The oocyte as an endocytic cell

Oocytes of Xenopus laevis grow primarily by sequestering vitellogenin (VTG) selectively from the maternal bloodstream. Morphological observations have demonstrated that an endocytic system is responsible for VTG uptake. Binding studies indicate the presence of 2-28 X 10(10) surface VTG receptors per oocyte. These are continuously internalized into endosomes whether or not they are occupied by VTG, and other macromolecules may become trapped in the process. VTG-containing endosomes give rise to dense transitional yolk bodies; these fuse with yolk platelets only after the cleavage of vitellogenin. In the absence of VTG, endosomes appear to fuse directly with yolk platelets. From these observations it is postulated that receptor occupancy can act as a transmembrane signal which directs the postendocytic compartmentation of proteins. Yolk platelet proteins do not undergo subsequent turnover, whereas adventitiously incorporated protein is gradually lost from the oocyte by a dual mechanism which may involve both lysosomal proteolysis and secretion from the oocyte as a consequence of membrane recycling. Although these observations may not apply to all growing oocytes, the X. laevis oocyte nevertheless appears to be a particularly attractive experimental system for studies of endocytic compartmentation and membrane receptor recycling.