Epithelium

Epithelium. The skin and the various mucous membranes of the body are covered with an investment of cells known as epithelial cells. These cells are of various shapes. They may be columnar, a single layer of such cells lining the mucous mem brane of the alimentary canal from the cardiac end of the stomach downwards, and also occurring in the tubules of the kidney, the small bronchi, and in many glands. They may be squamous or flattened, as in the superficial layers of the epithelium of the skin [Epidermis]; or they may be conical, fusiform, or polyhedral. A simple epithelium is one in which only a single layer of cells is present. In the stratified forms there are several layers one superimposed upon the other. Thus excellent examples of stratified squamous epithelia (pavement epithelia) are met with in the epidermis, in the tongue cavity of the mouth, pharynx, oesophagus, and on the surface of the cornea, while stratified columnar epithelium is met with in the mucous membrane of the larynx, trachea, and large bronchi. The situations in which simple columneer epithelium occurs have already been detailed. A single layer of flattened cells, simple squamous epithelium (endothelium, as it is often called), is met with on the surface of serous membranes, and in the lining walls of blood-vessels. An epithelial cell consists of a nucleated mass of protoplasm, and the individual cells are in close apposition one with another, being merely separated by an insignificant amount of intercellular substance (inteistitial cement substance). Ciliated cells present on their free surface a fringe of very fine hair-like processes termed cilia; these processes in the living cell are in rapid movement, as can be well seen, for exeimple, in a scraping from the roof of the mouth of a frog (D in figure); the movement produces an appearance such as is seen when the wind sweeps over a field of corn. In the human subject ciliated cells occur in the trachea and in other parts, but are not nearly so widely distributed as in some of the lower animals. Two other varieties of epithelial cells may be mentioned: goblet or chalice cells, which are met with in mucous glands, and in the mucous membrane of the respiratory and alimentary tracts: and the prickle cells which are found in the deeper layers of the stratified pavement epithelium of the skin. Some epithelial cells are filled with black pigment granules i (choroid and iris, and in the skin of negroes).