31-32) Epithelial Tissue and Types of Epithelia

Epithelial Tissue- is a sheet of cells that covers the body surfaces or lines a body cavity. It occurs in the body as covering and lining epithelium and glandular epithelium. The covering and lining epithelium forms the outer layer of the skin, dips into and lines the open cavities of the cardiovascular, digestive and respiratory systems, and covers the walls and organs of the closed ventral body cavity. Glandular epithelium fashions the glands of the body. Epithelium accomplishes many functions such as protection, absorption, flitration, excretion, secretion and sensory reception. Special characteristics of epithelial tissue that distinguishes it from over tissue types are cellularity, specialized contacts, polarity, being supported by connective tissue, is avascular (contains no blood vessels) and has regenerastion.

Classification of Epithelia- Each epithelium is given two names. The first name indicates the number of cell layers present and the second describes the shape of its cells.
-Based on the number of cell layers, there are simple and stratified epithelia. Simple Epithelia are composed of a single cell layer and typically found where absorption and filtration occur as well as where a thin epithelial barrier is desirable. Stratified Epithelia- consists of two or more cell layers stacked on top of the other. Stratified Epithelia are common in high-abrasion areas where protection is important such as the skin surface and lining of the mouth.
-All epithelial cells have six (somewhat irregular) sides. The apical surface view of an epithelial sheet looks like a honeycomb. This polyhedral shape allows the cells to be closely packed. Because epithelial cells vary in height, there are three common shapes of epithelial cells: squamous, cuboidal and columnar cells. Squamous cells are flattened and scalelike. Cuboidal cells are boxlike and approximately as tall as they are wide. Columnar cells are tall and column shaped. In each case, the shape of the nucleus conforms to that of the cell.