Temporal Changes Associated with the Degeneration of the Rat Oocyte1

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Abstract
The ultrastructure and ability of oocytes to mature in vitro were examined in oocytes collected from pregnant mare''s serum gonadotropin (PMSG)-primed immature rats. By 96 h after PMSG, cytoplasmic organelles tended to aggregate and the nuclear envelope increased in width and lost its trilaminar configuration. Prior to these ultrastructural alterations, the oocyte had an increased rate of germinal vesicle breakdown (GVB), lost its ability to form a polar body in vitro, no longer required pyruvate for GVB and fragmented when placed in vitro. Apparently, the viability of the oocyte gradually changes within the preovulatory follicle. These degenerative changes appear to be related to the physiological state of the follicle and the length of time that the oocyte remains within the follicle.