Distribution of the cytoplasmic activity inducing germinal vesicle breakdown in frog oocytes

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Abstract
Full‐grown oocytes of Rana pipiens were dissected out of the follicles and induced to undergo germinal vesicle breakdown (GVBD) in vitro by progesterone treatment. When the progesterone treatment was carried out after displacing the germinal vesicle (GV) from the animal half to the vegetal half by centrifugation (200–400 × G for 10 minutes), a considerable delay of GVBD took place although all the GVs eventually broke down. GVs displaced near the vegetal pole frequently migrated towards the animal pole and gave off a polar body in the equatorial zone of the oocyte. When an oocyte was tightly constricted along the equatorial line with thread so that the migration of the GV was prevented, the frequency with which the GV broke down in the vegetal half was considerably decreased depending on the tightness of constriction. The ooplasm in the animal and vegetal halves of the progesterone‐treated oocytes was tested for GVBD inducing activity by injecting it into untreated oocytes which contain an intact GV. It was found that the ooplasm from the vegetal half induced GVBD in the recipients with a lower frequency than that from the animal half. To localize the GVBD inducing activity in the ooplasm, oocytes were centrifuged after progesterone treatment at the interface between Ringer's solution and Ficoll solution. The GVBD inducing activity of the ooplasm of the animal pole was increased after removal of yolk and pigment granules by a brief centrifugation (4000 × G for 5–10 minutes). Prolonged centrifugation (4000 × G for 2 hours) brought about stratification of the ooplasm. From the centripetal to the centrifugal side, five layers were distinguished: oil cap, fluid hyaline ooplasm, gelatinous hyaline ooplasm, pigment granules and yolk. The highest activity of GVBD induction was found in the hyaline ooplasm. Based on these results, a mechanism of the GVBD induction in the frog oocyte is discussed.