Epithelium

Lore & Background

Epithelial tissue is classified by the number of cell layers and the shape of its cells. Simple epithelium is a single layer of cells, each in direct contact with the basement membrane, and is found where absorption and filtration occur. Stratified epithelium has two or more layers and is found where body linings must withstand mechanical or chemical insult. Pseudostratified columnar epithelium appears stratified due to nuclei at different heights but is a single layer. Transitional epithelium can change from squamous to cuboidal depending on tension.

Reader's Guide

Epithelial tissue is significant as the body's primary barrier and interface. It protects underlying tissues from radiation, desiccation, toxins, pathogens, and physical trauma. It regulates chemical exchange between tissues and body cavities, secretes hormones, sweat, mucus, and enzymes, provides sensation, and absorbs water and digested food in the digestive tract. All glands are made up of epithelial cells. Epithelial layers contain no blood vessels and receive nourishment via diffusion from underlying connective tissue through the basement membrane. Cell junctions are especially abundant in epithelial tissues, including tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions. Epithelia turn over at some of the fastest rates in the body, with cell division matching cell death to maintain constant cell numbers.

Did You Know?

• Epithelial tissues lack blood or lymph supply but are supplied by nerves.
• The outermost layer of human skin is composed of dead stratified squamous, keratinized epithelial cells.
• Pathologists do not consider endothelium and mesothelium (both derived from mesoderm) to be true epithelium, labeling their cancers sarcomas rather than carcinomas.
• Epithelial cells can adopt shapes of varying complexity from polyhedral to scutoidal to punakoidal.