Location of Epithelium
The four basic tissue types in the body are the epithelial, connective, muscular, and nervous tissue. These tissues exist and function in close association with one another. The epithelial tissue, or epithelium, consists of sheets of cells that cover the external surfaces of the body, line the internal cavities, form various organs and glands, and line their ducts. Epithelial cells are in contact with each other, either in a single layer or multiple layers. The structure of lining epithelium, however, differs from organ to organ, depending on its location and function. For example, epithelium that covers the outer surfaces of the body and serves as a protective layer differs from the epithelium that lines the internal organs. The overview illustration shows different types of epithelia in selected organs. Classification of Epithelium
Epithelium is classified according to the number of cell layers and the morphology or structure of the surface cells. A basement membrane is a thin, noncellular region that separates the epithelium from the underlying connective tissue. This membrane is easily seen with a light microscope. An epithelium with a single layer of cells is simple, and that with numerous cell layers is stratified. A pseudostratified epithelium consists of a single layer of cells that attach to a basement membrane, but not all cells reach the surface. An epithelium with flat surface cells is called squamous.When the surface cells are round, or as tall as they are wide, the epithelium is cuboidal. When the cells are taller than they are wide, the epithelium is called columnar. Epithelium is nonvascular, that is, it does not have blood vessels. Oxygen, nutrients, and metabolites diffuse from the blood vessels located in the underlying connective tissue to the epithelium.
Special Surface Modifications on Epithelial Cells
Epithelial cells in different organs exhibit special cell membrane...