Sinh học - Chapter 40: An introduction to animal structure and function

Occurring in sheets of tightly packed cells, epithelial tissue covers the outside of the body and lines organs and cavities within the body. The cells of a epithelium are closely joined and in many epithelia, the cells are riveted together by tight junctions. The epithelium functions as a barrier protecting against mechanical injury, invasive microorganisms, and fluid loss. The free surface of the epithelium is exposed to air or fluid, and the cells at the base of the barrier are attached to a basement membrane, a dense mat of extracellular matrix.