Evidence for recovery of surface membrane and its fusion with Golgi cisternae has been obtained previously in several glandular cells. This study was conducted to determine whether or not membrane is similarly retrieved from the surfaces of plasma cells from lymph nodes (of rats immunized with horseradish peroxidase [HRP]) and mouse myeloma cells (RPC 5.4 and X63 Ag 8 cell lines). Electron-dense tracers (cationic and anionic ferritin, HRP) were used to trace the pathways followed by surface membrane recovered by endocytosis, and immunocytochemistry was used to identify the secretory compartments.

When plasma cells or myeloma cells were incubated with cationized ferritin (CF), it bound to the cell surfaces and was taken up in endocytic vesicles, for the most part bound to the vesicle membrane. After 30-60 min, it was found increasingly within lysosomes and in several secretory compartments- notably in multiple stacked Golgi cisternae and secretory vacuoles. By immunocytochemistry the secretory product (immunoglobulins) and CF could be demonstrated in the same Golgi components. When myeloma cells were incubated with native (anionic) ferritin or in HRP, these tracers were taken up in much smaller amounts, primarily within the contents of endocytic vesicles. With continued incubation, they appeared only in lysosomes. When cells were doubly incubated, first in CF and then in HRP, both tracers were taken up (often within the same endocytic vesicle), but they maintained their same destinations as when incubated in a single tracer alone: the content marker, HRP, was localized exclusively within the lysosomal system, whereas the membrane marker, CF, was found within elements along the secretory pathway as well as within lysosomes.

The findings demonstrate the existence of considerable membrane traffic between the cell membrane and the Golgi cisternae and lysosomes in both normal plasma cells and myeloma cells. Because myeloma cells behave like the glandular cells studied previously with regard to pathways of retrieved surface membrane, they represent a suitable and promising system for further studies of mechanisms and pathways of membrane retrieval and recycling in secretory cells.

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