Single layer epithelium

When describing a single-layer unistratified epithelium, the term "unistratified" is most often omitted. Depending on the shape of the cells (epitheliocytes), there are:

• Flat single-layer epithelium;
• cuboidal epithelium;
• Cylindrical, or prismatic single-layer epithelium.

Single layered squamous epithelium, or mesothelium, lines the pleura, peritoneum and pericardium, prevents the formation of adhesions between the organs of the abdominal and chest cavities. When viewed from above, the mesothelial cells have a polygonal shape and uneven edges; they are flat in transverse sections. The number of cores in them ranges from one to three.

Binucleated 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. At pathological process, for example, pleurisy, pericarditis, through the mesothelium, an intensive release of fluid in the body cavity can occur. When the serous membrane is damaged, the mesothelial cells shrink, move away from each other, round and easily separate from the basement membrane.

It lines the tubules of the nephrons of the kidneys, small branches of the excretory ducts of many glands (liver, pancreas, etc.). In height and width, the cells of the cubic epithelium are most often approximately the same. In the center of the cell is a rounded nucleus.

Lines the cavity of the stomach, small and large intestines, gallbladder, excretory ducts of the liver and pancreas, and also forms the walls of some tubules of nephrons, etc. It is a layer of cylindrical cells located on the basement membrane in one layer. The height of epitheliocytes 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 intensively performed (alimentary canal, gallbladder), epithelial cells have a suction 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 cylindrical epithelium lining the stomach is the ability of cells to secrete mucus. Such an epithelium is called mucous. The mucus produced by the epithelium protects the gastric mucosa from mechanical, chemical and thermal damage.

Single-layer multi-row ciliated cylindrical epithelium is characterized by the presence of ciliated cilia, lines the nasal cavity, trachea, bronchi, fallopian tubes. The movement of cilia, along with other factors, contributes to the movement of eggs in the fallopian tubes, in the bronchi - dust particles from the 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 effects.

Stratified epithelium

Stratified epithelium there are three types:

• keratinizing;
• non-keratinizing;
• Transition.

The epithelium of the first two types covers the skin, cornea, and 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, an intensive exchange of substances between the body and the environment is carried out. Therefore, epithelial cells quickly die. It is estimated that only from the surface of the mucous membrane oral cavity in a healthy person, more than 5-10 5 epithelial cells are exfoliated every 5 minutes.

The restoration of the epithelium occurs due to mitosis of epithelial cells. Most of the cells of the single-layer epithelium are capable of dividing, and in the stratified epithelium, only cells of the basal and partly spiny layers have this ability.

Reparative regeneration of the epithelium occurs by intensive reproduction of the cells of the edges of the wound, which gradually move towards the site of the defect. Subsequently, as a result of the continuous reproduction of cells, the thickness of the epithelial layer in the wound area increases and, at the same time, maturation and differentiation of cells occur in it, acquiring a structure characteristic of cells of this type of epithelium. Of great importance for the processes of regeneration of the epithelium is the condition of the underlying connective tissue. Epithelialization of the wound occurs only after filling it with a young, rich in blood vessels, connective (granulation) tissue.

glandular epithelium

The glandular epithelium consists of glandular, or secretory, cells - glandulocytes. These cells synthesize and secrete specific products (secrets) on the surface of the skin, mucous membranes and in the cavity of internal organs or into the blood and lymph.

The 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 of them are of other origin (for example, the adrenal medulla develops from nervous tissue).

According to the structure, they are distinguished simple(with a non-branching excretory duct) and complex(with branched excretory duct) glands and by function - endocrine glands, or endocrine, and external secretion, or exocrine.

The endocrine glands are pituitary, pineal, thyroid, parathyroid, thymus, gonads, adrenals, and pancreatic islets. Exocrine glands produce a secret that is released into the external environment - on the surface of the skin or in cavities lined with epithelium (the cavity of the stomach, intestines, etc.). They participate in the performance of the function 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 secret.

Most exocrine glands are multicellular, with the exception of goblet cells (the only type of unicellular exocrine gland in the human body). Goblet cells are located inside the epithelial layer, produce and secrete mucus on the surface of the epithelium, which protects it from damage. These cells have an expanded apex, in which the secret accumulates, 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 a secret.

In some glands, derivatives of stratified epithelium, in addition to secretory epithelial cells are found that can contract. When contracting, they compress the secretory section and thereby facilitate the secretion from it.

Secretory cells - glandulocytes - most often lie in one layer on the basement membrane, but can also be located in several layers, for example, in sebaceous gland. Their shape changes depending on the phase of secretion. The nuclei are usually large, irregularly shaped, with large nucleoli.

In cells that produce a protein secret (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. A lamellar complex is well developed, which is directly related to the processes of secretion.

Numerous mitochondria are concentrated in places of the greatest cell activity, i.e., where the secret accumulates. In the cytoplasm of glandular cells there are various kinds of inclusions: protein grains, drops of fat and clumps of glycogen. Their number depends on the phase of secretion. Often intercellular secretory capillaries pass between the lateral surfaces of the cells. The cytolemma that limits their lumen forms numerous microvilli.

In many glands, the polar differentiation of cells is clearly visible, due to the direction of secretory processes - the synthesis of the secret, its accumulation and release into the lumen of the terminal section proceed 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 reticular apparatus lies at the tops.

In the formation of a secret, several successive phases are distinguished:

• Absorption of products for secretion synthesis;
• Synthesis and accumulation of a secret;
• Isolation of secretion and restoration of the structure of glandular cells.

The release of the secret occurs periodically, in connection with which regular changes in glandular cells are observed.

Depending on the method of secretion 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 secret leaves the cells into the cavity of the gland 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 together with the secret the top of the cell is separated. 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 secret along with the substances synthesized in them. In humans, according to the holocrine type, only the sebaceous glands of the skin secrete. With this type of secretion, the restoration of the structure of glandular cells occurs due to the intensive reproduction and differentiation of special poorly differentiated cells.

The secret of the exocrine glands can be proteinaceous, mucous, proteinaceous-mucous, sebaceous, the corresponding glands are also called. In the mixed glands there are two types of cells: some produce a protein, others - a mucous secret.

The excretory ducts of the exocrine glands consist of cells that do not have a secretory ability. In some glands (salivary, sweat), cells of the excretory ducts can take part in secretion processes. In glands that have developed from stratified epithelium, the walls of the excretory ducts are lined with stratified epithelium, and in glands that are derivatives of a single-layer epithelium, they are single-layered.

Epithelial tissue (synonymous with epithelium) is a tissue that lines the surface of the skin, the cornea of ​​the eye, serous membranes, the inner surface of the hollow organs of the digestive, respiratory and genitourinary systems, and also forms glands.

Epithelial tissue is characterized by high regenerative capacity. Different types epithelial tissue perform different functions and therefore have a different structure. So, the epithelial tissue, which performs mainly the functions of protection and delimitation from the external environment (skin epithelium), is always multilayered, and some of its types are equipped with a stratum corneum and participate in protein metabolism. Epithelial tissue, in which the function of external exchange is leading (intestinal epithelium), is always single-layered; it has microvilli (brush border), which increases the absorptive surface of the cell. This epithelium is also glandular, secreting a special secret necessary for the protection of the epithelial tissue and the chemical processing of substances penetrating through it. Renal and coelomic types of epithelial tissue perform the functions of absorption, formation of secrets,; they are also single-layered, one of them is equipped with a brush border, the other has pronounced depressions on the basal surface. In addition, some types of epithelial tissue have permanent narrow intercellular gaps (renal epithelium) or periodically occurring large intercellular openings - stomatomas (coelomic epithelium), which contributes to processes and absorption.

Epithelial tissue (epithelium, from Greek epi - on, over and thele - nipple) - border tissue lining the surface of the skin, cornea of ​​the eye, serous membranes, the inner surface of the hollow organs of the digestive, respiratory and genitourinary systems (stomach, trachea, uterus, etc. .). Most glands are of epithelial origin.

The boundary position of the epithelial tissue is due to its participation in metabolic processes: gas exchange through the epithelium of the alveoli of the lungs; absorption of nutrients from the intestinal lumen into the blood and lymph, excretion of urine through the epithelium of the kidneys, etc. In addition, the epithelial tissue also performs a protective function, protecting the underlying tissues from damaging effects.

Unlike other tissues, epithelial tissue develops from all three germ layers (see). From the ectoderm - the epithelium of the skin, oral cavity, most of the esophagus, the cornea of ​​​​the eye; from endoderm - epithelium gastrointestinal tract; from the mesoderm - the epithelium of the organs of the genitourinary system and serous membranes - the mesothelium. Epithelial tissue arises in the early stages of embryonic development. As part of the placenta, the epithelium is involved in the exchange between the mother and fetus. Taking into account the peculiarities of the origin of epithelial tissue, it is proposed to subdivide it into skin, intestinal, renal, coelomic epithelium (mesothelium, epithelium of the gonads) and ependymoglial (epithelium of some sensory organs).

All types of epithelial tissue are characterized by a number of common features: epithelial cells together form a continuous layer located on the basement membrane, through which the epithelial tissue is fed, which does not contain; epithelial tissue has a high regenerative capacity, and the integrity of the damaged layer, as a rule, is restored; epithelial tissue cells are characterized by a polarity of the structure due to differences in the basal (located closer to the basement membrane) and the opposite - the apical parts of the cell body.

Within the layer, the connection of neighboring cells is often carried out with the help of desmosomes - special multiple structures of submicroscopic sizes, consisting of two halves, each of which is located in the form of a thickening on the adjacent surfaces of neighboring cells. The slit-like gap between the halves of the desmosomes is filled with a substance, apparently, of a carbohydrate nature. If the intercellular spaces are expanded, then the desmosomes are located at the ends of the bulges of the cytoplasm of the contacting cells facing each other. Each pair of such bulges looks like an intercellular bridge under light microscopy. In the epithelium small intestine the gaps between adjacent cells are closed from the surface due to the fusion of cell membranes in these places. Such confluence sites have been described as end plates. In other cases, these special structures are absent, neighboring cells are in contact with their smooth or sinuous surfaces. Sometimes the edges of the cells overlap each other in a tiled manner. The basement membrane between the epithelium and the underlying tissue is formed by a substance rich in mucopolysaccharides and containing a network of thin fibrils.

The cells of the epithelial tissue are covered from the surface with a plasma membrane and contain organelles in the cytoplasm. In cells through which metabolic products are intensively excreted, the plasma membrane of the basal part of the cell body is folded. On the surface of a number of epithelial cells, the cytoplasm forms small, outward-facing outgrowths - microvilli. They are especially numerous on the apical surface of the epithelium. small intestine and the main divisions of the convoluted tubules of the kidneys. Here, microvilli are located parallel to each other and in the aggregate they look like a strip (cuticles of the intestinal epithelium and a brush border in the kidney). Microvilli increase the absorptive surface of cells. In addition, a number of enzymes were found in the microvilli of the cuticle and brush border.

On the surface of the epithelium of some organs (trachea, bronchi, etc.) there are cilia. Such an epithelium, which has cilia on its surface, is called ciliated. Due to the movement of the cilia, dust particles are removed from the respiratory organs, and a directed flow of fluid is created in the oviducts. The basis of the cilia, as a rule, consists of 2 central and 9 paired peripheral fibrils associated with centriole derivatives - basal bodies. The flagella of spermatozoa have a similar structure.

With a pronounced polarity of the epithelium, the nucleus is located in the basal part of the cell, above it are mitochondria, the Golgi complex, and centrioles. The endoplasmic reticulum and the Golgi complex are especially developed in secreting cells. In the cytoplasm of the epithelium, which experiences a large mechanical load, a system of special filaments, tonofibrils, is developed, creating, as it were, a framework that prevents cell deformation.

According to the shape of the cells, the epithelium is divided into cylindrical, cubic and flat, and according to the location of the cells - into single-layer and multilayer. In a single-layered epithelium, all cells lie on the basement membrane. If at the same time the cells have the same shape, that is, they are isomorphic, then their nuclei are located at the same level (in one row) - this is a single-row epithelium. If cells of different shapes alternate in a single-layer epithelium, then their nuclei are visible at different levels - a multi-row, anisomorphic epithelium.

In the stratified epithelium, only the cells of the lower layer are located on the basement membrane; the remaining layers are located above it, and the shape of the cells of different layers is not the same. Stratified epithelium is distinguished by the shape and condition of the cells of the outer layer: stratified squamous epithelium, stratified keratinizing (with layers of keratinized scales on the surface).

A special type of stratified epithelium is the transitional epithelium of the organs of the excretory system. Its structure changes depending on the stretching of the organ wall. In the distended bladder, the transitional epithelium is thinned and consists of two layers of cells - basal and integumentary. When the organ contracts, the epithelium thickens sharply, the shape of the cells of the basal layer becomes polymorphic, and their nuclei are located at different levels.

Integumentary cells become pear-shaped and layered on top of each other.

epithelial tissue- the outer surface of human skin, as well as the lining surface of the mucous membranes of internal organs, the gastrointestinal tract, the lungs, and most glands.

The epithelium is devoid of blood vessels, so nutrition occurs at the expense of adjacent connective tissues, which are powered by blood flow.

Functions of epithelial tissue

main function skin epithelial tissue - protective, that is, limiting the impact of external factors on internal organs. The epithelial tissue has layered structure, so keratinized (dead) cells are quickly replaced by new ones. It is known that epithelial tissue has increased regenerative properties, which is why human skin is quickly updated.

There is also intestinal epithelial tissue with a single layer structure, which has suction properties, due to which digestion occurs. In addition, the intestinal epithelium has the ability to release chemicals, in particular sulfuric acid.

human epithelial tissue covers almost all organs from the cornea of ​​​​the eye, to the respiratory and genitourinary systems. Some types of epithelial tissue are involved in protein and gas metabolism.

The structure of epithelial tissue

Cells of a single-layer epithelium are located on the basement membrane and form one layer with it. Stratified epithelial cells are formed from several layers, and only the lowest layer is the basement membrane.

According to the shape of the structure, epithelial tissue can be: cubic, flat, cylindrical, ciliated, transitional, glandular, etc.

Glandular epithelial tissue has secretory functions, that is, the ability to secrete a secret. The glandular epithelium is located in the intestine, it makes up sweat and salivary glands, endocrine glands, etc.

The role of epithelial tissue in the human body

The epithelium plays a barrier role, protecting internal tissues, and also promotes the absorption of nutrients. When eating hot food, part of the intestinal epithelium dies off and is completely restored overnight.

Connective tissue

Connective tissue- building matter that unites and fills the entire body.

Connective tissue is present in nature in several states at once: liquid, gel-like, solid and fibrous.

In accordance with this, blood and lymph, fat and cartilage, bones, ligaments and tendons, as well as various intermediate body fluids are distinguished. The peculiarity of the connective tissue is that there is much more intercellular substance in it than the cells themselves.

Types of connective tissue

— cartilaginous, is of three types:
a) Hyaline cartilage;
b) Elastic;
c) Fibrous.

— Bone(consists of forming cells - osteoblast, and destroying - osteoclast);

— fibrous, in turn happens:
a) Loose (creates a framework for organs);
b) Formed dense (forms tendons and ligaments);
c) Unformed dense (the perichondrium and periosteum are built from it).

— Trophic(blood and lymph);

— Specialized:
a) Reticular (tonsils are formed from it, Bone marrow, lymph nodes, kidneys and liver);
b) Fat (subcutaneous energy reservoir, heat regulator);
c) Pigmentary (iris, nipple halo, anus circumference);
d) Intermediate (synovial, cerebrospinal and other auxiliary fluids).

Connective tissue functions

These structural features allow the connective tissue to perform various functions:

1. Mechanical(supporting) function is performed by bone and cartilage tissues, as well as fibrous connective tissue of tendons;
2. Protective function is performed by adipose tissue;
3. transport function is performed by liquid connective tissues: blood and lymph.

Blood provides the transport of oxygen and carbon dioxide, nutrients, and metabolic products. Thus, the connective tissue connects the parts of the body together.

Connective tissue structure

Most of the connective tissue is an intercellular matrix of collagen and non-collagen proteins.

In addition to it - naturally cells, as well as a number of fibrous structures. by the most important cells we can name fibroblasts, which produce the substances of the intercellular fluid (elastin, collagen, etc.).

Important in the structure are also basophils (immune function), macrophages (fighters of pathogens) and melanocytes (responsible for pigmentation).

epithelial tissue

Epithelial tissue (epithelium) covers the surface of the body, lines the walls of hollow internal organs, forming a mucous membrane, glandular (working) tissue of the glands of external and internal secretion. The epithelium is a layer of cells lying on the basement membrane, the intercellular substance is almost absent. The epithelium does not contain blood vessels. Nutrition of epitheliocytes is carried out diffusely through the basement membrane.

Epithelial cells are tightly connected to each other and form a mechanical barrier that prevents the penetration of microorganisms and foreign substances inside the body. Epithelial tissue cells live for a short time and are quickly replaced by new ones (this process is called regeneration).

Epithelial tissue is also involved in many other functions: secretion (external and internal secretion glands), absorption (intestinal epithelium), gas exchange (lung epithelium).

The main feature of the epithelium is that it consists of a continuous layer of densely packed cells. The epithelium can be in the form of a layer of cells lining all surfaces of the body, and in the form of large clusters of cells - glands: liver, pancreas, thyroid, salivary glands, etc. In the first case, it lies on the basement membrane, which separates the epithelium from the underlying connective tissue . However, there are exceptions: epithelial cells in the lymphatic tissue alternate with elements of connective tissue, such an epithelium is called atypical.

The main function of the epithelium is to protect the relevant authorities from mechanical damage and infections. In those places where the body tissue is subjected to constant stress and friction and "wears out", epithelial cells multiply at a high speed. Often, in places of heavy loads, the epithelium is compacted or keratinized.

The epithelial cells are held together by a cementing substance containing hyaluronic acid. Since they do not fit the epithelium blood vessels, supply of oxygen and nutrients occurs by diffusion through the lymphatic system. Nerve endings can penetrate the epithelium.

Signs of epithelial tissue

The cells are arranged in layers

Ш Has a basement membrane

The cells are closely related to each other

Ø Cells have polarity (apical and basal parts)

Ø Absence of blood vessels

Ш Absence of intercellular substance

Ш High ability to regenerate

Morphological classification

Epithelial cells located in a layer can lie in many layers ( stratified epithelium) or in one layer ( single layer epithelium). According to the cell height epithelium flat, cubic, prismatic, cylindrical.

Single layer epithelium

Single layered cuboidal epithelium formed by cells of a cubic shape, is a derivative of three germ layers (external, middle and internal), located in the tubules of the kidneys, excretory ducts of the glands, bronchi of the lungs. The single-layer cubic epithelium performs absorption, secretory (in the tubules of the kidneys) and delimitation (in the ducts of the glands and bronchi) functions.

Rice.

Single layered squamous epithelium mesothelium, is of mesodermal origin, lines the surfaces of the pericardial sac, pleura, peritoneum, omentum, performing delimiting and secretory functions. The smooth surface of the mesatelia promotes the sliding of the heart, lungs, and intestines in their cavities. Through the mesothelium, the exchange of substances takes place between the fluid that fills the secondary cavities of the body and the blood vessels embedded in the layer of loose connective tissue.

Rice.

Single layer columnar (or prismatic) epithelium ectodermal origin, lines the inner surface of the gastrointestinal tract, gallbladder, excretory ducts of the liver and pancreas. The epithelium is formed by prismatic cells. In the intestines and gallbladder, this epithelium is called the border epithelium, as it forms numerous outgrowths of the cytoplasm - microvilli, which increase the surface of the cells and promote absorption. The cylindrical epithelium of mesodermal origin, lining the inner surface of the fallopian tube and uterus, has microvilli and ciliated cilia, the vibrations of which contribute to the advancement of the egg.

Rice.

Single layered ciliated epithelium -cells of this epithelium of various shapes and heights have ciliated cilia, the fluctuations of which contribute to the removal of foreign particles that have settled on the mucous membrane. This epithelium lines the airways and is of ectodermal origin. The functions of a single-layer multi-row ciliated epithelium are protective and delimiting.

Rice.

Stratified epithelium

The epithelium, according to the nature of the structure, is divided into integumentary and glandular.

Integumentary (surface) epithelium- these are border tissues located on the surface of the body, mucous membranes of internal organs and secondary cavities of the body. They separate the body and its organs from their environment and participate in the metabolism between them, carrying out the functions of absorption of substances and excretion of metabolic products. For example, through the intestinal epithelium, the products of digestion of food are absorbed into the blood and lymph, and through the renal epithelium, a number of products of nitrogen metabolism, which are slags, are excreted. In addition to these functions, the integumentary epithelium performs an important protective function, protecting the underlying tissues of the body from various external influences - chemical, mechanical, infectious, and others. For example, the skin epithelium is a powerful barrier to microorganisms and many poisons. Finally, the epithelium covering the internal organs creates the conditions for their mobility, for example, for the movement of the heart during its contraction, the movement of the lungs during inhalation and exhalation.

glandular epithelium- a kind of epithelial tissue, which consists of epithelial glandular cells, which in the process of evolution have acquired the leading property to produce and secrete secrets. Such cells are called secretory (glandular) - glandulocytes. They have exactly the same general characteristics like a covering epithelium. It is located in the glands of the skin, intestines, salivary glands, endocrine glands, etc. Among the epithelial cells are secretory cells, there are 2 types of them.

Ш exocrine - secrete their secret into the external environment or the lumen of the organ.

SH endocrine - secrete their secret directly into the bloodstream.

epithelial tissue cell function

Stratified epithelium is divided into three types: non-keratinized, keratinized and transitional. Stratified non-keratinized epithelium consists of three layers of cells: basal, styloid and flat.

Transition The epithelium lines organs that are subject to strong stretching - the bladder, ureters, etc. When the volume of the organ changes, the thickness and structure of the epithelium also changes.

The presence of a large number of layers allows you to perform a protective function. multilayer non-keratinizing the epithelium lines the cornea, oral cavity and esophagus, is a derivative of the outer germ layer (ectoderm).

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

III basal layer - contains stem cells, differentiated cylindrical and pigment cells (pigmentocytes).

Spiny layer - cells of a polygonal shape, they contain tonofibrils.

III granular layer - cells acquire a diamond shape, tonofibrils disintegrate and keratohyalin protein is formed inside these cells in the form of grains, this begins the process of keratinization.

The lustrous layer is a narrow layer, in which the cells become flat, they gradually lose their intracellular structure, and keratohyalin turns into eleidin.

Ш stratum corneum - contains horny scales, which have completely lost the structure of cells, contain the protein keratin. With mechanical stress and with a deterioration in blood supply, the process of keratinization intensifies.

AT thin skin, which is not under load, there is no granular and shiny layers. The main function of the stratified keratinizing epithelium is protective.

Epithelial tissues are a collection of differentiated cells that are closely located in the form of a layer on the basement membrane, on the border with the external or internal environment, and also form most of the body's glands.

Signs of epithelial tissue:

1. Cells are arranged in layers.

2. There is a basement membrane that performs mechanical (anchoring of epitheliocytes), trophic and barrier (selective transport of substances) functions.

3. Cells are closely related to each other.

4. Cells have polarity (apical and basal parts).

5. There are no blood vessels. Nutrition of epitheliocytes is carried out diffusely through the basement membrane from the side of the underlying connective tissue.

6. There is no intercellular substance.

7. High ability to regenerate. The restoration of the epithelium occurs due to mitotic division and differentiation of stem cells.

Histomorphology of superficial and glandular epithelium

There are two groups of epithelial tissues: surface epithelium (integumentary and lining) and glandular epithelium.

Surface epithelium - cover organs from the outside and from the inside, separate the body and its organs from their environment and participate in the metabolism between them, performing the functions of absorbing substances and releasing metabolic products. The integumentary epithelium performs a protective function, protecting the underlying tissues of the body from various external influences - chemical, mechanical, infectious and others. The epithelium covering the internal organs creates the conditions for their mobility, for example, for the movement of the heart during its contraction, the movement of the lungs during inhalation and exhalation.

Among the surface epithelium, two main groups are distinguished: single-layer and multilayer. In single-layer epithelium, all cells are connected with the basement membrane, and in multilayer epithelium, only one lower layer of cells is directly connected with it.

Single-layer epithelium can be of two types: single-row and multi-row. In a single-row epithelium, all cells have the same shape - flat, cubic or prismatic, and their nuclei lie on the same level, i.e. in one row. 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.

Stratified epithelium is stratified squamous keratinized, stratified squamous non-keratinized and transitional.

The glandular epithelium forms the secretory sections and excretory ducts of the exocrine glands, performs a secretory function, i.e. synthesizes and secretes specific products - secrets that are used in the processes occurring in the body.

Epithelium develops from all three germ layers.

Superficial epithelium.

Single layered epithelium. In the form of cells, they can be flat, cubic, prismatic.

Single layered squamous epithelium represented in the body by mesothelium and endothelium.

The mesothelium covers the serous membranes. Mesothelial cells are flat, have a polygonal shape and jagged edges. There are microvilli on the free surface of the cell. The secretion and absorption of serous fluid occurs through the mesothelium. Thanks to its smooth surface, sliding of the internal organs is easily carried out. The mesothelium prevents the formation of adhesions between the organs of the abdominal or thoracic cavities, the development of which is possible if its integrity is violated.

The endothelium lines the blood and lymphatic vessels and the chambers of the heart. It is a layer of flat cells - endotheliocytes, lying in one layer on the basement membrane. The endothelium, located in the vessels on the border with lymph or blood, is involved in the exchange of substances and gases between them and other tissues. If it is damaged, it is possible to change the blood flow in the vessels and the formation of blood clots in their lumen - blood clots.

Single layered cuboidal epithelium lines part of the renal tubules. The epithelium of the renal tubules performs the function of reabsorption of a number of substances from the primary urine into the blood.

Single layer prismatic epithelium characteristic of the middle section digestive system. It lines the inner surface of the stomach, small and large intestines, gallbladder, a number of ducts of the liver and pancreas.

In the stomach, in a single layer of prismatic epithelium, all cells are glandular, producing mucus, which protects the stomach wall from the rough influence of food and the digestive action of gastric juice.

In the small and large intestines, the epithelium is single-layered prismatic bordered. It consists of:

Cylindrical epithelial cells are the most numerous cells of the intestinal epithelium, performing the main absorption function of the intestine. On the apical surface of the cells there is a border formed by microvilli. The total number of microvilli on the surface of one cell varies widely - from 500 to 3000. Microvilli are covered on the outside with glycocalyx, which adsorbs enzymes involved in parietal (contact) digestion. Due to microvilli, the active surface of intestinal absorption increases 30-40 times.

Grocery cells are essentially unicellular mucous glands located among columnar epitheliocytes. They produce mucins that perform a protective function and help move food through the intestines. The number of cells increases towards the distal intestine. The shape of the cells changes in different phases of the secretory cycle from prismatic to goblet.

Paneth cells, or exocrinocytes with acidophilic granules, are constantly located in the crypts (6-8 cells each) of the jejunum and ileum. In the apical part of these cells, acidophilic secretory granules are determined. The cells secrete a secret rich in the enzyme peptidase, lysozyme, etc. It is believed that the secret of the cells neutralizes the hydrochloric acid of the intestinal contents, participates in the breakdown of dipeptides to amino acids, and has antibacterial properties.

Endocrinocytes. Among endocrine cells, there are several types of cells that secrete various hormones: melatonin, serotonin, enteroglucagon; cholecystokinin; produce somatostatin. Endocrinocytes make up about 0.5% of the total number of intestinal epithelial cells. These cells are updated much more slowly than epithelial cells. Renewal of the cellular composition of the intestinal epithelium occurs in 4-5 days duodenum and somewhat slower (for 5-6 days) in the ileum.

Young differentiated cells are involved in the regeneration of the epithelium.

Single layered epithelium line the airways (nasal cavity, trachea, bronchi) and fallopian tubes. Consists of ciliated, goblet and basal cells.

Ciliated (or ciliated) cells are high, prismatic in shape, there are cilia on the apical surface, which, with the help of flexion movements (the so-called "flickers"), clean the inhaled air from dust particles, pushing them towards the nasopharynx. Goblet cells secrete mucus onto the surface of the epithelium. Basal cells are low, lie on the basement membrane, belong to the cambial cells, which divide and differentiate into ciliated and goblet cells, thus participating in the regeneration of the epithelium.

Stratified squamous nonkeratinized epithelium covers the outside of the cornea of ​​​​the eye, lines the mucous membrane of the oral cavity and esophagus. It distinguishes three layers: basal, spiny and flat (superficial).

The basal layer consists of prismatic epithelial cells located on the basement membrane. Among them are stem cells capable of mitotic division.

The spinous layer consists of cells of irregular polygonal shape. The upper layers of the epithelium are formed by squamous cells. Finishing their life cycle, the latter die off and fall off (exfoliate) from the surface of the epithelium.

Stratified squamous keratinized epithelium covers the surface of the skin, forming its epidermis. It consists of five layers:

1.Basal.

2. Prickly.

3. Grainy.

4. Brilliant.

5. Horny.

Basal layer consists of keratinocytes, melanocytes, Langerhans cells and lymphocytes. Keratinocytes are cylindrical and capable of dividing. Melanocytes (pigment cells) form the pigment melanin, which has the ability to retain ultra-violet rays. Melanin does not allow UV rays to penetrate deep into the epidermis, where they can cause damage to the genetic apparatus of rapidly dividing cells of the basal layer. Langerhans cells act as macrophages in the epidermis. They have processes forming a kind of network in the epidermis. Due to this, they can capture environmental antigens in large quantities and transfer them to intraepidermal helper lymphocytes. In addition, these cells can migrate from the epidermis to the dermis, and then from the skin to the regional lymph node, and carry antigens on their surface. They are able to migrate from the epidermis to the dermis and to the regional lymph nodes. They perceive antigens in the epidermis and "present" them to intraepidermal and regional lymphocytes. lymph nodes thus triggering immunological reactions. T-lymphocytes penetrate into the basal and prickly layers of the epidermis from the dermis, perform a protective function.

Spiny layer consists of keratinocytes and Langerhans cells. Keratinocytes, forming 5-10 layers, have a variety of shapes. They are connected to each other with the help of numerous desmosomes that look like spikes.

Granular layer consists of two or three rows of spindle-shaped cells. In their cytoplasm there are many grains of keratogealin. The formation of keratohyalin (sulfur-containing protein) is the beginning of the synthesis of the horny substance of keratin. The cells of the granular layer are still alive, but they cannot divide. They gradually lose their organelles and nucleus. The cytoplasm contains lipids and hydrolytic enzymes. Lipids are secreted into the intercellular spaces and prevent the diffusion of water through the skin and the loss of body fluids.

glitter layer consists of 3-4 rows of flat dead cells. Their nuclei are destroyed. The grains of keratohyalin merge and undergo chemical transformations, refracting light eleidin is formed, therefore the layer is called brilliant.

stratum corneum- outer and most powerful. It consists of many rows of keratinized squamous cells containing keratin and air bubbles that help retain heat. Keratin is resistant to acids and alkalis.

transitional epithelium lines the mucous membrane of the urinary organs - the pelvis of the kidneys, ureters, Bladder, the walls of which are subject to significant stretching when filled with urine. There are three layers of cells in the epithelium:

1. Basal - formed by small rounded cells.

2. Intermediate - polygonal cells.

3. Superficial - consists of very large cells that have a dome-shaped or flattened shape, depending on the state of the organ wall. When the wall is stretched due to the filling of the organ with urine, the epithelium becomes thinner and its surface cells flatten. During the contraction of the wall of the organ, the thickness of the epithelial layer increases sharply.

glandular epithelium. They are characterized by a pronounced secretory function. The glandular epithelium consists of glandular, or secretory, cells. They carry out the synthesis and isolation of specific products. The shape of the cells is very diverse and varies depending on the phase of secretion. In the cytoplasm of cells that produce protein secrets, the granular endoplasmic reticulum is well developed. In cells synthesizing non-protein secrets, an agranular endoplasmic reticulum is expressed. Numerous mitochondria accumulate in the places of the greatest activity of cells, i.e. where the secret is formed.

For the formation of a secret from the blood and lymph, various inorganic compounds, water and low molecular weight organic substances enter the glandular cells from the basal surface: amino acids, monosaccharides, fatty acids. Secrets are synthesized from these products in the endoplasmic reticulum. They move to the zone of the Golgi apparatus, where they gradually accumulate, undergo chemical restructuring and take the form of granules that are released from the cells.

The mechanism of secretion in different glands is not the same, and therefore there are three types of secretion: merocrine, apocrine and holocrine.

At merocrine type of secretion during secretion excretion, glandular cells completely retain their structure (cells of the salivary and pancreas glands). At apocrine type of secretion, there is a partial destruction of glandular cells (cells of the mammary gland). Holocrine the type of secretion is characterized by the complete destruction of glandular cells (cells of the sebaceous glands).

Restoration of the structure of glandular cells occurs either by intracellular regeneration (with mero- and apocrine secretion) or by cellular regeneration, i.e. cell division (with holocrine secretion).

Glandular epithelial tissue forms glands - organs consisting of secretory cells that produce and secrete specific substances of various chemical nature. The glands are divided into two groups:

Glands of internal secretion, or endocrine.

Glands of external secretion, or exocrine.

Both those and other glands can be unicellular and multicellular.

Endocrine glands produce hormones that enter directly into the blood or lymph. Therefore, they consist only of glandular cells and do not have excretory ducts.

exocrine glands develop secrets that are released into the external environment, i.e. on the surface of the skin or in the cavity of organs. Exocrine glands consist of two parts: secretory, or end, divisions and excretory ducts. According to the structure of the terminal sections, glands are distinguished: branched and unbranched, as well as tubular, alveolar or mixed (tubular-alveolar).

According to the number of excretory ducts, glands are distinguished: simple and complex. Simple glands have a non-branching excretory duct, complex glands have a branching one.

In the excretory duct, the glands open - in unbranched glands, one at a time, and in branched glands, several terminal sections.

Chemical composition the secret may be different, in connection with this, the exocrine glands are divided into several types: protein (or serous), mucous, protein-mucous (or mixed), sebaceous, saline (for example: sweat and lacrimal).

Questions for self-control

1. General characteristics of epithelial tissues.

2.Single-layer epithelium.

3. Stratified squamous keratinized epithelium. Structure. Functions.

4. Transitional epithelium. Structure. Functions.

5. Glandular epithelium: structure, principles of classification of glands. Sources of development. Features of the structure of the glands according to the method of formation of secretion.

BIBLIOGRAPHY

1. Afanasiev Yu.I. Histology. M.. “Medicine”, 2001

2. Bykov V.L. Cytology and general histology. - St. Petersburg: "Sotis", 2000

3. Bykov V.L. Private histology of man. - St. Petersburg: "Sotis", 1999

4. Afanasiev Yu.I. Laboratory studies in the course of histology, cytology and embryology. - M .: Medicine, 1999

5. Volkova O.V. Histology, cytology and embryology. Atlas. - M .: Medicine, 1999

Lecture 4

CONNECTIVE TISSUE