The epithelial tissue (also called epithelium ) consists of layers of cells closely adjacent to each other. The cell membranes are so tightly joined and "glued" to make it a perfect waterproofing fabric ; it means that the epithelial tissue is suitable for preventing the passage of substances from the inside of the body to the outside, and vice versa.
Because of this characteristic, the epithelial one is a tissue particularly exposed to wear, to which it responds with a marked regenerative capacity (mitotic activity). At the same time, however, it must also be resistant, so as to protect the surfaces of the organs it covers. The epithelia, in fact, cover the exposed surfaces of the organism (the epidermis is an epithelial tissue), but also the cavities and the internal ducts (they constitute for example the innermost layer of blood vessels, the one in direct contact with the blood). In addition, they form the glands that spill substances inside (endocrine glands) and outside (exocrine glands) of the body.
The epithelia rest on an underlying layer of connective tissue, to which they are fixed by the interposition of the basement membrane ; it is a dense substance formed by filamentous proteins and adhesive polysaccharides, through which they spread the nutrients destined for the epithelium. The epithelial tissue, in fact, is devoid of blood vessels .
There are different types of epithelial tissue, further differentiated according to the number of cell layers that compose them and based on the shape of the cells that make them up.
The three main types of epithelial tissue are
- The epithelial covering fabric : as the name implies, it covers the body surfaces and has the function of protection (against physical, chemical and / or biological agents); moreover, it regulates the passage of certain substances (absorption, diffusion, excretion);
- The glandular or secreting epithelial tissue : has the task of processing and secreting particular substances;
- The sensory epithelial tissue or neuroepithelium : it allows to capture and transmit certain stimuli coming from the external world (eg taste and olfactory receptors).
