Breakdown of the germinal vesicle in pig oocytes in vivo and in vitro.

The breakdown of the pig germinal vesicle (GV) was studied in follicular oocytes matured in vivo and in vitro. For better characterization, the whole process was divided into four well-defined stages, based on the chromatin changes, and on nucleolus and nuclear membrane disappearance. In the intact germinal vesicle (GV I) nuclear membrane and nucleolus are clearly visible and chromatin forms a ring or horseshoe around the nucleolus. In the GV II a fre orceinpositive structures (chromocenters) on the nuclear membrane can be detected. For the GV III slightly stained chromatin clumps, localized especially around the nucleolus, and the beginning of strand formation are typical. In the last stage - GV IV - the nuclear membrane is less distinct and the nucleolus disappears completely. Chromatin is seen as an irregular network or as individual bivalents. According to these criteria GV breakdown in vivo was completed in most oocytes between 20 to 24 hours after HCG injection. In culture a similar stage of development was reached between 16 to 20 hours. This difference in the progress of nuclear changes appeared at the beginning of nuclear maturation and remained unchanged throughout the whole period studied.