epitelit

Epitelia are a type of biological tissue that covers the surfaces of the body, lines internal cavities and organs, and forms glands. These tissues are characterized by tightly packed cells that are arranged in continuous sheets. The primary functions of epithelia include protection, secretion, absorption, excretion, filtration, diffusion, and sensory reception.

Epithelial cells are avascular, meaning they do not contain blood vessels. Instead, they receive nutrients by diffusion from underlying connective tissues. They are also highly regenerative, with cells constantly being replaced. The arrangement of epithelial cells can vary, leading to different classifications. Simple epithelia consist of a single layer of cells, while stratified epithelia have multiple layers. The shape of the cells can be squamous (flat), cuboidal (cube-shaped), or columnar (tall and rectangular). These classifications are often combined, such as simple squamous epithelium or stratified cuboidal epithelium.

The apical surface of an epithelial cell faces a free space, which could be the external environment or the lumen of an organ. The basal surface is attached to the underlying basement membrane, a thin layer of connective tissue. Junctions between epithelial cells, such as tight junctions, desmosomes, and gap junctions, provide structural support and regulate the passage of substances between cells. Glands, which are derived from epithelial tissue, are specialized for secretion and can be exocrine (secreting into ducts) or endocrine (secreting hormones directly into the bloodstream).