

The unilamellar squamous epithelium that covers the mucous membranes of organs and serous membranes is called lining.

Unilamellar cubic epitheliumlines the excretory ducts of the glands, kidney tubules, 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 cells in the kidney tubules are microvilli.

Monolayer prismatic (cylindrical) epitheliumhas a larger cell height than its 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 towards 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 this or that organ. For example, the columnar epithelium of the gastric mucosa produces mucus and is called mucous epithelium, the intestinal epithelium is called rimmed, 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 viewed with an electron microscope. Microvilli increase the suction surface of the cell up to 30 times.

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

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

Goblet cellsare unicellular glands that secrete mucus that covers the epithelium. This contributes to the adhesion of harmful particles, microorganisms, viruses, which are trapped along with the inhaled air.

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