Evidence for direct membrane retrieval following cortical granule exocytosis in Xenopus oocytes and eggs

Rapid exocytosis is typically followed by rapid resorption of exocytosed membrane; however, whether membrane retrieval occurs via indirect endocytosis of numerous small vesicles or direct resealing of the original, larger exocytotic vesicles is controversial. Here we show that cortical granule (CG) exocytosis in Xenopus oocytes and eggs is followed by rapid formation of endosomes as large as the CGs. Large endosomes are translucent, and their formation has the same developmental and pharmacological profile as CG exocytosis. Time course analyses show that large endosomes are not derived from small endosomes. Large endosome formation is triggered by stimuli that do not trigger increases in intracellular-free calcium and is insensitive to perturbation of microtubules by treatment with nocodazole. Perturbation of the f-actin cytoskeleton with latrunculin, however, sharply reduces large endosome formation. We conclude that CG membrane is directly retrieved in Xenopus oocytes and eggs and suggest that this retrieval is not directly dependent on an increase in intracellular-free calcium, but is dependent on the actin cytoskeleton.