Hypertrophy and hyperplasia of alveolar type II cells in response to silica and other pulmonary toxicants.

Alveolar Type II cells serve two major functions in the lung, both of which are essential for the preservation of normal lung function. First, Type II cells synthesize and secrete pulmonary surfactant, and second, they function as progenitor cells for maintaining the alveolar epithelium. The Type II cell population of the lung is quite sensitive to the deposition of toxicants in the distal lung, responding in two principal ways. Damage to the Type I epithelium stimulates Type II cells to proliferate and subsequently differentiate to replace the injured Type I cells. Second, a portion of the Type II cell population may become hypertrophic. Both of these events are frequent findings in the diseased or damaged lung. The Type II cell changes are often associated with increases in surfactant pools. In those cases where ultrastructural characteristics of hypertrophic Type II cells were examined, the appearance of these cells was consistent with that of an activated cell type. Alterations in the lamellar body compartment are a common finding in hypertrophic Type II cells, with increases in both lamellar body size and number. It is likely that the hypertrophic, or activated, Type II cells account for the increased levels of surfactant found in the lungs after exposure to a variety of toxic agents. We examined, in detail, Type II cell hyperplasia and hypertrophy induced by silica deposition. Both Type II cell hyperplasia and hypertrophy were prominent responses. The proliferative response led to an approximate doubling of the number of Type II cells in the lung.(ABSTRACT TRUNCATED AT 250 WORDS)