Endocytosis in alveolar type II cells: effect of charge and size of tracers

The type II cell of the alveolar epithelium of adult rats has been studied by electron microscopy to determine its ability to endocytose electron-dense tracers of differing molecular charge and size. The tracers studied were native (anionic) ferritin, cationic ferritin, 70-kilodalton dextran, and colloidal carbon. Alveolar macrophages ingested all tracers in large amounts, while type II cells took up significant amounts of cationic ferritin only. This tracer was observed, in sequence, within small pinocytic vesicles, large electron-lucent multivesicular bodies, small electron-dense multivesicular bodies, and, by 30 min after instillation, within the nonlamellar matrix of lamellar bodies. By 2 hr all lamellar bodies in any labeled cell contained cationic ferritin. Cationic ferritin also appeared to be transported from alveoli to interstitium by vesicles of type II, but not type I, cells. Native ferritin and dextran were observed in the same organelles as cationic ferritin but in much smaller amounts; colloidal carbon was not taken up by type II cells. These tracers were not observed in the alveolar interstitium. Type II cells therefore appear to internalize preferentially the tracer that binds to the cell membrane. Once within the cell, the tracer may enter a pathway that terminates in a lamellar body or, in the case of cationic ferritin, may be ferried across the cell.