Acute and subacute effects of injury on the canine alveolar septum

The effect of papain on the prevalence and distribution of alveolar macrophages, alveolar septal interstitial tissue gaps and epithelial cells in normal canine pulmonary alveoli was studied by light and electron microscopy. Serial sections of whole alveoli from control animals and from animals sacrificed 4 h, two weeks and one month after the instillation into one lung of crude papain in saline solution containing India ink as a marker were compared. In dogs, as in humans, there is normally a zonal distribution of alveolar macrophages and type 2 cells at alveolar junctional sites. We hypothesize that early alveolar septal injury takes place at these junctional sites, judging from concentration of alveolar macrophages and interstitial septal gaps at these sites following papain exposure, and also that septal repair activities are greatest at these sites, in view of the preponderance and high prevalence of type 2 cells occupying interstitial septal gaps in junctional zones. Consequently, the type 2 cell may play a role beyond that of merely replacing type 1 epithelial cells following alveolar septal injury.