This is a tissue bordering between the body and the environment that performs several functions: protective, secretory, nutritional, etc. The epithelium does not have blood vessels, is abundantly equipped with nerve endings, covers the entire external surface of the body and lines the internal cavities, forms the majority of the glands. The cells are tightly adjacent to each other, there is very little intercellular substance between them, nutrition is carried out by diffusion from a deeper connective tissue through a thin layer, or the basement membrane on which the epithelium is located. Among the epithelial tissues, two groups are distinguished: integumentary epithelium and glandular. The integumentary epithelium by the nature of the arrangement of cells is divided into single-layer and multi-layer (Fig. 6, 7). Depending on the height and shape of the cells, a single-layered squamous epithelium is distinguished (in the wall of the peritoneum, pleura),
The stratified epithelium is divided into stratified squamous (keratinized and non-keratinized) and stratified transitional. The stratified squamous epithelium covers the entire surface of the skin (epidermis), the mucous membrane of the oral cavity, esophagus, ruminants, pancreas, the final part of the rectum, and the cornea of the eye. Stratified transitional epitheliumlining the inner wall of the bladder and ureters, the walls of which are distended when filled with urine, the epithelium becomes thinner, the surface cells flatten, the number of rows of cells becomes smaller. In a reduced state, the thickness of the epithelial layer increases sharply. Differs from stratified squamous epithelium by superficially lying non-keratinized cells. Epithelial cells wear out relatively quickly and die. They have a high ability to physiological regeneration, and therefore quickly recover.
Integumentary multilayer epithelium:
a – multilayer squamous epithelium of the cornea of the eye: 1 – cells of the surface layer; 2 – cells of the prickly layer; 3 – cells of the cylindrical layer; 4 – basement membrane; 5 – connective tissue; b – stratified squamous epithelium of the skin (epidermis): 1 – stratum corneum; 2 – a brilliant layer; 3 – granular layer; 4 – a prickly layer; 5 – a cylindrical layer; 6 – basement membrane; 7 – excretory duct of the sweat gland;
- 8 – connective tissue; in– multilayer transitional epithelium:
- 1 – covering cells; 2– rounded and club-shaped cells;
- 3 – connective tissue
Drops or granules of various organic substances are synthesized in the cytoplasm of glandular epithelial cells , which can periodically be evacuated outside the cell. Such clusters are called secret , the process of isolating it is called secretion. Glands are unicellular and multicellular. The first include goblet cells that secrete mucus, they are also found among prismatic cells of the mucous membrane of the trachea, bronchi, and intestines.
The multicellular glands are divided into exocrine, or endocrine glands, which secrete secretion through the excretory ducts into any cavity of the body or outside the body, and endocrine, or endocrine glands,not having excretory ducts. The secret (hormone) secreted by the latter enters directly into the bloodstream through the walls of the blood capillaries. Exocrine glands have terminal, secretory departments and a system of excretory tubular departments. There are simple and complex exocrine glands. Simple glands have a non-branching excretory duct, complex glands branching. In the unbranched glands, it opens one at a time, and in the branched glands – several end sections in the form of a tube, vesicle (alveoli) or an intermediate type (alveoli tube).