Epithelium(Latin epithelium, from other Greek - nipple of the mammary gland), or epithelial tissue- layer of cells lining the surface (epidermis) and body cavities, as well as mucous membranes internal organs, alimentary tract, respiratory system, genitourinary tract. In addition, it forms most of the glands of the body.

Morphological classification of epithelium:

1. Single layer epithelium can be single-row or multi-row. U single-row single-layer epithelium all cells have the same shape - flat, cubic or prismatic, their nuclei lie at the same level, that is, in one row. In multirow single-layer epithelium, hematoxylin-eosin-stained, prismatic and intercalary cells are distinguished; the latter, in turn, are divided according to the principle of the ratio of the nucleus to the basement membrane into high intercalary and low intercalary cells.
2. Stratified epithelium It can be keratinizing, non-keratinizing and transitional. Epithelium in which keratinization processes associated with cell differentiation occur upper layers into flat horny scales, called stratified squamous keratinizing epithelium. In the absence of keratinization, the epithelium is called stratified squamous non-keratinizing epithelium.
3. Transitional epithelium lines organs subject to strong stretching - the bladder, ureters, etc. When the volume of an organ changes, the thickness and structure of the epithelium also changes.

Ontophylogenetic classification of epithelium:

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

1. Epidermal type of epithelium is formed from the ectoderm, has a multilayer or multirow structure, is adapted to perform primarily protective function.
2. Endodermal type of epithelium develops from the endoderm, is single-layered prismatic in structure, carries out the processes of absorption of substances, and performs a glandular function.
3. Coelonephrodermal type of epithelium develops from the mesoderm, single-layer, flat, cubic or prismatic in structure; performs a barrier or excretory function.
4. Ependymoglial type of epithelium represented by a special epithelium lining, for example, the cavities of the brain. The source of epithelial formation is the neural tube.
5. Angiodermal type of epithelium formed from mesenchyme, lining blood vessels from the inside.

Types of epithelium

Single layer epithelium

1. Single layer squamous epithelium(endothelium and mesothelium). The endothelium lines the inside of blood vessels, lymphatic vessels, and the cavities of the heart. Endothelial cells are flat, poor in organelles and form the endothelial layer. The metabolic function is well developed. They create conditions for blood flow. When the epithelium is damaged, blood clots form. The endothelium develops from mesenchyme. The second type - mesothelium - develops from mesoderm. Lines all serous membranes. Consists of flat polygonal cells connected to each other by uneven edges. Cells have one, rarely two, flattened nuclei. There are short microvilli on the apical surface. They have absorptive, excretory and delimiting functions. The mesothelium ensures the free sliding of internal organs relative to each other. The mesothelium secretes a mucous secretion onto its surface. The mesothelium prevents the formation of connective tissue adhesions. They regenerate quite well due to mitosis.
2. Single layer cuboidal epithelium develops from endoderm and mesoderm. On the apical surface there are microvilli that increase the working surface, and in the basal part the cytolemma forms deep folds, between which mitochondria are located in the cytoplasm, so the basal part of the cells looks striated. Lines the small excretory ducts of the pancreas, bile ducts and renal tubules.
3. Single layer columnar epithelium found in the organs of the middle part of the digestive canal, digestive glands, kidneys, gonads and genital tract. In this case, the structure and function are determined by its localization. Develops from endoderm and mesoderm. The gastric mucosa is lined with single-layer glandular epithelium. It produces and secretes a mucous secretion that spreads over the surface of the epithelium and protects the mucous membrane from damage. The cytolemma of the basal part also has small folds. Single-layer columnar epithelium has high regeneration.
4. The kidney tubules and intestinal mucosa are lined bordered epithelium. In the bordered epithelium of the intestine, border cells - enterocytes - predominate. At their top there are numerous microvilli. In this zone, parietal digestion and intensive absorption of food occur. Mucous goblet cells produce mucus on the surface of the epithelium, and small endocrine cells are located between the cells. They secrete hormones that provide local regulation.
5. Single layer multirow ciliated epithelium. It lines the airways and is of endodermal origin. In it, cells are of different heights, and the nuclei are located at different levels. The cells are arranged in a layer. Under the basement membrane lies loose connective tissue with blood vessels, and the epithelial layer is dominated by highly differentiated ciliated cells. They have a narrow base and a wide top. At the top there are flickering cilia. They are completely immersed in mucus. Between the ciliated cells are goblet cells - these are single-celled mucous glands. They produce a mucous secretion onto the surface of the epithelium. There are endocrine cells. Between them there are short and long intercalary cells; these are stem cells, poorly differentiated, due to which cell proliferation occurs. The ciliated cilia perform oscillatory movements and move the mucous film along the airways to the external environment.

Stratified epithelium

Multilayered squamous non-keratinizing epithelium.

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

Stratified squamous keratinizing epithelium- epidermis, it lines the skin. In thick skin (palm surfaces), which is constantly under stress, the epidermis contains 5 layers:

1. basal layer - contains stem cells, differentiated cylindrical and pigment cells (pigmentocytes).
2. stratum spinosum - polygonal cells containing tonofibrils.
3. granular layer - the cells acquire a rhomboid shape, the tonofibrils disintegrate and inside these cells the protein keratohyalin is formed in the form of grains, this is where the process of keratinization begins.
4. The stratum lucidum is a narrow layer, in which the cells become flat, they gradually lose their intracellular structure, and keratohyalin turns into eleidin.
5. stratum corneum - contains horny scales that have completely lost their cell structure and contain the protein keratin. With mechanical stress and deterioration of blood supply, the process of keratinization intensifies.

In thin skin that does not experience stress, there is no granular and shiny layer. Multilayer cubic and cylindrical epithelium are extremely rare - in the area of ​​the conjunctiva of the eye and the area of ​​​​the junction of the rectum between single-layer and multilayer epithelium.

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

Glandular epithelium- a type of epithelial tissue that consists of epithelial glandular cells, which in the process of evolution acquired the leading property of producing and secreting secretions. Such cells are called secretory (glandular) - glandulocytes. They have exactly the same general characteristics How covering epithelium. Located in the glands of the skin, intestines, salivary glands, glands internal secretion etc. Among the epithelial cells there are secretory cells, there are 2 types of them:

• exocrine - release their secretion into the external environment or the lumen of an organ;
• endocrine - release their secretions directly into the bloodstream.

Characteristic features of the epithelium

There are five main characteristic features of the epithelium:

Epithelia are layers (less often cords) of cells - epithelial cells. There is almost no intercellular substance between them, and the cells are closely connected to each other through various contacts.

Epithelia are located on basement membranes that separate epithelial cells from the underlying connective tissue.

The epithelium has polarity. Two sections of cells - basal (lying at the base) and apical (apical) - have different structure.

The epithelium does not contain blood vessels. Epithelial cells are nourished diffusely through the basement membrane from the side of the underlying connective tissue.

Epithelia have a high ability to regenerate. Epithelial restoration occurs due to mitotic division and differentiation of stem cells.

Epithelial tissue is also called border or integumentary tissue, since it is located mainly in those places of the body where it comes into contact with the external environment, with the contents of organs, gland secretions, etc.

Epithelial tissue arises from early stages(15-day embryo) embryonic development. All three germ layers (ectoderm, mesoderm and endoderm) take part in its formation.

Epithelial tissue is characterized by a number of specific features. It is dominated by cells, with almost no intercellular substance. The cells are arranged in the form of layers, closely touching each other with their surfaces or cytoplasmic protrusions in the form of bridges that extend into the recesses of neighboring cells. Moreover, they are characterized by polarity - differences in the structure of the proximal and distal sections. The cells are located on a thin plate - the basement membrane, under which there is necessarily a layer of loose fibrous connective tissue. Through this membrane, nutrients enter and metabolic products are removed; it prevents the growth of epithelial cells into the depths of the underlying tissues. Epithelial tissue does not have blood vessels.

This fabric has a high ability to regenerate. If integrity is damaged, it is easily restored.

Epithelial tissue performs protective, metabolic and secretory functions. Protective function is that the epithelium protects all tissues located underneath it from mechanical, chemical, and thermal influences. Thus, most microorganisms do not penetrate through intact skin.

Exchange function is that through the cells of epithelial tissue, the exchange of substances between the body and environment, for example, the absorption of nutrients from the intestines into the blood and lymph, the transfer of oxygen from the lungs to the blood, the release of metabolic products from the kidneys, etc.

Secretory function determined by the ability of individual cells to produce specific substances that are important for the body. Thus, mucus produced by epithelial cells of the stomach protects its wall from exposure gastric juice, enzymes of the cells of the gastrointestinal tract are involved in the digestive process, hormones - substances of the endocrine glands - regulate metabolic processes, growth and development of the body (this epithelium is called glandular, since most glands are formed from it).

Depending on the origin and function, epithelial tissue has a different structure. Several classifications of epithelium have been proposed. According to its functional significance, it is divided into integumentary and glandular.

Covering epithelium. It, in turn, can be divided according to the number of layers into single-layer and multilayer, and according to the shape of the cells - into flat, cubic and cylindrical.

Single layer squamous epithelium also called mesothelium. Its cells are flat, when viewed from the surface they have an irregular shape, the boundaries between them are clearly defined, the nucleus is usually located in the middle of the cell. Some cells have 2 or even 3 nuclei. On the free surface of single-layer squamous epithelial cells there are microvilli. The mesothelium covers the serous membranes, peritoneum, pleura, and pericardial sac, which creates an even, smooth surface of the organs, prevents their fusion and ensures free sliding. Violation of its integrity can lead to adhesions, which limit the mobility of organs and lead to changes in their functions (Fig. 3).

Rice. 3. Single-layer squamous epithelium (top view): 1 - cell border; 2 - cell nuclei; 3 - blood vessel under the epithelium

Single layer cuboidal epithelium has cells with the same dimensions in all dimensions, resembling a cube in shape. The nuclei are located in the center of the cell. Such epithelium is found in the kidney tubules (Fig. 4).

Rice. 4. Single-layer cubic and single-layer cylindrical epithelium (kidney tubules): 1 - single-layer cubic epithelium (nuclei - in the center of the cell); 2 - single-layer cylindrical epithelium (nuclei are located closer to the base of the cell); 3 - connective tissue; 4 - blood vessels

Single layer columnar epithelium contains cells that are elongated, cylindrical in shape, with nuclei lying closer to their base. On the free surface of the cylindrical epithelium there are many microvilli, which in the intestine form the so-called suction border. Single-layer columnar epithelium covers the stomach, small and large intestines, ducts of the liver and pancreas, and kidney tubules. Among the columnar epithelial cells in the intestine there are many goblet cells that secrete mucus.

A type of single-layer cylindrical epithelium is ciliated, or ciliated, epithelium, a characteristic feature of which is the presence on the free surface of outgrowths from the cytoplasm - cilia, which are in continuous movement. Each cell can have up to 250 cilia. In 1 second, the eyelash makes 16-17 vibrations. This epithelium covers the fallopian tubes and respiratory tract. Flicker of eyelashes fallopian tubes ah promotes the advancement of the egg, and in respiratory tract- pushing dust particles into the external environment.

Stratified squamous epithelium There are three types: keratinizing, non-keratinizing and transitional.

Multilayer flat keratinizing The epithelium covers the surface of the skin, forming the so-called epidermis, which has several dozen layers of cells. In this case, the cells of the germinal layer (deeply lying) continuously multiply and have a cylindrical shape. The closer they are to the surface, the flatter they become. A specific process occurs in the cells of this epithelium - the process of keratinization, which consists in the fact that their cytoplasm becomes denser as it approaches the surface of the body, the nucleus disappears and the cell dies. Connecting with neighboring cells, it forms horny scales, which are rejected from the surface of the skin. In the deep layers of cells of multilayered squamous epithelium, a pigment can be formed - a coloring substance that creates a kind of impenetrable screen for ultraviolet rays, protecting the tissues located underneath from their adverse effects.

Multilayer flat non-keratinizing the epithelium covers the cornea of ​​the eye, the mucous membrane of the oral cavity, pharynx and esophagus. The cells of this epithelium are not susceptible to keratinization. After a certain cycle, the flat cells of the upper layers die and are rejected from the surface. Thus, it has been established that under normal conditions, over 500 thousand epithelial cells fall off the oral mucosa every 5 minutes (Fig. 5).

Rice. 5. Multilayered squamous keratinizing epithelium: 1 - epithelium (a - basal layer (germ), b - layer of spinous cells (germ), c - granular layer, d - stratum lucidum, d - stratum corneum); 2 - connective tissue

Multilayer flat transitional epithelium covers the inner surface of organs that dramatically change their volume. It lines the renal calyces, renal pelvis, and bladder. When the organ is stretched, the epithelium becomes almost flat; when it collapses, it turns into cubic and even cylindrical.

Glandular epithelium. Glandular epithelial cells are capable of synthesizing and secreting specific products - secretions (juices). The glandular epithelium received its name due to the fact that glands are formed from it, most of which are independent organs (salivary glands, pancreas, thyroid and etc.). Based on the number of secreted cells, unicellular and multicellular glands are distinguished. The latter, in turn, are divided according to their structure into simple and complex, according to their shape - into tubular, alveolar and alveolar-tubular, and according to the method of secretion - into exocrine glands and endocrine glands. Each complex gland is an organ with a specific structure.

Example unicellular glands are the goblet cells of the gastric mucosa. A simple alveolar gland has the shape of a vesicle ( sebaceous glands skin), and simple tubular - the shape of a tube (sweat glands).

Complex alveolar or scarred glands have numerous branches in the form of vesicles or tubes. For example, the parotid salivary gland belongs to the complex alveolar glands, and the submandibular gland belongs to the alveolar-tubular glands.

Exocrine glands are characterized by the fact that the secretion they produce is released through a duct into the organ cavity or onto the surface of the skin (liver, stomach glands, skin).

Endocrine glands ( endocrine glands) do not have ducts, and their secretion, otherwise called hormone or hormone, is released directly into the blood.

Epithelial tissues, or Epithelia (from Greek epi - above and thele - nipple, thin skin) - Border tissues Which are located on the border with the external environment, cover the surface of the body, line its cavities, the mucous membranes of the internal organs and form most of the glands. Distinguish Three types of epithelia:

1) Integumentary epithelia (form various linings),

2) Glandular epithelia (form glands)

3) Sensory epithelia (perform receptor functions and are part of the sense organs).

Functions of epithelium:

1 Demarcation, barrier - The main function of epithelium, all others are its partial manifestations. Epithelia form barriers between the internal environment of the body and the external environment; the properties of these barriers (mechanical strength, thickness, permeability, etc.) are determined by the specific structural and functional characteristics of each epithelium. A few exceptions to the general rule are epithelia, which delimit two areas of the internal environment - for example, lining body cavities (mesothelium) or blood vessels (endothelium).

2 Protective - Epithelia provide protection to the internal environment of the body from the damaging effects of mechanical, physical (temperature, radiation), chemical and microbial factors. The protective function can be expressed in different ways (for example, epithelia can form thick layers, form an outer low-permeable, physically and chemically stable stratum corneum, secrete a protective layer of mucus, produce substances with an antimicrobial effect, etc.).

3 Transport - May manifest as substance transfer Through Sheets of epithelial cells (for example, from the blood through the endothelium of small vessels into surrounding tissues) or On their surface(for example, mucus transport by the ciliated epithelium of the respiratory tract or ovitis by the ciliated epithelium of the fallopian tube). Substances can be transported across the epithelial layer by mechanisms of diffusion, transport mediated by carrier proteins, and vesicular transport.

ABOUT Suction- many epithelia actively absorb substances; The most striking examples are the epithelium of the intestine and renal tubules. This function is essentially a special version of the transport function.

© Secretory - Epithelia are the functional leading tissues of most glands.

© excretory - Epithelia are involved in removing from the body (with urine, sweat, bile, etc.) end products of metabolism or (exogenous) compounds introduced into the body (for example, drugs).

ABOUT Sensory (receptive) - Epithelia, being on the border of the internal environment of the body and the external environment, perceive signals (mechanical, chemical) emanating from the latter.

General morphological features Eliteliev include:

J) Arrangement of cells (epithelial cells) in closed layers, Which form Planar linings, Roll up into Tubes Or form Bubbles (follicles); This feature of epithelia is determined by signs (2) and (3);

2) The minimum amount of intercellular substance, Narrow intercellular spaces;

3) The presence of developed intercellular connections, Which determine the strong connection of epithelial cells with each other in a single layer;

4) Border position (usually between the tissues of the internal environment and the external environment);

5) Cell polarity- As a consequence of sign (4). In epithelial cells there are Apical pole(from the Greek apex - top), free, directed to the external environment, and basal pole, Facing the tissues of the internal environment and associated with Basal membrane. Multilayered epithelia are characterized by Vertical anisomorphy(from the Greek an - negation, iso - identical, morphe - form) - unequal morphological properties of cells of different layers of the epithelial layer;

6) Location on the basement membrane - a special structural formation (see structure below), which is located between the epithelium and the underlying loose fibrous connective tissue;

7) Absence Vessels; Nutrition of the epithelium is carried out by Diffusion of substances through the basement membrane from connective tissue vessels. Various distances of individual layers of multilayered epithelia from the nutritional source probably enhance (or maintain) their vertical anisomorphy;

8) High regeneration ability- Physiological and reparative - carried out thanks to Cambia(including stem and semi-stem cells) and is due to the borderline position of the epithelium (determining a significant need for active renewal of rapidly deteriorating epithelial cells). Cambial elements in some epithelia are concentrated in certain areas of them (localized cambium), In others, they are evenly distributed among other cells (diffuse cambium).

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

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

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

Surface epithelium differs from other tissues in six main ways:

1) 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 the following functions: barrier, trophic, metabolic, anti-invasive, morphogenetic; attaches a layer of epithelium to itself; 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 connected through intercellular contacts:

a) dense (zonula accludens),

b) toothed or finger-shaped (junctio intercellularis denticulatae),

c) desmosomes (desmosoma), etc.;

4) absence of blood vessels, since the epithelium is supplied from 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 the opposite direction, which is explained by the border position of the tissue; in the cytolemma of the basal part of the cell there is sometimes 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) integumentary epithelial tissue has a high ability to regenerate.

Classification of epithelial surface tissues. Epithelial surface tissues are classified according to 2 criteria:

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

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

Morphological classification. The surface epithelium is divided into single-layer and multilayer.

Single layer epithelium in turn, they are divided into single-row and multi-row, or pseudo-multilayer. Single row epithelium divided into flat, cubic and prismatic, or columnar. Multirow epithelium always prismatic.

Stratified epithelium are divided 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 of 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. Single-layer epithelium differs from multilayered epithelium in that all its cells are located on the basement membrane, while While in multilayered epithelium, only one basal layer of cells is connected to 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, there are 5 types of epithelial tissues:

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) coelonephrodermal epithelium - develops from the mesoderm and lines the pleura, peritoneum, pericardium, and 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 mesenchyme, lines the chambers of the heart, blood and lymphatic vessels.

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

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

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

Mesothelium(mesothelium) develops from the leaves of the splanchnotome, lining the peritoneum, pleura, and pericardium. Mesotheliocyte cells have a flattened irregular shape, the edges of the cells are indented; cells contain one, sometimes several flattened nuclei, the cytoplasm is poor in organelles of general importance, it contains pinocytotic vesicles, indicating the metabolic function; on the luminal surface there are microvilli that increase the surface of the cells. The function of the mesothelium is to provide a smooth surface to the serous membranes. This facilitates the sliding of organs in the abdominal, thoracic and other cavities; through the mesothelium, substances are exchanged between the serous cavities and the underlying connective tissue of their walls. The mesothelium secretes fluid contained in these cavities. When the mesothelium is damaged, adhesions can form between the serous membranes, impeding the movement of organs.

Single layer cuboidal epithelium(epithelium cuboideum simplex) is present in the renal tubules and 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. On the apical surface there are 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 folds of the cytolemma, between which 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 basal striations indicates the reabsorption (reverse absorption) of water. The source of development of the renal epithelium is the mesoderm, or more precisely, nephrogenic tissue.

Columnar epithelium(epithelium columnare) is located in the small and large intestines 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 ER, Golgi complex and mitochondria are well developed; in the apical part there are secretory granules containing mucous secretion. Thus, the surface epithelium of the gastric mucosa is glandular. Therefore its functions:

1) secretory, i.e. the production of mucous secretion that envelops the gastric mucosa;

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

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

Columnar (marginal) epithelium of the small and large intestines(epithelium columnare cum limbus striatus) lines the mucous membrane of the small and large intestines, develops from the intestinal endoderm; characterized by having a prismatic shape. The cells of this epithelium are connected to each other using tight junctions, or endplates, i.e., 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 ​​the lateral surfaces of these cells, closer to their base, there are desmosomes, finger-like, or jagged, contacts. On the apical surface of columnar epitheliodites 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 a suction, or striated, border (limbus striatus). Functions of the bordered epithelium: 1) parietal digestion; 2) absorption of breakdown products. Thus, a sign confirming the absorptive function of this epithelium is: 1) the presence of an absorptive border and 2) single-layeredness.

The epithelium of the small and large intestines 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 cells), lacking 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 granules.

Pseudostratified (multi-row) epithelium(epithelium pseudostratificatum) is single-layered, since all its cells lie on the basement membrane. Why then is this epithelium called multirow? Because its cells have different shapes and sizes, and, therefore, their nuclei are located at different levels and form rows. The cores of the most small cells(basal, or short intercalary) are located closer to the basal membrane, the nuclei of medium-sized cells (long intercalary) are localized higher, the nuclei of the tallest cells (ciliated) are furthest from the basal membrane. Multirow 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 multirow epithelium there are 4 types of cells:

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 tallest cells of the pseudostratified epithelium of the mucous membrane of the respiratory tract. The nuclei of these cells are oval in shape and, as already mentioned, are furthest from the basement membrane. Their cytoplasm contains 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 body (modified centriole). The cilia, carrying out oscillatory movements directed against the inhaled air, remove dust particles deposited 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 multirow.

Small intercalary cells respiratory tract - the smallest, have triangular shape, with a wide basal end lying on the basement membrane. 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 intercalary cells higher than the small intercalary ones, but their apical part does not reach the surface of the epithelium.

Goblet cells(exocrinocytus caliciformis) are glandular cells (single-celled glands). Until the moment these cells have time to accumulate secretions, they have a prismatic shape. Their cytoplasm has a flattened nucleus, smooth ER, the ILGI complex and mitochondria are well developed. Granules of mucous secretion accumulate in their apical part. As these granules accumulate, the apical part of the cell expands and the cell takes on the appearance of a glass, which is why it is called goblet. The function of goblet cells is to secrete a mucous secretion, which, enveloping the mucous membrane of the trachea and bronchi, protects it from chemical and physical influences.

Endocrinocytes As part of the multirow 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.

Chapter 6. EPITHELIAL TISSUE

Chapter 6. EPITHELIAL TISSUE

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

6.1. GENERAL MORPHOLOGICAL CHARACTERISTICS AND CLASSIFICATIONS

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Scheme 6.1. Morphological classification of types of surface epithelium

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

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

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

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

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

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

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

6.1.1. Single layer epithelia

Single row epithelia

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

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

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

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

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

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

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

Single-layer columnar (prismatic) epithelium(epithelium simplex columnare). This type of epithelium is characteristic of the middle section 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 connected to each other using desmosomes, gap communication junctions, lock-type junctions, and tight junctions (see Chapter 4). Thanks to the latter, the contents of the stomach, intestines and other hollow organs cannot penetrate into the intercellular gaps of the epithelium.

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

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

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

Multirow epithelia

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

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

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

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

6.1.2. Stratified epithelia

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

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

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

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

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

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

Rice. 6.6. Stratified squamous keratinizing epithelium:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

6.2. Glandular epithelia

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

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

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

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

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

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

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

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

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

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

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

Holocrine type secretion is accompanied by the accumulation of secretion (fat) in the cytoplasm and 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 through intracellular regeneration (with mero- and apocrine secretion), or with the help of cellular regeneration, i.e., division and differentiation of cambial cells (with holocrine secretion).

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

Glands

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

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

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

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

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

Scheme 6.2. Morphological classification of exocrine glands

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

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

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

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

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

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

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

Rice. 6.11. Types of exocrine glands:

1 - simple tubular glands with unbranched end sections;

2 - simple alveolar gland with an unbranched end section;

3 - simple tubular glands with branched end sections;

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

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

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

Control questions

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

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

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

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