Hyperplasia of type II pneumocytes in pulmonary lymphangioleiomyomatosis.

Background: Little is known of the morphology of the pneumocytes lining the parenchymal cysts in lymphangioleiomyomatosis (LAM).
Objective: To evaluate the structural characteristics of the alveolar epithelial cells in LAM.
Methods: Immunohistochemical and electron microscopic studies were performed on lung tissue from 22 women with pulmonary LAM.
Results: Epithelial cells that reacted with PE-10 (a mouse monoclonal antibody that recognizes the surfactant apoprotein A in type II pneumocytes) and TTF-1 (an antibody that identifies nuclear transcription factor found in type II pneumocytes) were the predominant cell type lining the surfaces of lesions of LAM and normal areas of lung. Scanning and transmission electron microscopic studies confirmed that these cells were type II pneumocytes as demonstrated by (1) apical microvilli, (2) electron-dense lamellar bodies, and (3) cytoplasmic projections that extended from the basal surfaces into the underlying connective tissue, where they made extensive contact with interstitial connective tissue cells. A few cells had morphologic characteristics of type I pneumocytes, including large flat surfaces lacking microvilli. Cells that appeared intermediate between type I and type II pneumocytes were observed occasionally.
Conclusions: These observations and the reactivity of these cells with antibody to proliferating cell nuclear antigen demonstrate that extensive hyperplasia of type II pneumocytes is a major characteristic of LAM.