What is epithelium?

Epithelium refers to large sheets of cells that form the outer covering of all surfaces of the body exposed to the external environment as well as organs within the body cavity. Surfaces of the body that are exposed to the external environment include the skin and respiratory, digestive, reproductive, and urinary tracts. All of these are covered by epithelium. Internal body cavities and organs that do not connect to the exterior of the body or to the external environment include blood vessels, air sacs in the lungs, glandular ducts, sweat and salivary glands, kidneys, and urinary bladder. All of these are lined by the epithelium. 

The cells of the epithelium are tightly packed together with no spaces between the cells. This prevents the movement of substances through intercellular spaces and is a critical feature of the epithelium as it covers vital organs in the body. 

There are 8 different types of epithelial tissues - Simple squamous, Stratified Squamous, Simple Cuboidal, Stratified Cuboidal, Simple Columnar, Stratified Columnar, Pseudostratified Columnar and Transitional epithelia or urothelium. They are differentiated by their shape and arrangement of the cells. However, they all have the same characteristics – cellularity, avascularity, polarity, attachment, and regeneration. 

Epithelium performs a wide range of functions including absorption, secretion, protection, sensation, transportation, and receptor function. Epithelium is also called epithelial tissue.