Meiotic spindle morphogenesis in in vivo and in vitro matured mouse oocytes: insights into the relationship between nuclear and cytoplasmic quality.

BACKGROUND This work addresses the hypothesis that events occurring within the follicle soon after the LH surge are essential for coordinating morphogenesis of the spindle and cytoplasm in mouse oocytes matured in vivo (IVO); we further tested whether in vitro maturation (IVM) fails to support these events. METHODS Oocytes collected at 1, 2, 3, 4 and 5 h post-hCG or after IVM were analyzed for chromatin, nuclear lamina, microtubules (MTs) and centrosomal proteins by conventional fluorescence and confocal microscopy. In addition, these parameters were monitored in oocytes maintained in 50 microM roscovitine, followed by IVM, or in oocytes retrieved at 1.5 and 5 h post-hCG in vivo and cultured up to 16 h. RESULTS A G2/M delay was observed in IVO oocytes based upon persistence of cytoplasmic MTs, nuclear lamina and centrosomes at the cortex; rapid meiotic progression in IVM oocytes was related to loss of these markers, indicating that a global activation of MPF occurred in culture. Also, maturating-promoting factor (MPF) inactivation resulted in cultured oocytes that exhibited IVO characteristics after drug removal. IVO-like characteristics were also exhibited by oocytes retrieved at 5 but not at 1.5 h after hCG treatment, even though these oocytes were subsequently cultured. CONCLUSIONS The results emphasize the importance of coupling MT remodeling and cell cycle components during oocyte maturation to achieve a balanced coordination of nuclear and cytoplasmic maturation that under physiological conditions occurs within the first 5 h of LH stimulation.