Lung, Alveolar/Bronchiolar Epithelium - Hyperplasia

Alveolar/bronchiolar epithelial hyperplasia is hyperplasia of the epithelial cells in the centriacinar region (terminal bronchiole/alveolar duct and adjacent alveoli) of the lung (Figure 1, Figure 2, Figure 3, Figure 4, and Figure 5). It is this consistent location that differentiates this lesion from alveolar epithelial hyperplasia, which is randomly located in the alveolar parenchyma. The cells are generally cuboidal with round to oval nuclei. The majority of the cells are nonciliated, with a variable number of ciliated cells, and some appear to have apical blebs. Given the location of this lesion, it may be a precursor to alveolar/bronchiolar neoplasia. As with alveolar/bronchiolar neoplasms, it is unclear what type of cell is proliferating in this lesion. The proliferating cells may originate in the terminal bronchioles or in the alveolar ducts. The morphology of the cells is similar to that of the terminal bronchiole cells. However, in alveolar/bronchiolar neoplasms, in some of the cells immunohistochemical staining is consistent with Clara cells, and in other cells it is consistent with alveolar epithelial cells. As with alveolar epithelial hyperplasia, this lesion can occur concurrently with, and may be secondary to, inflammation (Figure 4 and Figure 5).