epitéliát

Epitélium is a fundamental type of animal tissue that covers the external surfaces of the body, lines internal cavities and passages, and forms glands. It is characterized by cells that are tightly packed together with very little extracellular space. This arrangement creates a continuous sheet that serves as a protective barrier, facilitating or inhibiting the passage of substances.

Epithelial tissues are classified based on two main criteria: the number of cell layers and the shape of the cells. Regarding the number of layers, epithelia can be simple (a single layer of cells) or stratified (multiple layers of cells). Simple epithelia are typically involved in absorption, secretion, and filtration, while stratified epithelia provide protection against abrasion and other mechanical stresses.

Cell shapes are described as squamous (flat and thin), cuboidal (cube-shaped), or columnar (taller than they are wide). Combinations of these classifications, such as simple squamous epithelium or stratified cuboidal epithelium, describe specific types of epithelial tissues with distinct functions. For instance, simple squamous epithelium lines blood vessels and air sacs in the lungs, where efficient diffusion is crucial. Simple columnar epithelium, found in the digestive tract, often possesses microvilli to increase surface area for absorption.

In addition to forming protective coverings and linings, epithelial cells can differentiate to perform specialized functions. This includes secretion of mucus, hormones, and enzymes, as well as absorption of nutrients and ions. The apical surface of epithelial cells, which faces a lumen or external environment, may have modifications like cilia or microvilli. The basal surface rests on a basement membrane, a thin layer of connective tissue that anchors the epithelium.