epitelyum

Epithelium is a fundamental tissue type in the animal kingdom. It forms the outer covering of the body, the lining of organs and cavities, and the glandular tissue. Epithelial cells are tightly packed together, forming continuous sheets that act as a barrier. This arrangement is crucial for protection, secretion, absorption, and filtration.

The structure of epithelial tissue varies depending on its function. It can be classified based on the number of cell layers and the shape of the cells. Simple epithelium consists of a single layer of cells, while stratified epithelium has multiple layers. Cell shapes are typically described as squamous (flat), cuboidal (cube-shaped), or columnar (column-shaped). Pseudostratified columnar epithelium, for example, appears to have multiple layers but is actually a single layer with nuclei at different levels. Transitional epithelium, found in the urinary bladder, is a specialized type that can stretch.

Epithelial cells have a distinct polarity, with an apical surface facing an open space (like a lumen) and a basal surface attached to an underlying basement membrane. This basement membrane provides structural support and anchors the epithelium to connective tissue. Epithelial tissues are avascular, meaning they lack blood vessels, and nutrients are supplied by diffusion from underlying connective tissues. Epithelial cells have a high regenerative capacity, allowing them to repair damage quickly.