Epithelial tissue (textus epithelialis) covers the surface of the body and lining the mucous membranes, separating the body from the external environment (the cover epithelium). From the epithelial tissue, glands (glandular epithelium) are formed. In addition, the sensory epithelium is isolated, the cells of which are changed to perceive specific irritations in the organs of hearing, balance and taste.
Classification of epithelial tissue. Depending on the position relative to the basal membrane, the cover epithelium is subdivided into a single-layered and multilayered epithelium. All cells of single-layered epithelium lie on the basement membrane. The cells of the multilayer epithelium form several layers, and only the cells of the lower (deep) layer lie on the basal membrane. The single-layered epithelium, in turn, is divided into single-stranded, or isomorphic (flat, cubic, prismatic), and multilayer (pseudo-layered). The nuclei of all cells of the single-rowed epithelium are located on the same level, and all cells have the same height.
Depending on the shape of the cells and their capacity for keratinization, a multilayer keratinizing (planar), multi-layered non-corneum (flat, cubic and prismatic) and transitional epithelium are distinguished.
All epithelial cells have common structural features. Epitheliocytes are polar, their apical part differs from the basal part. Epitheliocytes of the covering epithelium form strata that are located on the basal membrane and are devoid of blood vessels. In epithelial cells there are all organelles of general purpose. Their development, the structure is associated with the function of epithelial cells. So, the cells secreting protein are rich in elements of a granular endoplasmic reticulum; cells, producing steroids, - elements of an ungrain endoplasmic reticulum. Suction cells have many microvilli, and epitheliocytes covering the mucous membrane of the respiratory tract are provided with cilia.
The integument 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 band of the cochlear duct), gas exchange (respiratory alveolocytes).
Single-layered epithelium. Single-layered simple flat, simple cubic, simple columnar and pseudo-layered epithelium.
A single-layered flat epithelium is a layer of thin flat cells lying on the basement membrane. In the zone of occurrence of nuclei there are protrusions of the free surface of the cell. Epitheliocytes are polygonal in shape. Flat epitheliocytes form the outer wall of the capsule of the glomerulus of the kidney, cover the back of the cornea of the eye, lining all the blood and lymphatic vessels, the heart cavities (endothelium) and the alveoli (respiratory epitheliocytes), and cover the facing surfaces of the serous membranes (mesothelium).
Endotheliocytes have an elongated (sometimes fusiform) shape and a very thin layer of cytoplasm. The nucleated part of the cell is thickened, bulging into the lumen of the vessel. Microvilli are located mainly above the nucleus. The cytoplasm contains micropinocytosis 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 a lot of microvilli, in some cells there are 2-3 nuclei. Mesotheliocytes facilitate the mutual sliding of internal organs and prevent the formation of adhesions (adhesions) between them. Respiratory (respiratory) epithelial cells of 50-100 microns in size, their cytoplasm is rich in micropinocytosis vesicles and ribosomes. Other organelles are poorly represented.
A simple cubic epithelium is formed by a single layer of cells. There are no desiccated cubic epitheliocytes (in collecting tubules of the kidney, distal tubules of nephrons, bile ducts, cerebral plexuses of the brain, pigment epithelium of the retina of the eye, etc.) and ciliary (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.
A simple single-layer columnar (prismatic) epithelium covers the mucosa of the digestive tract, beginning from the entrance to the stomach and up to the anus, the walls of the papillary ducts and collecting tubules of the kidneys, striated ducts of the salivary glands, uterus, and fallopian tubes. Columnar epithelial cells are high prismatic polygonal or rounded cells. They closely adjoin each other by a complex of intercellular connections, which are located near the surface of the cells. A rounded or ellipsoidal nucleus is usually located in the lower (basal) third of the cell. Often prismatic epithelial cells are provided with a variety of microvilli, stereocilia or cilia. Microvilli cells prevail in the epithelium of the intestinal mucosa and gallbladder.
The pseudo-layered (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 distinguishes 3 types of cells:
- basal epithelioiites forming the lower (deep) row of cells. They are the source of epithelial renewal (daily updated to 2% of the cells of the population);
- intercalating epitheliocytes, slightly differentiated, not having cilia or microvilli and not reaching the lumen of the organ. They are located between the surface cells;
- surface epitheliocytes - elongated cells that reach the lumen of the organ. These cells have a rounded nucleus and well-developed organelles, especially the Golgi complex and the endoplasmic reticulum. The apical cytolemma is covered with m and crocks and, cilia.
The ciliated cells cover the nasal mucosa, the trachea, the bronchi, the non-desiccated mucosa of the male urethra, the excretory ducts of the glands, the ducts of the epididymis and the vas deferens.
Multilayered epithelium. To this type of epithelium is included the non-corneum and cornified squamous epithelium, multilayered cubic and columnar epithelium.
Multilayered planar non-corneumal epithelium covers the mucous membrane of the mouth and esophagus, the transitional zone of the anal canal, the vocal cords, the vagina, the female urethra, and the outer surface of the cornea of the eye. This epithelium distinguishes 3 layers:
- the basal layer is formed by large prismatic cells that lie on the basement membrane;
- The spiny (intermediate) layer is formed by large process polygonal cells. The basal layer and the lower part of the spiky layer form a germinal (germinative) layer. Epitheliocytes divide mitotically and, advancing towards the surface, flatten out and replace the sloughing cells of the surface layer;
- the surface layer is formed by flat cells.
Multilayered flat keratinized epithelium covers the entire surface of the skin, forming its epidermis. In the epidermis of the skin, 5 layers are distinguished:
- the basal layer is the deepest. It contains prismatic forms of cells lying on the basement membrane. In the cytoplasm, located above the nucleus, there are melanin granules. Between the basal epitheliocytes there are pigment-containing cells - melanocytes;
- Chypovaty layer is formed by several layers of large polygonal prickly epitheliocytes. The lower part of the prickly layer and the basal layer form a growth layer, the cells of which divide mitotically and advance to the surface;
- The granular layer consists of oval epitheliocytes, keratogialin-rich granules;
- The shining layer has a pronounced refractive power due to the presence of flat, denuclearized epithelial cells containing keratin;
- The stratum corneum is formed by several layers of keratinizing cells - horny scales containing keratin and air bubbles.
Superficial horny scales disappear (sluschyvayutsya), in their place move the cells from the deep layers. The stratum corneum has a weak thermal conductivity.
The multilayered cubic epithelium is formed by several layers (from 3 to 10) cells. The surface layer is represented by cells of a cubic form. Cells have microvilli and are rich in glycogen granules. Under the surface layer are several layers of elongated spindle-shaped cells. Directly on the basement membrane are polygonal or cubic cells. This type of epithelium is rare. It is located in small sections on a short stretch between the multinucleated prismatic and multilayered flat nonkeratinizing epithelium (mucous membrane of the back of the vestibule of the nose, epiglottis, part of the male urethra, excretory ducts of the sweat glands).
The multilayer columnar epithelium also consists of several layers (3-10) of cells. Surface epithelial cells are prismatic and often carry cilia on their surface. The deep-lying epithelial cells are cylindrical and cubic. This type of epithelium occurs in several parts of the excretory ducts of the salivary glands 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, bladder, the beginning of the urethra, when the mucous membrane of the organs is stretched, the number of layers changes (decreases). The cytomegma of the surface layer is folded and asymmetric: its outer layer is denser, the inner layer is thinner. In an empty bladder cells are high, on the drug can be seen up to 6-8 rows of nuclei. In a full bladder the cells are flattened, the number of rows of nuclei does not exceed 2-3, the cytolemma of the surface cells is smooth.
The glandular epithelium. Cells of glandular epithelium (gland-dulocytes) form the parenchyma of multicellular glands and unicellular glands. The glands are divided into exocrine, having excretory ducts, and endocrine, with no 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 produced by them through the 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 cover of the tubular internal organs is formed from the primary endodermal layer, but also glands, unicellular and multicellular. From the cells remaining in the forming integumentary epithelium, unicellular intraepithelial glands (mucous membranes) are formed. 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, from the primary ectodermal layer along with the epidermis, skin sweat and sebaceous glands are formed. Some glands retain their connection with the surface of the body due to the duct - these are the exocrine glands, other glands lose this connection in the development process and become endocrine glands.
In the human body, there are many unicellular goblet-shaped exocrines of the nocites. They are located among other epithelial cells covering the mucous membrane of the hollow organs of the digestive, respiratory, urinary and reproductive systems. These exocrins 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 glass in shape. A narrow, chromatin rich nucleus lies in the narrowed basal part of the cell, in its pedicle. Above the nucleus is a well-developed Golgi complex, over which, in the expanded part of the cell, there are many secretory granules emerging from the cell according to the mecric type. After isolation of secretory granules, the cell becomes narrow.
In the synthesis of mucus involved ribosomes, endoplasmic reticulum, 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 by means of transport vesicles. The carbohydrate component of mucus is synthesized by the Golgi complex, and protein binding with carbohydrates occurs here. In the Golgi complex, presecretory granules are formed, which are separated and transformed into secretory granules. Their number increases in the direction of the apical section of the secretory cell, to the lumen of the hollow (tubular) internal organ. The secretion of mucus granules from the cell to the surface of the mucosa is usually accomplished by exocytosis.
Exocrinocytes also form the initial secretory departments of exocrine multicellular glands, which produce various secrets, and their tubular ducts through which the secret is secreted. The morphology of exocrine cells depends on the nature of the secretory product and the phase of secretion. The glandular cells are polarized structurally and functionally. Their secretory drops, or granules, are concentrated in the apical (supernuclear) zone and are secreted through the apical cytolemma covered with microvilli. Cells are rich in mitochondria, elements of the Golgi complex and endoplasmic reticulum. The granular endoplasmic reticulum predominates in protein-synthesizing cells (for example, the glandulocyte of the parotid salivary gland), ungrainous - in cells synthesizing lipids or carbohydrates (for example, in the cortical endocrine cells of the adrenal gland).
The secretory process in exocrine cells occurs cyclically, in which 4 phases are isolated. In the first phase, the cells necessary for synthesis are supplied to the cell. In the second phase, a synthesis of substances takes place in the granular endoplasmic reticulum, which by means of transport bubbles move to the surface of the Golgi complex and merge with it. Here, the substances that are secreted first accumulate in the vacuoles. As a result, the condensing vacuoles are transformed 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 recovery of exocrine cells.
There are 3 types of secretion:
- Meclorin (eccrine), in which secretory products are secreted by exocytosis. It is observed in serous (protein) glands. With this type of secretion, the structure of the cells is not disturbed;
- apocrine type (for example, lactocytes) is accompanied by destruction of the apical part of the cell (macroapokrino type) or the tops of microvilli (micro-apocrine type);
- holocrine type, in which the glandulocytes are completely destroyed and their contents are included in the secretion (eg, sebaceous glands).
Classification of multicellular exocrine glands. Depending on the structure of the initial (secretory) department, they distinguish tubular (resemble a tube), acinous (reminiscent of a pear or an elongated bunch of grapes) and alveolar (rounded), and tubular-acinous and tubular alveolar glands.
Depending on the number of ducts glands are divided into simple, having one duct, and complex. In the complex glands, several ducts flow into the main (common) excretory duct, each opening several primary (secretory) divisions.