Light and Electron Microscope Studies on Experimental Pneumonitis Induced by Blasticidin-S, with Special Reference to Alveolar Regeneration

In the experimental pneumonitis of rabbits induced by the agricultural chemical, Blasticidin-S, regeneration of the alveoli was observed by light and electron microscopy. The animals were singly injected with BC-S solution through the trachea into the alveoli and sacrificed at certain intervals. The non-ciliated epithelial cells at the terminal portion of the respiratory bronchiole began to proliferate three days following BC-S administration. These proliferated epithelial cells extruded outward to form glandular structures five and six days after the treatment. These structures increased in number and blood capillaries developed beneath their epithelia seven and eight days after the treatment. The epithelial cells lining the glandular structures began to differentiate cytologically after nine and ten days, forming both types of alveolar cells, i.e., great and squamous alveolar epithelial cells twelve and fourteen days after the treatment. Blood-air barriers were accomplished by this stage. After the ninth day the epithelial cells, proliferating and branching off, began to form new alveoli also within the destroyed masses of the pulmonary tissues.In this experimental study it was concluded that the regeneration of alveoli originates from the non-ciliated epithelial cells in the terminal portion of the respiratory bronchioles. These cells seemed to correspond to the special non-ciliated cells found in the normal rabbit which differed in fine structure from ordinary non-ciliated cells, Clara cells, and were supposed to be cells of an undifferentiated type. The detailed processes of the differentiation of both types of alveolar epithelial cells were described and discussed.