Mitotic index of the bronchial and alveolar lining of the normal rat lung.

Cell turnover in the rat airway epithelium has been studied at 5 levels of the bronchial tree after administration of either colchicine or tritiated thymidine, and in male and female rats of 3 age groups. This is the first study to include all these features of the same species. In the young animals the Mitotic Index (number of cells in division per 1,000 nuclei) decreased progressively to the periphry and was higher in the male than in the female, although the rate of weight gain was the same for both. In the oldest animals no difference was observed between the various airway levels or between the sexes. In the intermediate group the proximal to distal decrease was apparent but the difference with sex was not. The concentration of cells per unit length of airway is higher in the trachea and main bronchus than in intrapulmonary airways. (In the intrapulmonary airways virtually no basal cells are present.) This means that a given Mitotic Index represents replacement of a larger area of epithelium in small airways than in large ones.