The chronological and morphological changes of the nucleus during mouse oocyte maturation and fertilization were systematically studied. Although most oocytes went through GVBD 2-4 hrs after culture, 13.6% remained at GV stage 8 hrs after culture.TEM observation revealed that nucleoli of oocytes which failed to go through GVBD were composed of fibrillar-granular component,small vacuoles and fibrillar centers or showed small vacuoles on nuclear surface. During GVBD, the nucleoli became smaller and smaller and finally disappeared with the nuclear-associated chromatin dislocated to the periphery. Nuclear membrane with attached chromatin became folded and electronic dense cores appeared in the center of chromatin clumps at the same time.The last event of GVBD was the disruption of nuclear membrane.At the end of the 5th hr after culture, meiosis progressed to prometaphase I.Chromosomes,distributed in the original GV area free of organelles,were surrounded by large quantity of mitochondria and small SER vesicles. At the end of the 12th hr after culture,48. 1% of the oocytes emitted PB1.Decondensing sperm head and early male pronuclcus(mPN)with condensed nucleoli were found 1-2 hrs after insemination.The formation and enlargement of female PN(fPN) occurred a little earlier than that of mPN. 33.3% finished syngamy at 8-9 hrs after insemination.The process of nucleolus formation was reverse to that in GVBD. The oolemma modification caused by cortical reaction could effectively inhibit polyspermy.in contrast,there were sperm binding to the oolemma where CGs failed to be released. In addition, PB2 was emitted 2-5 hrs after insemination. The difference between PB1 and PB2 as well as the abstriction of polar body were also discussed.
