Sheet of cells that covers a body surface or lines a body cavity.
Occurs in body as covering and lining epithelium and glandular epithelium
Form boundaries between different environments
Forms many functions like protection, absorption, filtration, excretion, secretion, and sensory reception
Special characteristics: polarity, specialized contacts (fit close together to form continuous sheets), supported by connective tissue, avascular but innervated (it is supplied by nerve fibers but contains no blood vessels) and an ability to regenerate
Located in the kidneys, in the lungs and forms the walls of air sacs
2 types: Endothelium which is slick, friction reducing lining, allows efficient exchange of nutrients and wastes between the bloodstream and surrounding tissue cells. Mesothelium which is found in serous membranes which line the ventral body cavity and covers its organs.