من طرف Epithelial tissues
Epithelial tissues cover or line all body parts both externally and internally. With cells tightly packed together and with little intercellular space, the epithelial tissues form effective barriers between underlying tissues and external media. Thus, the epithelial tissues are extremely important in regulating the exchange of materials between different body parts. The portions of cell membranes found on the outer surface of an epithelial tissue are specialized to act as the regulators of the tissue strictly managing the flow of materials into and out of the epithelium. In contrast, those portions of cell membranes that are adjacent only to other epithelial cell membranes allow a less restricted flow of materials between epithelial cells. The outside surfaces of epithelial tissues often are highly specialized in other ways possessing cilia, hairs, or glands, which may arise from individual cells or groups of cells that have invaginated and which are capable of covering the outer surface of an epithelial tissue with mucous or waxy secretions. Epithelial tissues are separated from underlying tissues by extra cellular basement membranes.
Three types of epithelial cells exist: Squamous cells are thin and flat
cuboidal cells have a square shape in section and a polygonal shape when looking at the surface of a tissue
and columnar cells are much taller than they are wide.
Simple epithelial tissues are only one cell-layer thick and are named on the basis of their cell type: Simple squamous, simple cuboidal, and simple columnar epithelia are the three types of simple epithelial tissues.
Psuedostratified epithelial tissues appear to have more than one layer of cells, but, actually, all the cells of such a tissue reach the underlying tissue just as the cells of a simple epithelial tissue do.
Stratified epithelial tissues have two or more layers of cells.
In stratified epithelial tissues, the outermost layer of cells determines the name of the tissue: Stratified squamous, stratified cuboidal, and stratified columnar epithelia are the three types of stratified epithelial
tissues.
