mucosa A lining of mostly endodermal origin, covered in epithelium, which is involved in absorption and secretion. They line various body cavities that are exposed to the external environment and internal organs. It is at several places continuous with skin: at the nostrils, the lips, the ears, the genital area, and the anus. The sticky, thick fluid secreted by the mucous membranes and gland is termed mucus. The term mucous membrane refers to where they are found in the body and not every mucous membrane secretes mucus[WP]. region of mucosa organ mucosa mucosa of organ mucosa of organ part mucosal region tunica mucosa mucous membrane organ component layer