Lung, Epithelium, Alveolus - Hyperplasia

Alveolar epithelial hyperplasia is a proliferation of type II pneumocytes and may be primary (Figure 1, Figure 2, Figure 3, and Figure 4) or secondary to type I pneumocyte injury or inflammation (Figure 5). Type II pneumocytes are thought to be the progenitors of type I cells. Type I pneumocytes are especially vulnerable to oxidant injury, and proliferation of type II pneumocytes is often seen following injury and loss of type I cells. When alveolar epithelial hyperplasia is secondary to type I pneumocyte damage, it is usually associated with inflammation, as opposed to primary alveolar epithelial hyperplasia, which is typically not associated with inflammation. Foci of hyperplasia are characterized by an increase in closely spaced, cuboidal type II cells lining alveolar septa. The cells lack cellular atypia, seldom contain mitotic figures, and may or may not have prominent cytoplasmic vacuoles. The alveolar architecture is maintained, and the focus does not compress the surrounding parenchyma. These morphologic characteristics differentiate hyperplasia from adenoma, in which cellular atypia, mitotic figures, and compression of adjacent tissue may be present and the normal architecture is distorted. Accumulations of alveolar macrophages may be associated with primary hyperplasia (although the accumulations are most severe when associated with alveolar bronchiolar carcinomas). In some cases, some of the cells in a focus of alveolar epithelial hyperplasia (primary or secondary) may have features of atypia (Figure 6), such as enlarged or oddly shaped nuclei or binucleation. Primary alveolar epithelial hyperplasia is thought to be the precursor lesion to alveolar/bronchiolar adenomas and carcinomas, especially when there are features of cellular atypia.