Immunohistochemical localization of pulmonary surfactants in fetal rabbit lung

Differentiation and proliferation of the alveolar epithelial cells in fetal rabbit lung were studied by using immunohistochemical techniques. To observe the differentiation of type II alveolar epithelial cells, an antibody to purified rabbit surfactant was prepared and its specificity was checked by Western blot analysis. This antibody reacted with the 29-35 kDa protein group, which was presumed to be the major surfactant associated protein group in rabbit. After immunohistochemical staining, surfactant stained type II cells first appeared during the canalicular stage of fetal lung development. And they were detected prominently in the proximal portion of primitive alveolar zones and the number of stained cells increased rapidly in the succeeding gestational days. Clara cells were also reacted with this antibody during the alveolar stage. The distribution of DNA-synthesizing cells in the developing lung indicated that proliferation of alveolar epithelial cells is reduced earlier in the proximal portion than in the distal portion of primitive alveolar zones. These results suggest that differentiation of type II cells begins during the canalicular stage and it begins earlier in the proximal portion of primitive alveolar zones, which has already ceased prominent proliferation.