epiteliile

Epithelia, also known as epithelial tissue, are one of the four basic types of animal tissue, along with connective tissue, muscle tissue, and nervous tissue. Epithelia cover the surfaces of the body, line internal cavities and organs, and form glands. Their primary functions include protection, secretion, absorption, excretion, filtration, diffusion, and sensory reception. Epithelial cells are tightly packed, forming continuous sheets. They are avascular, meaning they do not contain blood vessels, and receive nutrients by diffusion from underlying connective tissue. Epithelial cells are polarized, with an apical surface facing a lumen or external environment and a basal surface attached to a basement membrane.

Epithelial tissues are classified based on the number of cell layers and the shape of the cells. In terms of layers, they can be simple (a single layer of cells) or stratified (multiple layers of cells). In terms of cell shape, they can be squamous (flat), cuboidal (cube-shaped), or columnar (column-shaped). Combinations of these classifications exist, such as simple squamous epithelium, stratified cuboidal epithelium, and pseudostratified columnar epithelium, which appears stratified but is actually a single layer of cells with varying heights. Transitional epithelium is a specialized type found in the urinary system that can stretch and recoil. Glandular epithelia are specialized for secretion and can be exocrine, secreting substances onto an epithelial surface or into a duct, or endocrine, secreting hormones directly into the bloodstream.