Mouse oocyte maturation: the effect of modified nucleocytoplasmic ratio

Fully grown mouse oocytes isolated from large antral follicles and cultured in vitro complete their maturation rip to the second metaphase with extrus ion of the first polar body (1PB) with a 40/50 proportion (80 %). When thei r cytoplasmic volume is, however, reduced before the onset of culture, the frequency of oocytes completing maturation gradually decreases. In the half oocytes, 66 % (33/50) extruded 1PB, while in third oocytes the proportion was 57 % (28/49) and in quater oocytes no polar bodies were extruded. The t ime course of germinal vesicle breakdown was also delayed in comparison to the decreased cytoplasmic volume. Moreover, the isolated germinal vesicles surrounded with a thin cytoplasmic rim only remained intact after a prolong ed culture. The full competence of complete maturation can be restored by f usion of an additional cytoplast to the manipulated nucleate parts. We post ulate that a critical nucleocytoplasmic volume ratio is absolutely necessar y for normal maturation in mammalian oocytes. (C) Inra/Elsevier, Paris.