épithéliums

Épithéliums are one of the four basic types of animal tissue, along with connective tissue, muscle tissue, and nervous tissue. Epithelial tissue lines the surfaces of the body, both externally and internally. It forms the protective outer layer of the skin, the lining of the digestive tract, respiratory tract, and urogenital tract, and also constitutes glands and their ducts. The primary functions of epithelia include protection, secretion, absorption, excretion, filtration, diffusion, and sensory reception.

Epithelial cells are typically arranged in sheets, with very little extracellular matrix between them. They are characterized by close cell-to-cell adhesion and have a distinct apical surface, which faces the lumen or external environment, and a basal surface, which rests on a basement membrane. This basement membrane anchors the epithelium to the underlying connective tissue. Epithelial tissues are avascular, meaning they do not contain blood vessels. Instead, they receive nutrients through diffusion from the underlying connective tissue.

Epithelia are classified based on their shape and the number of cell layers. The basic shapes are squamous (flat), cuboidal (cube-shaped), and columnar (tall and rectangular). Epithelia can be simple (a single layer of cells) or stratified (multiple layers of cells). Special types include pseudostratified columnar epithelium, which appears to have multiple layers but all cells contact the basement membrane, and transitional epithelium, which can stretch. Glandular epithelia are specialized for secretion and can be exocrine (secreting onto a surface) or endocrine (secreting hormones into the bloodstream).