LAMELLAR BODIES OF RAT ALVEOLAR TYPE-2 CELLS HAVE LATE ENDOSOMAL MARKER PROTEINS ON THEIR LIMITING MEMBRANES

Alveolar type 2 cells are known to take up surfactant phospholipids an d proteins from the alveolar space and recycle them into secretory org anelles via a receptor-mediated endocytic pathway. To clarify the intr acellular route(s) through which materials ingested by the cells are p rocessed, we examined the immunocytochemical localization of late endo somal and lysosomal membrane markers, rab 7 and lamp 1 proteins, withi n rat alveolar type 2 cells. The limiting membranes of lamellar bodies (LBs) showed positive immunoreactivity for both proteins, whereas mul tivesicular bodies (MVBs) exhibited positive immunoreactivity only for lamp 1 protein on free vesicles in the MVB lumen. From these findings , it is suggested that LBs are not only secretory granules, but also c onstitute one of the late endosomal compartments of the cells and that MVBs of this cell type may be targeted to cell organelle(s) other tha n lysosomes.