Maturation of the mammalian oocyte is characterized in part by dissolution of the nuclear envelope, or germinal vesicle breakdown (GVB). By fluorescence microscopy after vital uptake of acridine orange (AO), redistribution and perinuclear accumulation of organelles corresponding to lysosomes occur before GVB in rat oocytes undergoing meiotic maturation in vitro. In follicle-enclosed oocytes explanted during the preovulatory gonadotropin surge (GS) and individually cultured as such in chemically defined medium at approximately 22 degrees C, lysosomes aggregated into disperse clusters after 30 min; by 60 min, perinuclear concentration of lysosomes and their essential disappearance from the cortical ooplasm were observed. GVB occurred within 120 min. In contrast, follicle-enclosed oocytes explanted before the GS displayed a generally homogeneous distribution of lysosomes and intact GV for up to 5 h in culture. In oocytes aspirated from follicles before the GS, partially denuded of granulosa cells, and cultivated without added hormone, most lysosomes concentrated around the GV within 60 min, with GVB occurring generally by 120 min. Luteinizing hormone (LH) added in vitro to the isolated preparation at 3 or 30 x 10(-8) M sharply accelerated these events. The effects of LH, not seen with 1.5 x 10(-8) M hormone, were blocked by anti-LH IgG. Up to 60 x 10(-8) M follicle-stimulating hormone or 80 x 10(-8) M prolactin were ineffective in accelerating lysosome redistribution or GVB. After GVB, lysosomes became once again uniformly dispersed and unresponsive, even to 60 x 10(-8) M added LH, a finding consistent with tachyphylaxis of target cells by independent criteria. The present data, all statistically significant at P less than 0.05, demonstrate that mobilization of lysosomes before GVB is a specific response to factors that promote resumption of meiotic maturation of rat oocytes.

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