What do the epithelial cells do
epithelium, Epithelial tissue, Cover epithelium, Cover fabric, Plural: Epithelia. Collective reference for all cover and finishing fabrics. E. On the one hand they provide mechanical protection and sealing, on the other hand they control the exchange of substances between the internal tissues with their intercellular spaces and the outside world. Corresponding to these tasks, the cell membranes of neighboring epithelial cells are usually intimately interlocked with one another, and the intercellular gap between them is very narrow (10-20 nm) and also closed by special intercellular connections (cement or end strips, desmosomes, tight junctions). E. cells are typically built polar: they have a morphologically and physiologically distinguishable outer and a basal side facing the inner tissue, on which a basal lamina is usually deposited. Epithelia can consist of one or more layers of cells. Multi-layer E. are mostly used as outer body covering (epidermis) to protect against mechanical injuries and - especially in terrestrial animals and plants - to seal against fluid loss through evaporation.
in the Animal kingdom multilayered epidermis occur in invertebrates only in exceptional cases, but are the rule in vertebrates. There they can (with an average thickness of 10-20 nm) on mechanically highly stressed areas of skin such as the soles of the feet reach a thickness of a hundred or more cell layers, whereby the cells flatten and keratinize more and more towards the surface. Constant cell loss through death and peeling of cells on the outside is caused by continuous replenishment from the dividing basal layer (Stratum basale or germinativum) balanced. In the epidermis of terrestrial vertebrates, most of the cell layers die off prematurely and bake to form a tough, impermeable horny layer (keratinized squamous epithelium). The tensile strength of horny epithelia is increased by the abundant formation of desmosomes. They have no vascular supply; they are fed exclusively by diffusion from underlying tissue. The cornification corresponds functionally to the deposition of a cuticle in invertebrates, which is also carried out by E. cells.
Moist, multilayered E. for example of the mucous membranes in the oral cavity and esophagus are usually thinner and consist of living, non-keratinizing, flat cells up to the surface. In the epidermis of aquatic animals and in the ureter of mammals, such multilayered E. only reach a thickness of three to five cell layers.
In invertebrates and as the lining of hollow organs and body cavities in vertebrates, single-layer E. Depending on the cell shape, they are called Squamous epithelium, as a cubic (isoprismatic) Pavement epithelium or as a high prismatic Columnar epithelium labeled (see Fig.). There are smooth transitions between these types. That is also included in the single-layer E. multi-row E., Whose cells of different heights, some with long peduncles, have a connection to the basal lamina, but the staggered arrangement of their nuclei simulates a multilayered structure. Such E. are very elastic (Transitional epithelium) and line hollow organs that are subject to strong and rapid changes in volume, such as the urinary bladder. The endothelium of the blood vessels is also a single-layer E. E., which have secretory (glands) and resorptive functions (Transport epithelia, e.g. in the intestine), have characteristic surface enlargements: a trimming with microvilli (or with flagella or eyelashes, Ciliated epithelium) on the outside and a system of deep, often branched plasmalemma folds (basal labyrinth) on the basal side (see Fig.).
The development of E. in the animal kingdom is basically not tied to a specific cotyledon. Multi-layer E. are, however, predominantly of ectodermal origin, while endodermal and mesodermal E., in which the protective function against secretory tasks usually takes a back seat, are usually single-layer.
Epithelium: Basic forms of the epithelial tissue: a Squamous epithelium, b cubic (isoprismatic) epithelium, c Columnar epithelium, d multilayered epithelium
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