Introduction
An epithelium is defined as a tissue that covers or lines the free surfaces of the body. It is one of four fundamental body tissues, with the other three basic tissues being muscle, nervous and connective tissues. Histologically, an epithelium is characterized as being composed of cells sitting upon a basement membrane. The cells are usually arranged in one or more layers with scant intercellular material, or they may sometimes be arranged in cords as in some endocrine glands.
One of the surfaces of an epithelium is exposed either to air or fluid, whereas the other surface is generally attached to a basement membrane that rests upon an underlying connective tissue. With rare exceptions, epithelium is not penetrated by blood vessels and must rely on diffusion processes from the underlying blood supply of the connective tissue for the exchange of nutrients, gases, and wastes (e.g., the endothelial lining of a blood vessel).
Epithelial tissues show numerous specializations of the free surface of cells or of the component cells themselves. For example, the glands of the skin and intestinal tract are derived from epithelium. The embryonic epithelium invaginates into the underlying connective tissue during embryonic development to differentiate into glandular structures. Remember that epithelial tissues are specialized to function in secretion, protection, excretion, absorption, lubrication, sensation, and even reproduction.
One should be able to classify any variety of epithelium according to its structure. An epithelium is usually named according to the appearance of the surface cells. The shape of the cells is important in naming the epithelium. Sometimes the shape referred to is the three dimensional one as in the case of squamous cells (scale-like) and sometimes the two dimensional appearance is referred to as it is seen when the cell is cut vertically to its base. Thus “cuboidal” cells which may appear as squares in sections are actually prismatic in shape with eight sides if seen in their entirety. “Columnar” cells are more elongated prisms. If a cell is surrounded and compressed equally on all sides by other cells, it is usually fourteen sided and appears as a polyhedron in a section.
Identify the following representative varieties of epithelial and be able to interpret the appearance even if it is not cut vertically to the basement membrane.