| Causes and mechanism for the ageing-associated decline in female fertility of human and animals have always been a subject of studies and controversies. It has been showed that poor oocyte quality resulting from errors in meiosis is one of major causes for the ageing-associated decline in female fertility. Germinal vesicle ( GV ) transfer is a technique developed in recent years, which may be a valuable research procedure to study cell cycle regulation and characterize the cytoplasmic-nuclear interplay during maturation and fertilization of oocytes. Because GVs can be exchanged between oocytes of different quality or age, it is expected that GV transfer may be used to solve genetic or developmental anomalies in oocyte maturation, e.g. the increased incidence of aneuploidy in oocytes of women of advanced age. Besides, effect of the amount of cytoplasm on oocyte maturation and early embryo development has also interested many researchers. Using KM mouse as a animal model for studies on the maternal age effect, we compared quantity, quality and in vitro developmental competence of oocytes from different age groups of mice to elucidate the possible mechanism of the decline in ageing-associated fertility. In order to find out the cytoplasmic and nuclear mechanisms regulating meiosis and the relations between meiotic errors and age-associated changes in the oocytes, GVs were exchanged between GV oocytes from different age groups of mice. To elucidate effect of modified nucleocytoplasmic ratio on oocyte maturation, early embryo development and meiosis, different amount of cytoplasm was removed from cells and the GV oocytes were made into 4/5, 3/4,2/3, 1/2, 1/3, 1/4 and 1/5 oocytes. The results are as follows:Number of newborn pups decreased gradually and date of pupbirth was prolonged with maternal age of KM mice. The mice showed ageing-associated decrease in oocyte pool, as indicated by significantly decreased number of GV oocytes from ovaries, MII oocytes from oviducts and follicles in ovary sections in the aged mice. The mice showed also ageing-associated decline in oocyte quality, as indicated by increased frequency of premature chromatid separation, spindle disruption and/or chromosome misalignments in MII oocytes of aged mice. The GV oocytes from aged mice normally matured in vitro. The percentages of pronuclear-stage embryos, 2-cell embryos and blastocyst derived from MII oocytes of agedmice by in vitro fertilization ( IVF ) were comparable with ones of young mice, but pronuclei formed 1 h later in aged mice than in young mice. Bovine sperm extract or Sr2+-induced Ca2+ oscillations in MII oocytes and distribution of protein kinase C a ( PKC a ) in both Mil oocytes and early embryos were not differed between young and old mice.. Electrofusion rates were equivalent among 9 types of GV-cytoplast complexes constructed by GV exchange between 6-8-week-old mice and 6-8-week-old, 3-month-old, 6 -month-old, 9-month-old and 12 -month-old mice respectively. Maturation rates of 9 types of reconstructed GV oocytes were not differed with maternal age. After maturation of reconstructed GV oocytes, the percentages of pronuclear-stage embryos and 2-cell embryos developed from 9 types of reconstructed oocytes by IVF or artificial activation were not influenced by cytoplasmic or nuclear changes resulting from oocytes of different age groups of mice. The oocytes reconstructed by GV exchange between young and young or young and aged mice showed normal bovine sperm extract or Sr2+-induced Ca2+ oscillations in MII stage and normal PKC a distribution in MII oocytes and early embryos. Normal meiosis appeared in the oocytes reconstructed by GV exchange between young and young mice. Like the oocytes of aged mice, the reconstituted oocytes by GVs of aged mice and enucleated GV oocytes of young mice showed significantly increased frequency of premature chromatid separation, spindle disruption and/or chromosome misalignment. In contrast, the reconstituted oocytes by GVs of young mice and enucleated GV oocytes of aged mice b...
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