Endocytosis and multivesicular body formation in rabbit luteal cells during pseudopregnancy

Horseradish peroxidase (HRP) was injected directly into the corpora lutea of rabbits on different days of pseudopregnancy. Young luteal cells have also been incubated "in vitro" in a medium containing horseradish peroxidase. In the "in vivo" experiments the 5 to 9 days old luteal cells take up far more horseradish peroxidase than the older ones (14-20 days). Different types of endocytic vacuoles, MvB's and also some DB's are already peroxidase positive shortly (15-20 min) after injection of the tracer. In the "in vitro" experiments the cells are more heavily loaded. The normal morphology of pale, dense and terminal MvB's is also described and staining with PTA at low pH provides further information on changes occurring in the MvB's. The various findings on multivesicular bodies are compared and two possible pathways for endocytic vacuoles are proposed: one to DB's the other to MvB's. The related phenomena of the uptake of material and the internalization of plasma membrane are discussed in the light of the possible function of endocytosis in luteal cells.