This variety of epithelium consists of thin cells arranged edge to edge to form a single layer. With the exception of certain regions on the surfaces of the pulmonary alveoli, the cells are more cytoplasmic and granular than are those of endothelium which this tissue in other respects closely resembles.
During foetal life the smaller air-passages and alveoli of the lung are lined by a pavement-epithelium, the cells of which are nearly as thick as those of some varieties of cubical epithelium. When, however, the lung is expanded by the respiratory acts following birth, many of the cells lining the alveoli become greatly extended and flattened until their bodies are thin and membranous and their nuclei inconspicuous or even destroyed. These greatly flattened epithelial cells are found covering those portions of the alveolar walls in which the capillary bloodvessels are situated and permit a ready interchange of gases between the air in the alveolar cavities and the blood circulating in their walls. Many of the epithelial cells covering the tissues in the meshes between the capillaries retain the cytoplasmic and granular character possessed before birth and appear capable of multiplying and, perhaps, replacing such of the thinner cells as may be thrown off or destroyed.
It will be evident, from the foregoing descriptions, that there is no sharp structural line separating cubical from pavement-epithelium. Functionally, pavement-epithelium is a much less active tissue than the cubical variety.