Epithelial tissue

Epithelial tissue (textus epithelialis) covers the surface of the body and lines the mucous membranes, separating the body from the external environment (integumentary epithelium). Glands are formed from epithelial tissue (glandular epithelium). In addition, sensory epithelium is distinguished, the cells of which are modified to perceive specific stimuli in the organs of hearing, balance and taste.

Classification of epithelial tissue. Depending on the position relative to the basement membrane, the integumentary epithelium is divided into single-layer and multilayer. All cells of the single-layer epithelium lie on the basement membrane. Cells of the multilayer epithelium form several layers, and only the cells of the lower (deep) layer lie on the basement membrane. Single-layer epithelium, in turn, is divided into single-row, or isomorphic (flat, cubic, prismatic), and multi-row (pseudo-multilayer). The nuclei of all cells of the single-layer epithelium are located at the same level, and all cells have the same height.

Depending on the shape of the cells and their ability to keratinize, a distinction is made between stratified keratinizing (flat), stratified non-keratinizing (flat, cubic and prismatic) and transitional epithelium.

All epithelial cells have common structural features. Epithelial cells are polar, their apical part differs from the basal one. Epithelial cells of the covering epithelium form layers that are located on the basal membrane and lack blood vessels. Epithelial cells contain all the organelles of general purpose. Their development and structure are associated with the function of epithelial cells. Thus, cells that secrete protein are rich in elements of the granular endoplasmic reticulum; cells that produce steroids are rich in elements of the non-granular endoplasmic reticulum. Absorbing cells have many microvilli, and epithelial cells covering the mucous membrane of the respiratory tract are equipped with cilia.

The integumentary epithelium performs barrier and protective functions, the function of absorption (epithelium of the small intestine, peritoneum, pleura, nephron tubules, etc.), secretion (amniotic epithelium, epithelium of the vascular strip of the cochlear duct), gas exchange (respiratory alveolocytes).

Single-layer epithelium. Single-layer epithelium includes simple flat, simple cubic, simple columnar and pseudo-stratified epithelium.

A single-layer flat epithelium is a layer of thin flat cells lying on a basement membrane. In the area of the nuclei, there are protrusions of the free surface of the cell. The epithelial cells are polygonal in shape. Flat epithelial cells form the outer wall of the renal glomerulus capsule, cover the cornea of the eye from behind, line all blood and lymphatic vessels, the cavities of the heart (endothelium) and alveoli (respiratory epithelial cells), and cover the surfaces of the serous membranes facing each other (mesothelium).

Endothelial cells have an elongated (sometimes spindle-shaped) shape and a very thin layer of cytoplasm. The nucleated portion of the cell is thickened and protrudes into the lumen of the vessel. Microvilli are located mainly above the nucleus. The cytoplasm contains micropinocytic vesicles, single mitochondria, elements of the granular endoplasmic reticulum and the Golgi complex. Mesotheliocytes covering the serous membranes (peritoneum, pleura, pericardium) resemble endotheliocytes. Their free surface is covered with numerous microvilli, some cells have 2-3 nuclei. Mesotheliocytes facilitate mutual sliding of internal organs and prevent the formation of adhesions (fusions) between them. Respiratory epithelial cells are 50-100 μm in size, their cytoplasm is rich in micropinocytic vesicles and ribosomes. Other organelles are poorly represented.

Simple cubic epithelium is formed by a single layer of cells. A distinction is made between non-ciliated cubic epithelial cells (in the collecting ducts of the kidney, distal straight tubules of the nephrons, bile ducts, vascular plexuses of the brain, pigment epithelium of the retina, etc.) and ciliated (in terminal and respiratory bronchioles, in ependymocytes lining the cavities of the ventricles of the brain). The anterior epithelium of the lens of the eye is also a cubic epithelium. The surface of these cells is smooth.

Simple single-layer columnar (prismatic) epithelium covers the mucous membrane of the digestive tract, starting from the entrance to the stomach and up to the anus, the walls of the papillary ducts and collecting ducts of the kidneys, striated ducts of the salivary glands, uterus, fallopian tubes. Columnar epithelial cells are tall prismatic polygonal or rounded cells. They are tightly adjacent to each other by a complex of intercellular connections, which are located near the surface of the cells. The round or ellipsoid nucleus is usually located in the lower (basal) third of the cell. Often, prismatic epithelial cells are equipped with many microvilli, stereocilia or cilia. Microvillous cells prevail in the epithelium of the intestinal mucosa and gallbladder.

Pseudomultilayered (multi-row) epithelium is formed mainly by cells with an oval nucleus. The nuclei are located at different levels. All cells lie on the basement membrane, but not all of them reach the lumen of the organ. This type of epithelium is divided into 3 types of cells:

1. basal epithelioites, which form the lower (deep) row of cells. They are the source of epithelial renewal (up to 2% of the population cells are renewed daily);
2. intercalated epithelial cells, poorly differentiated, lacking cilia or microvilli and not reaching the lumen of the organ. They are located between the superficial cells;
3. Surface epithelial cells are elongated cells that reach the lumen of the organ. These cells have a round nucleus and well-developed organelles, especially the Golgi complex and endoplasmic reticulum. The apical cytolemma is covered with blood villi and cilia.

Ciliated cells cover the mucous membrane of the nose, trachea, bronchi, non-ciliated cells cover the mucous membrane of part of the male urethra, excretory ducts of glands, ducts of the epididymis and vas deferens.

Multilayered epithelium. This type of epithelium includes nonkeratinizing and keratinizing flat epithelium, multilayered cubic and columnar epithelium.

The stratified squamous nonkeratinizing epithelium covers the mucous membrane of the mouth and esophagus, the transition zone of the anal canal, the vocal cords, the vagina, the female urethra, and the outer surface of the cornea. This epithelium has 3 layers:

1. the basal layer is formed by large prismatic cells that lie on the basement membrane;
2. the spinous (intermediate) layer is formed by large polygonal cells with processes. The basal layer and the lower part of the spinous layer form the germinal (germinative) layer. The epithelial cells divide mitotically and, moving towards the surface, flatten and replace the exfoliating cells of the superficial layer;
3. The superficial layer is formed by flat cells.

The multilayered flat keratinizing epithelium covers the entire surface of the skin, forming its epidermis. The epidermis of the skin has 5 layers:

1. the basal layer is the deepest. It contains prismatic cells lying on the basal membrane. The cytoplasm located above the nucleus contains melanin granules. Between the basal epithelial cells lie pigment-containing cells - melanocytes;
2. The spinous layer is formed by several layers of large polygonal spinous epithelial cells. The lower part of the spinous layer and the basal layer form the germ layer, the cells of which divide mitotically and move toward the surface;
3. the granular layer consists of oval epithelial cells rich in keratohyalin granules;
4. the shiny layer has a pronounced light-refracting ability due to the presence of flat, anuclear epithelial cells containing keratin;
5. The stratum corneum is formed by several layers of keratinized cells - horny scales containing keratin and air bubbles.

The superficial horny scales fall off (slough off), and cells from the deeper layers move into their place. The horny layer has poor thermal conductivity.

Stratified cuboidal epithelium is formed by several layers (from 3 to 10) of cells. The superficial layer is represented by cubic cells. The cells have microvilli and are rich in glycogen granules. Several layers of elongated spindle-shaped cells are located under the superficial layer. Polygonal or cubic cells lie directly on the basement membrane. This type of epithelium is rare. It is located in small areas over a short distance between multinucleated prismatic and stratified squamous nonkeratinizing epithelium (mucous membrane of the posterior part of the nasal vestibule, epiglottis, part of the male urethra, excretory ducts of sweat glands).

The stratified columnar epithelium also consists of several layers (3-10) of cells. The superficial epithelial cells have a prismatic shape and often bear cilia on their surface. The deeper epithelial cells are cylindrical and cubic. This type of epithelium is found in several areas of the excretory ducts of the salivary and mammary glands, in the mucous membrane of the pharynx, larynx and male urethra.

Transitional epithelium. In the transitional epithelium covering the mucous membrane of the renal pelvis, ureters, urinary bladder, beginning of the urethra, when the mucous membrane of the organs is stretched, the number of layers changes (decreases). The cytolemma of the superficial layer is folded and asymmetric: its outer layer is denser, the inner one is thinner. In an empty urinary bladder, the cells are high, up to 6-8 rows of nuclei are visible on the preparation. In a filled bladder, the cells are flattened, the number of rows of nuclei does not exceed 2-3, the cytolemma of the superficial cells is smooth.

Glandular epithelium. Glandular epithelial cells (glandulocytes) form the parenchyma of multicellular glands and unicellular glands. Glands are divided into exocrine, which have excretory ducts, and endocrine, which do not have excretory ducts. Endocrine glands secrete the products they synthesize directly into the intercellular spaces, from where they enter the blood and lymph. Exocrine glands (sweat and sebaceous, gastric and intestinal) secrete the substances they produce through ducts on the surface of the body. Mixed glands contain both endocrine and exocrine parts (for example, the pancreas).

During embryonic development, not only the epithelial covering of tubular internal organs, but also glands, unicellular and multicellular, are formed from the primary endodermal layer. Unicellular intraepithelial glands (mucous) are formed from the cells remaining in the forming integumentary epithelium. Other cells intensively divide mitotically and grow into the underlying tissue, forming exo-epithelial (extra-epithelial) glands: for example, salivary, gastric, intestinal, etc. In the same way, sweat and sebaceous glands are formed from the primary ectodermal layer along with the epidermis. Some glands maintain a connection with the body surface thanks to a duct - these are exocrine glands, other glands lose this connection during development and become endocrine glands.

The human body contains many single-celled goblet exocrine cells. They are located among other epithelial cells covering the mucous membrane of the hollow organs of the digestive, respiratory, urinary and reproductive systems. These exocrine cells produce mucus, which consists of glycoproteins. The structure of goblet cells depends on the phase of the secretory cycle. Functionally active cells resemble a goblet in their shape. A narrow, chromatin-rich nucleus lies in the narrowed basal part of the cell, in its stalk. Above the nucleus is a well-developed Golgi complex, above which, in the expanded part of the cell, there are many secretory granules secreted from the cell according to the merocrine type. After the secretory granules are secreted, the cell becomes narrow.

Mucus synthesis involves ribosomes, the endoplasmic reticulum, and the Golgi complex. The protein component of mucus is synthesized by polyribosomes of the granular endoplasmic reticulum, which is located in the basal part of the cell. This component is then transferred to the Golgi complex using transport vesicles. The carbohydrate component of mucus is synthesized by the Golgi complex, where proteins are bound to carbohydrates. Presecretory granules are formed in the Golgi complex, which separate and turn into secretory granules. Their number increases toward the apical section of the secretory cell, toward the lumen of the hollow (tubular) internal organ. Secretion of mucus granules from the cell to the surface of the mucous membrane is usually accomplished by exocytosis.

Exocrine cells also form the initial secretory sections of exocrine multicellular glands that produce various secretions, and their tubular ducts through which the secretion is released. The morphology of exocrine cells depends on the nature of the secretory product and the secretion phase. Glandular cells are polarized structurally and functionally. Their secretory drops, or granules, are concentrated in the apical (supranuclear) zone and are released through the apical cytolemma covered with microvilli. The cells are rich in mitochondria, elements of the Golgi complex and endoplasmic reticulum. Granular endoplasmic reticulum predominates in protein-synthesizing cells (e.g., glandulocytes of the parotid salivary gland), nongranular - in cells synthesizing lipids or carbohydrates (e.g., in the cortical endocrinocytes of the adrenal gland).

The secretory process in exocrine cells occurs cyclically, and four phases are distinguished. In the first phase, the substances necessary for synthesis enter the cell. In the second phase, the synthesis of substances occurs in the granular endoplasmic reticulum, which move to the surface of the Golgi complex with the help of transport vesicles and merge with it. Here, the substances to be secreted initially accumulate in vacuoles. As a result, the condensing vacuoles turn into secretory granules, which move in the apical direction. In the third phase, secretory granules are released from the cell. The fourth phase of the secretory cycle is the restoration of exocrine cells.

There are 3 possible types of secretion:

1. merocrine (eccrine), in which secretory products are released by exocytosis. It is observed in serous (protein) glands. With this type of secretion, the structure of the cells is not disturbed;
2. apocrine type (for example, lactocytes) is accompanied by the destruction of the apical part of the cell (macroapocrine type) or the tips of the microvilli (microapocrine type);
3. holocrine type, in which glandulocytes are completely destroyed and their contents become part of the secretion (for example, sebaceous glands).

Classification of multicellular exocrine glands. Depending on the structure of the initial (secretory) section, there are tubular (resembling a tube), acinous (resembling a pear or an elongated bunch of grapes) and alveolar (rounded), as well as tubular-acinous and tubular-alveolar glands.

Depending on the number of ducts, glands are divided into simple, having one duct, and complex. In complex glands, several ducts flow into the main (common) excretory duct, into each of which several initial (secretory) sections open.

[ 1 ], [ 2 ], [ 3 ], [ 4 ]