Epitheleal Tissue

Biological containment and beyond

Found ubiquitously on and throughout the body, epithelial tissues cover all body surfaces, line body cavities and hollow organs, and are the most prevalent tissue types found in glands. Collectively referred to as the epithelium, these cells are responsible for performing a wide spectrum of biological functions including:

• Secretion
• Absorption
• Filtration
• Excretion
• Protection
• Diffusion
• Sensory reception

Because epithelial tissue forms body linings and coverings, its cells are, by necessity, packed tightly together with very little intercellular matrix in between. One surface of each cell is exposed (not touching other cells) while the opposite surface or “underside” is anchored to the connective tissue beneath by a protein-and-carbohydrate basement membrane.

Shapes and arrangements

Epithelial cells take one of three principal forms depending on location and function, and can be arranged in a single layer (simple epithelium) or multiple layers (stratified epithelium). Combinations of these shapes and depths produce a variety of common epithelial tissue types.

• Squamous epithelium contains cells that appear flattened or scale like; examples include capillary walls and the linings of the pericardial cavity and lung alveoli, allowing passive diffusion of substances across the epithelium
• Cuboidal epithelium cells are cube-shaped with round, centralized nuclei; often have absorptive, secretory or excretory functions; examples include collecting ducts found in the pancreas, kidney and salivary glands
• Columnar epithelium consists of column-shaped cells with heights at least four times their width; can be secretory, absorptive or excretory; examples include lining of stomach, colon and rectum; ciliated columnar cells are found in the female reproductive tract and uterus
• Pseudostratified columnar epithelium is actually a type of simple, monolayered epithelium where irregular cell thickness and nuclei appearing at different heights can mimic the appearance of multiple layers; found lining nasal, tracheal and bronchial airways

True stratified epithelial tissue can also be keratinized, meaning that the outermost layer consists of cells that have died, lost their cytoplasm, and now contain a durable protein called keratin. The epidermal layer of mammalian skin is an example of keratinized epithelium.