The epithelium of the internal cavities of the body is derived. General characteristics and classification of epithelia
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; The epithelium is nourished 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).
Characteristic morphological features of epithelial tissues
Epithelial tissues are a collection of polarly differentiated cells, closely adjacent to each other, located in the form of a layer on the basement membrane; they lack blood vessels and have very little or no intercellular substance.
Functions. Epithelia cover the surface of the body, secondary body cavities, the internal and external surfaces of hollow internal organs, and form the secretory sections and excretory ducts of the exocrine glands. Their main functions: delimiting, protective, suction, secretory, excretory.
Histogenesis. Epithelial tissues develop from all three germ layers. Epithelia of ectodermal origin are predominantly multilayered, while those developing from endoderm are always single-layered. Both single-layer and multilayer epithelia develop from the mesoderm.
Classification of epithelial tissues
1. Morphofunctional classification takes into account the structural features and functions performed by one or another type of epithelium.
Based on their structure, epithelia are divided into single-layer and multilayer. The main principle of this classification is the ratio of cells to the basement membrane (Table 1). The functional specificity of single-layer epithelia is usually determined by the presence of specialized organelles. For example, in the stomach the epithelium is single-layered, prismatic, single-row glandular. The first three definitions characterize structural features, and the last indicates that gastric epithelial cells perform a secretory function. In the intestine, the epithelium is single-layered, prismatic, single-row, bordered. The presence of a brush border in epithelial cells suggests an absorptive function. In the airways, in particular in the trachea, the epithelium is single-layered, prismatic, multirow ciliated (or ciliated). It is known that cilia in this case play a protective function. Multilayer epithelia perform protective and glandular functions.
Table 1. Comparative characteristics of single-layer and multilayer epithelia.
|
SINGLE LAYER EPITHELIAS |
MULTILAYERED EPITHELIA |
|
All epithelial cells are in contact with the basement membrane: |
Not all epithelial cells are in contact with the basement membrane: |
|
1) single-layer flat; 2) single-layer cubic (low prismatic); 3) single-layer prismatic (cylindrical, columnar) Happens: |
1) multilayer flat non-keratinizing contains three layers of different cells: basal, intermediate (spinous) and superficial; 5 layers: basal, spinous, granular, shiny and horny; The basal and spinous layers constitute the germinal layer of the epithelium, since the cells of these layers are capable of division. |
Ontophylogenetic classification (according to N. G. Khlopin). This classification takes into account from which embryonic rudiment a particular epithelium developed. According to this classification, epidermal (skin), enterodermal (intestinal), coelonephrodermal, ependymoglial and angiodermal types of epithelium are distinguished.
For example, cutaneous epithelium covers the skin, lines the oral cavity, the esophagus, the glandular chambers of the multichamber stomach, the vagina, the urethra, and the border section of the anal canal; intestinal-type epithelium lines the single-chamber stomach, abomasum, and intestines; epithelium of the coelonephrodermal type lines the body cavities (mesothelium of the serous membranes), forms renal tubules; ependymoglial type of epithelium lines the ventricles of the brain and the central canal of the spinal cord; angiodermal epithelium lines the cavities of the heart and blood vessels.
Single-layer and multilayer epithelia are characterized by the presence of special organelles - desmosomes, hemidesmosomes, tonofilaments and tonofibrils. In addition, single-layer epithelia may have cilia and microvilli on the free surface of cells (see section “Cytology”).
All types of epithelia are located on the basement membrane (Fig. 7). The basement membrane consists of fibrillar structures and an amorphous matrix containing complex proteins - glycoproteins, proteoglycans and polysaccharides (glycosaminoglycans).
Rice. 7. Scheme of the structure of the basement membrane (according to Yu. K. Kotovsky).
BM – basement membrane; WITH - Light plate; T – dark plate. 1 – cytoplasm of epithelial cells; 2 – core; 3 – hemidesmosomes; 4 – keratin tonofilaments; 5 – anchor filaments; 6 – plasmalemma of epithelial cells; 7 – anchoring filaments; 8 – loose connective tissue; 9 - Hemocapillary.
The basement membrane regulates the permeability of substances (barrier and trophic function) and prevents invasion of the epithelium into the connective tissue. The glycoproteins it contains (fibronectin and laminin) promote the adhesion of epithelial cells to the membrane and induce their proliferation and differentiation during the regeneration process.
By location and function of the epithelium are divided into: superficial (cover organs from the outside and inside) and glandular (form the secretory sections and excretory ducts of the exocrine glands).
Surface epithelia are border tissues that separate the body from the external environment and participate in the exchange of substances and energy between the body and the external environment. They are located on the surface of the body (integumentary), the mucous membranes of internal organs (stomach, intestines, lungs, heart, etc.) and secondary cavities (lining).
Glandular epithelia have pronounced secretory activity. Glandular cells - glandulocytes are characterized by a polar arrangement of organelles of general importance, well-developed ER and Golgi complex, and the presence of secretory granules in the cytoplasm.
The process of functional activity of a glandular cell associated with the formation, accumulation and release of secretions beyond its boundaries, as well as restoration of the cell after secretion is released, is called Secretory cycle.
During the secretory cycle, initial products (water, various inorganic substances and low molecular weight organic compounds: amino acids, monosaccharides, fatty acids, etc.) enter the glandulocytes from the blood, from which, with the participation of organelles of general importance, a secret is synthesized and accumulated in the cells, and then by exocytosis is released into the external ( Exocrine glands ) or internal ( Endocrine glands ) Wednesday.
Secretion is released (extrusion) by diffusion or in the form of granules, but can also be by converting the entire cell into a common secretory mass.
Regulation of the secretory cycle is carried out with the participation of humoral and nervous mechanisms.
Epithelial regeneration
Various types of epithelia are characterized by high regenerative activity. It is carried out due to the cambial elements, which divide by mitosis, constantly replenishing the loss of worn-out cells. Glandular cells, which secrete according to the merocrine and apocrine type, are, in addition, capable of maintaining their vital functions not only through reproduction, but also due to intracellular regeneration. In holocrine glands, constantly dying glandulocytes are replaced during the secretory cycle due to the division of stem cells located on the basement membrane (cellular regeneration).
Now that we've sorted it out, it's time to move on to the next large group - epithelial ones. There are different types of epithelial tissues To make it easier to navigate through them, we present below diagram 2. This diagram has already been given in the general characteristics of epithelial tissues.
Single layer epithelium are divided into two groups: not all epithelial cells are of the same “growth”, that is, their nuclei are located in a row (single-row single-layer), or there are “undergrowths” and “overgrowths”, the nuclei of which are not at the same level, but on different ones (multi-row single-layer ).
Single row epithelium(Fig. 17), depending on the shape, can be flat (vessels and heart are lined with endothelium, serous membranes have a mesothelial lining, part of the renal nephron is built by flat epithelial cells, and so on), cubic (renal tubules) and cylindrical, or prismatic. 
Multirow epithelium(Fig. 18) lines the respiratory tract. All epithelial cells are in contact with the basement membrane. To make it easier for you to understand, imagine a very crowded street. People scurry past each other: some to work, some from work, some on a date, some - wherever they look. You are standing on the steps at the entrance to a large supermarket and looking down at the crowd a little from above. Do you see everyone passing by? Hardly. Teenagers 12-14 years old may not be noticed by you, and little children, led by their mothers, will probably remain out of your sight, although everyone, regardless of age, steps on the same asphalt. So it is with multirow epithelium. The longest epithelial cells are visible on the outside, while the short and medium ones are obscured. The nuclei of all cells form 3 rows (hence the name). Those cells that, like pine trees in the forest, “reached the sun” and look into the lumen of the cavity (bronchus, for example), have special cilia that constantly perform oscillatory movements. Therefore, multirow single-layer epithelium is also called ciliated epithelium. 
Another feature that is present when comparing ciliated and columnar epithelial cells is the location of the so-called goblet cells. They secrete mucus that coats the cells, thereby protecting them from chemical and mechanical damage. Actually, it is to the goblet cells (along with small glands) that the mucous membranes owe their name.
IN stratified epithelium not all cells border the basement membrane. Continuing the proposed analogy, let us assume that some mothers, out of fear that the child would be run over by passers-by, took the babies in their arms, and some exemplary fathers, demonstrating to mothers their involvement in caring for their offspring, placed their only-begotten children on their shoulders. In other words, the connection between children's sandals, shoes, sneakers and the asphalt skin of the earth was severed.
As can be seen from Diagram 2, there are three type of stratified epithelium. There are so many layers of cells in each of them that you can lose count. The keratinizing epithelium (Fig. 19) forms the most superficial layer of the skin - the epidermis (the same one that slides off an overzealous tanner). Note that the upper layer of this type of epithelium, having successively gone through all stages of aging, is represented by dead cells that gradually exfoliate. The non-keratinizing epithelium (Fig. 20), located on the mucous membranes of the esophagus, mouth and cornea of the eye, in all its layers, including the most superficial, contains cells that may differ from each other in shape, size and ability to divide (picture I). 
Picture I. stratified non-keratinizing epithelium
Transitional epithelium(Fig. 21) stands apart. It is the only one that is not static and is capable of changing the thickness of its own layer; a similar property is manifested in the transitional epithelium, depending on the circumstances. When the bladder is empty, the layer of transitional epithelium lining it from the inside is quite thick (A), but when urine stretches the bladder, the epithelial lining thins (B). This type of epithelium (picture II) also occurs in the renal pelvis and ureters. 
Picture II. Transitional epithelium
Glandular epithelium, as already indicated, plays the role of bricks for building glands. Its main function is the production of certain substances. Production, or rather separation, is translated into Latin as secretion (secretio), but what is “separated” is therefore a secret. Glands located in the skin and walls of hollow organs, as a rule, have excretory ducts that carry secretions either outside (sweat, earwax, milk) or into the organ cavity (tracheal mucus, saliva, gastrointestinal enzymes) and are called exocrine glands. If the gland does not have ducts for removing secretions and what it produces goes directly into the blood of the capillaries surrounding it and is carried by the bloodstream, then they speak of an endocrine gland. When the secretion of such a gland affects the functioning of individual systems of the body or the entire body, it is called a hormone (oxytocin, thyroxine, adrenaline, insulin and many others). When it can “intervene” only in the environment and do things within a radius of a few millimeters to 2-4 cm, it is called a mediator (heparin, histamine, as well as serotonin, prostaglandins, quinines, etc., already known to you). However, in cases where the mediator is secreted not by one glandular cell, not by three, but by hundreds of glands, then its effect will no longer be local at all.
Glands can be multicellular, for example, mucous or sweat, and even form entire organs (pituitary gland, adrenal gland, pancreas). But they can be represented by only one cell, because what is a goblet cell if not a unicellular gland. The principle of secretion is the same for any gland. First, they accumulate the necessary substances that enter through the basement membrane from the blood. Then they form their own secret from the resulting components. Next, the elimination stage begins, and not in all glands it is “painless”. For example, the cells that “produce” saliva do not suffer from this at all, while the cells of the mammary glands, along with their tasty secretion, lose part of the cytoplasm, and the epithelial cells that synthesize sebum are completely destroyed. Finally, the fourth phase of secretion involves “licking the wounds” and restoring the original state of the glandular cells.
Exocrine glands may have some structural features that serve as the basis for their simple classification. They are divided into simple (Fig. 22) and complex (Fig. 23) according to the way their excretory duct branches. And the terminal sections can have a tubular or sac-like (alveolar) shape, and they can also branch. Ultimately, there are many variations. Exocrine glands can be distinguished as simple tubular unbranched (1) and branched (3), simple alveolar unbranched (2) and branched (4), and can be complex tubular and/or complex alveolar (5). 
Single layer epithelium
When describing single-layer single-row epithelium, the term “single-row” is most often omitted. Depending on the shape of the cells (epithelial cells), they are distinguished:
- Flat single-layer epithelium;
- Cuboidal single-layer epithelium;
- Cylindrical or prismatic single-layer epithelium.
Single layer squamous epithelium, or mesothelium, lines the pleura, peritoneum and pericardium, prevents the formation of adhesions between the organs of the abdominal and thoracic cavities. When viewed from above, mesothelial cells have a polygonal shape and uneven edges; in cross sections they are flat. The number of cores in them ranges from one to three.
Binuclear cells are formed as a result of incomplete amitosis and mitosis. Using electron microscopy, it is possible to detect the presence of microvilli at the top of the cells, which significantly increases the surface of the mesothelium. During a pathological process, for example pleurisy, pericarditis, intense release of fluid into the body cavity can occur through the mesothelium. When the serous membrane is damaged, the mesothelial cells contract, move away from each other, become rounded and easily separate from the basement membrane.
Lines the tubules of the nephrons of the kidneys, small branches of the excretory ducts of many glands (liver, pancreas, etc.). Cuboidal epithelial cells are most often approximately the same in height and width. In the center of the cell there is a rounded nucleus.
It lines the cavity of the stomach, small and large intestines, gall bladder, excretory ducts of the liver and pancreas, and also forms the walls of some nephron tubules, etc. It is a layer of cylindrical cells located on the basement membrane in one layer. The height of epithelial cells is greater than their width, and they all have the same shape, so their nuclei lie at the same level, in one row.
In organs where absorption processes are constantly and intensely occurring (digestive canal, gall bladder), epithelial cells have an absorptive border, which consists of a large number of well-developed microvilli. These cells are called bordered. The border also contains enzymes that break down complex substances into simple compounds that can penetrate the cytolemma (cell membrane).
A feature of the single-layer columnar epithelium lining the stomach is the ability of the cells to secrete mucus. This epithelium is called mucous. The mucus produced by the epithelium protects the gastric mucosa from mechanical, chemical and thermal damage.
Single-layer multirow ciliated columnar epithelium, characterized by the presence of ciliated cilia, lines the nasal cavity, trachea, bronchi, and fallopian tubes. The movement of cilia, along with other factors, contributes to the movement of eggs in the fallopian tubes, and in the bronchi - dust particles from exhaled air into the nasal cavity.
Goblet cells. In the single-layer cylindrical epithelium of the small and large intestines, there are cells that have the shape of a glass and secrete mucus, which protects the epithelium from mechanical and chemical influences.
Stratified epithelium
Stratified epithelium there are three types:
- keratinizing;
- Non-keratinizing;
- Transition.
The epithelium of the first two types covers the skin, cornea, lines the oral cavity, esophagus, vagina and part of the urethra; transitional epithelium - renal pelvis, ureters, bladder.
Epithelial regeneration
The integumentary epithelium is constantly exposed to the external environment. Through it, intensive metabolism occurs between the body and the environment. Therefore, epithelial cells die quickly. It is estimated that more than 5-10 5 epithelial cells are exfoliated from the surface of the oral mucosa of a healthy person every 5 minutes.
Epithelial restoration occurs due to mitosis of epithelial cells. Most cells of single-layer epithelium are capable of division, and in multilayer epithelium only cells of the basal and partly spinous layers have this ability.
Reparative regeneration of the epithelium occurs through intensive proliferation of cells at the edges of the wound, which gradually move towards the site of the defect. Subsequently, as a result of the continuous proliferation of cells, the thickness of the epithelial layer in the wound area increases and, at the same time, maturation and differentiation of cells take place in it, acquiring a structure characteristic of cells of this type of epithelium. The condition of the underlying connective tissue is of great importance for the processes of epithelial regeneration. Epithelization of the wound occurs only after it is filled with young connective (granulation) tissue rich in blood vessels.
Glandular epithelium
The glandular epithelium consists of glandular, or secretory, cells - glandulocytes. These cells synthesize and secrete specific products (secrets) onto the surface of the skin, mucous membranes and in the cavities of internal organs or into the blood and lymph.
Glands in the human body perform a secretory function, being either independent organs (pancreas, thyroid, large salivary glands, etc.) or their elements (glands of the fundus of the stomach). Most glands are derivatives of the epithelium, and only a few are of a different origin (for example, the adrenal medulla develops from nervous tissue).
By structure they distinguish simple(with a non-branching excretory duct) and complex(with a branched excretory duct) glands and by function - the glands of internal secretion, or endocrine, and external secretion, or exocrine.
Endocrine glands include pituitary gland, pineal gland, thyroid, parathyroid, thymus, gonads, adrenal glands and pancreatic islets. Exocrine glands produce a secretion that is released into the external environment - onto the surface of the skin or into cavities lined with epithelium (stomach cavity, intestines, etc.). They participate in performing the functions of the organ of which they are an element (for example, the glands of the alimentary canal are involved in digestion). Exocrine glands differ from each other in location, structure, type of secretion and composition of the secretion.
Most exocrine glands are multicellular formations, with the exception of goblet cells (the only type of unicellular exocrine glands in the human body). Goblet cells are located inside the epithelial layer and produce and secrete mucus onto the surface of the epithelium, which protects it from damage. These cells have an expanded apex, in which secretions accumulate, and a narrow base with a nucleus and organelles. The remaining exocrine glands are multicellular exoepithelial (located outside the epithelial layer) formations, in which a secretory, or terminal, section and an excretory duct are distinguished.
Secretory department consists of secretory, or glandular, cells that produce secretions.
In some glands, derivatives of multilayered epithelium, in addition to secretory ones, there are epithelial cells that can contract. By contracting, they compress the secretory department and thereby facilitate the release of secretions from it.
The cells of the secretory sections - glandulocytes - most often lie in one layer on the basement membrane, but can also be located in several layers, for example in the sebaceous gland. Their shape changes depending on the phase of secretion. The nuclei are usually large, irregular in shape, with large nucleoli.
In cells that produce protein secretions (for example, digestive enzymes), the granular endoplasmic reticulum is especially well developed, and in cells that produce lipids and steroids, the non-granular endoplasmic reticulum is better expressed. The lamellar complex is well developed and is directly related to secretion processes.
Numerous mitochondria are concentrated in places of greatest cell activity, i.e., where secretions accumulate. In the cytoplasm of glandular cells there are various types of inclusions: protein grains, drops of fat and lumps of glycogen. Their number depends on the phase of secretion. Intercellular secretory capillaries often pass between the lateral surfaces of cells. The cytolemma, which limits their lumen, forms numerous microvilli.
In many glands, polar differentiation of cells is clearly visible, due to the direction of secretory processes - the synthesis of secretion, its accumulation and release into the lumen of the terminal section proceeds in the direction from the base to the apex. In this regard, the nucleus and ergastoplasm are located at the bases of the cells, and the intracellular mesh apparatus lies at the apices.
In the formation of a secretion, several successive phases are distinguished:
- Absorption of products for secretion synthesis;
- Synthesis and accumulation of secretions;
- Secretion secretion and restoration of the structure of glandular cells.
The release of secretion occurs periodically, and therefore regular changes in the glandular cells are observed.
Depending on the method of secretion, merocrine, apocrine and holocrine types of secretion are distinguished.
With merocrine type of secretion(the most common in the body), glandulocytes completely retain their structure, the secretion leaves the cells into the gland cavity through holes in the cytolemma or by diffusion through the cytolemma without violating its integrity.
With apocrine type of secretion granulocytes are partially destroyed and the top of the cell is separated along with the secretion. This type of secretion is characteristic of the mammary and some sweat glands.
Holocrine type of secretion leads to the complete destruction of glandulocytes, which are part of the secretion along with the substances synthesized in them. In humans, only the sebaceous glands of the skin secrete according to the holocrine type. With this type of secretion, restoration of the structure of glandular cells occurs due to intensive reproduction and differentiation of special poorly differentiated cells.
The secretion of the exocrine glands can be proteinaceous, mucous, proteinaceous, sebaceous, and the corresponding glands are also called. In mixed glands there are two types of cells: some produce protein, others produce mucous secretion.
The excretory ducts of the exocrine glands consist of cells that do not have secretory ability. In some glands (salivary, sweat), cells of the excretory ducts can take part in secretion processes. In the glands that developed from multilayered epithelium, the walls of the excretory ducts are lined with multilayered epithelium, and in the glands that are derivatives of single-layered epithelium, they are lined with single-layered epithelium.
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 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 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 nuclei of the smallest 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, triangular in 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, 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.
