Identification of Cells Labeled with Tritiated Thymidine in the Pulmonary Alveolar Walls of the Mouse1,2,3

Abstract

A detailed investigation with both the light and the electron microscope was carried out to determine what kinds of cells in the pulmonary alveoli are capable of cell division. The dividing cells were labeled with tritiated thymidine, and their behavior was studied for 30 days. It was shown that the number of labeled cells decreased by more than one-half between the fifth and tenth days, and decreased further between the tenth and twentieth, and the twentieth and thirtieth days. Electron microscopy revealed that the labeled cell population included all kinds of cells normally found in the alveolar walls. The largest group of labeled cells were leukocytes in the vascular system. The large decrease in the number of labeled cells between the fifth and tenth days was shown to be related to decrease in the number of labeled leukocytes in the capillaries of the lung. Endothelial cells and type 2 alveolar cells were classified as stem-type, renewing cell populations. The free alveolar macrophages were classified as a dividing transit renewing cell population.