Release of lamellar bodies from alveolar type 2 cells.

The release of lamellar bodies from rat alveolar type 2 cells has been studied during a 12hr dark/12hr light cycle. A proportion of the type 2 cell population was ultrastructurally abnormal. The cellular changes formed a spectrum raninging from mild to severe, culminating in rupture of the cell and release of its contents into the alveolar air space. The majority of aberrant cells were observed in animals killed during the dark period. A holocrine secretory mechanism is suggested for the release of lamellar bodies containing the surface active phospholipids of the alveolar lining. The release of lamellar material by a process of exocytosis was also observed, but only rarely, in spite of the numerous lamellar bodies seen in each cell cross-section.