Alveolar differentiation in rabbits

: Lung tissue of the white New Zealand rabbit was examined by transmission and scanning electron microscopy from the 23rd to the 30th day post conception (p.c.). The following results were obtained: 1. About day 23 p.c., when the development of capillaries increases, the "canalicular period" starts. This is followed by the "terminal sac period" characterized by the beginning of alveolarisation. On day 28 p.c. typical alveoles can be found. 2. The Pneumoplast is the stem cell of the pneumocyte type I as well as type II. They differentiate parallel in either one or the other. This stem cell of the entodermal origin has a single cilium. During the period of single cilia growth the cell is not mitotic. 3. The maturation of the lamellar body, typical of the pneumocyte type II, can be accomplished in a direct as well as in a indirect way of synthesis. Transitions between both are possible. 4. The most important factors of differentiation are collagenic induction substances beside nerval and humoral factors. Those humoral factors can be transported easier into the cells with advancing capillarisation as a result of the shortened distance of diffusion.