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 up to the second metaphase with extrusion of the first polar body (1 PB) with a 40/50 proportion (80 %). When their 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 time 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 prolonged culture. The full competence of complete maturation can be restored by fusion of an additional cytoplast to the manipulated nucleate parts. We postulate that a critical nucleocytoplasmic volume ratio is absolutely necessary for normal maturation in mammalian oocytes. © Inra/Elsevier, Paris. mouse / oocyte / germinal vesicle / cytoplasm Resume h Maturation de l’ovocyte de souris : effet d’un rapport nucleo-cytoplasmique reduit. Des ovocytes murins ayant acheve leur croissance et extraits de grands follicules a antrum achevent leur maturation in vitro jusqu’a la metaphase II avec emission du premier globule polaire (GPI) dans la proportion 40/50, soit 80 %. Cependant, quand leur volume cytoplasmique est reduit avant le debut de la culture, la frequence des ovocytes qui terminent leur maturation decroit correlativement. Dans des demi ovocytes, 66 % (33/50) expulsent le GPI, dans les tiers d’ovocytes la proportion est 28/49 (57 %) et dans les quarts d’ovocytes il n’y a pas d’emission de GP. La rupture de la vesicule germinative est aussi retardee en relation avec la reduction du cytoplasme ; les vesicules germinatives isolees, entourees seulement d’une fine couche de cytoplasme restent intactes apres culture prolongee. La competence complete a la maturation peut etre restauree par la fusion d’un cytoplaste aux fractions nuclees. Nous suggerons qu’un rapport nucleo-cytoplasmique critique est absolument necessaire pour la maturation normale des ovocytes de mammiferes. © Inra/Elsevier, Paris. souris / ovocyte / vesicule germinale / cytoplasme * Correspondence and reprints E-mail: gott@vuzv.cz