Alveolar epithelial responses in experimental streptococcal pneumonia

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Abstract
We describe an experimental model of pneumonia in Wistar rats evoked by Streptococcus sanguis. The lesion developed rapidly as a confluent bronchopneumonia of the single‐lobed left lung. Except at the extreme base, where an abscess formed, the pneumonic process thereafter resolved, and most of the lung appeared microscopically and ultrastructurally normal 8 days after infection.Sequential electron microscopic studies revealed that in the areas of lung which subsequently resolved, damage was restricted to type 1 pneumocytes. Within 24 h of infection, the unaffected type 2 pneumocytes were observed to proliferate, transform into elongated pneumocytes of intermediate morphology, and then undermine and strip off the damaged type 1 cells from the subjacent basement membrane. Thereafter, the intermediate type pneumocytes completed their transformation into definitive type 1 cells, thus completing the repair process. We hypothesize that this represents an accelerated form of the normal type 1 replacement mechanism, and that uncomplicated epithelial repair following acute alveolar damage is possible if type 2 pneumocytes escape significant damage, so that they retain their capacity to proliferate and differentiate into type 1 epithelium.