Coculture with cumulus cells improves maturation of mouse oocytes denuded of the cumulus oophorus: observations of nuclear and cytoplasmic events

Objective: To study the mechanisms by which cumulus cells (CCs) promote oocyte maturation by observing the effect of removing the cumulus oophorus on nuclear and cytoplasmic events during in vitro maturation. Design: Experimental animal study. Setting: Academic institution. Animal(s): Mice of the Kun-ming breed. Intervention(s): Cumulus-free oocytes were cultured alone (DOs) or with a CC monolayer (coDOs), and the nuclear and cytoplasmic events were compared with those of oocytes matured in vivo or in vitro with the cumulus intact (COCs). Main Outcome Measure(S): Nuclear progression, spindle assembly, behavior of cortical granules (CGs) and mitochondria, levels of glutathione (GSH), and dynamics of maturation-promoting factor (MPF) activity during oocyte maturation under different conditions. Result(s): Cumulus removal increased MPF activity and accelerated the transition from the G2 to the M phase and the redistribution of CGs. Spindle assembly and mitochondrial congregation were impaired. In addition, removal of the cumulus caused a precocious exocytosis of CGs, leading to zona hardening and reduced penetrability of oocytes by sperm. After DOs were matured on the CC monolayer, however, these parameters were much improved, and the DOs acquired characteristics closer to those of cumulus-invested oocytes matured in vivo or in vitro. The level of intracellular GSH of DOs, on the other hand, did not differ from that of oocytes matured as COCs, suggesting that the mouse DOs can synthesize GSH on their own. Conclusion(s): While denuding mouse oocytes of CCs impaired in vitro cytoplasmic maturation, coculture with CCs promoted maturation, possibly through the regulation of MPF activity and meiotic progression. (Fertil Steril (R) 2008;90:2376-88. (C) 2008 by American Society for Reproductive Medicine.)