Epitheellagen

Epitheellagen is a term used to describe the organization and layering of epithelial tissue in animals. It refers to how epithelial cells are arranged into one or more cell layers that cover surfaces and line cavities, forming a protective, secretory, or absorptive barrier. Epithelial tissue is commonly classified by the number of cell layers (simple, stratified, pseudostratified) and by cell shape (squamous, cuboidal, columnar). Special forms include transitional epithelium.

The basal cell layer rests on the basement membrane and displays apico-basal polarity. Epithelial cells are joined by junctional complexes such as tight junctions, adherens junctions, and desmosomes. The apical surface often bears microvilli or cilia, reflecting tissue-specific roles. Examples include simple squamous epithelium lining blood vessels and body cavities; simple cuboidal epithelium in many glands and ducts; simple columnar epithelium in the intestinal mucosa with microvilli for absorption; stratified squamous epithelium forming the epidermis; and transitional epithelium lining the urinary tract.

Growth and renewal: epithelia originate from embryonic germ layers and renew continuously through stem and progenitor cells in the basal layer. Regeneration after injury depends on signaling and the integrity of the basement membrane.

Functions include protection, selective permeability, secretion, sensation, and filtration. Epithelia can be altered by disease or injury, leading to changes such as metaplasia, hyperplasia, or neoplasia.

See also: epithelium, epithelial tissue, basement membrane.