PULMONARY ALVEOLAR CELL INCLUSIONS: THEIR DEVELOPMENT IN THE RAT.

Cytoplasmic inclusions in the special pulmonary alveolar cells of adult, newborn, and fetal rats were investigated by electron microscopy and histochemistry, and lung extracts were analyzed for surface-tension activity. The inclusions were positive in the periodic acid-Schiff test after digestion with diastase and were not stained with toluidine blue; they formed myelin figures and possessed alkaline phosphatase activity. Both the typical surface tension activity and the inclusions developed 2 days before birth. These properties differentiated the inclusions from mitochondria.