Change of the mitochondrial distribution in mouse ooplasm during in vitro maturation

Mitochondria (mt) have been reported to be closely related to the maturation of mammalian oocytes, but their function in oocyte maturation has not been elucidated. In this study, we examined the kinetics of mt and chromatin configuration during in vitro maturation of mouse oocytes to clarify the relationship between oocyte maturation and mitochondrial distribution morphologically. Oocytes were recovered from 6-to 8-wk-old ICR strain female mice. Germinal vesicle (GV) -stage oocytes were divided into 4 groups and cultured: group A, oocytes collected after pregnant mare serum gonadotropin (PMSG) injection; and group B, oocytes collected after PMSG-human chorionic gonadotropin injection. Groups A and B were subdivided into 2 groups: denuded oocytes (DO) and cumulus-enclosed-oocytes (CEO). At 0, 4, 8, 12 and 16 h from the onset of the culture, oocytes were fixed and stained to visualize alpha-tubulin, chromatin and mt using confocal laser scanning microscopy (CLSM). It was observed that mt aggregated around the nucleus from the GV-stage through progression to germinal vesicle breakdown (GVBD). With the movement of the nucleus, mt were concentrated around the nucleus and polarized. The maturation rate (the rate of the first polar body extrusion) and the fertilization rate of CEO were significantly higher than that of DO in both groups A (p<0.01) and B (p<0.05). During the GV-stage to GVBD, the rate of mitochondrial aggregation around the nucleus tended to be high in group A (CEO). The rates of mitochondrial polarization in MI and MII oocytes were 76.1% with in-vitro maturation (IVM) and 86.7% with in-vivo-maturation, respectively; the rate was significantly higher in in-vivo-maturation-oocytes than in IVM-oocytes (p<0.01). From the present results it can be considered that aggregaton of mitochondria around the nucleus was essential for maturation, fertilization and development.