This variety resembles stratified epithelium in forming layers several cells in thickness, but differs in the character of its superficial cells. These do not undergo the horny change peculiar to stratified epithelium, but continue to increase in size, forming a covering of very large cells lying upon those beneath. Under these largest superficial cells are pyriform cells lying with their larger, rounded ends next to the topmost layer, while their deeper and more attenuated ends lie between the oval or round cells that form the one or two deepest layers of the epithelium and rest upon the underlying tissues.
Transitional epithelium is found lining the renal pelves, ureters, and bladder. Its structure permits of a considerable stretching of the tissues beneath without rupture of the epithelial layer over them, the cells of which become flattened to cover the increased surface, to return to their first condition when the viscus which they line is emptied. This is notably the case in the bladder, the epithelial lining of which may be taken as a type of this variety of tissue.
The functional activities of epithelium are in marked contrast to the comparatively inert character of endothelium. The cytoplasmic nature of the epithelial cell, when contrasted with the poverty in cytoplasm of the cell in endothelium, would lead us to expect this difference in the cellular activities of the two tissues. At the beginning of this chapter a sketch of the manifold functions of epithelium was given. It is a fair general statement of its usefulness to say that epithelium is chiefly concerned in bringing about chemical changes in substances brought to it. Sometimes these substances are elaborated into fresh cell-constituents, and the activity of the tissue is displayed chiefly in an active multiplication and growth of its cells. This is especially true in the stratified variety, where protection is provided by a constantly renewed supply of cells. In other cases the substances received by the cells are elaborated into definite compounds destined to form the essential constituents of a secretion. This secretory function of epithelium is an extremely important one, and for its performance that tissue is usually arranged in a special structure or organ, called a gland. A brief statement of the general characters and classification of these organs may here appropriately find a place.