Cumulus cells are required for the increased apoptotic potential in oocytes of aged mice

Recent studies with female ICR mice have suggested that oocyte DNA fragmentation is one reason for poor oocyte quality and lower fertility associated with ageing. Since it was not determined if this increased 'apoptotic' potential in aged oocytes is due to changes within the oocyte itself or within the microenvironment of cumulus cells (CC) surrounding the germ cell, we sought to clarify if CC were required to affect the rate of apoptosis in oocytes maintained in vitro. Intact cumulus-oocyte complexes (COC) were retrieved by superovulation of virgin female ICR mice at 7 weeks ('young') or 34-35 weeks ('aged') of age. One-half of the COC in each group were incubated at 37 degrees C in human tubal fluid medium under paraffin oil for 24 h. The other half of the COC in each group were denuded of CC and incubated under the same conditions (denuded oocytes; DO). Following incubation, COC were stripped of adherent CC by gentle pipetting. All DO were then fixed and checked by light microscopy for morphological changes characteristic of apoptosis. In young mice, the presence of CC had no significant effect on oocyte death rate (18 +/- 9% and 14 +/- 6% apoptotic oocytes in COC and DO, respectively; P > 0.05). However, in aged mice the percentage of CC-enclosed oocytes that underwent apoptosis was significantly greater as compared to the death rate in DO (48 +/- 3% versus 19 +/- 8% apoptotic oocytes, respectively; P < 0.05). This increased death potential was due to the presence of CC since the occurrence of apoptosis in DO of aged versus young mice was not significantly different (19 +/- 8% versus 14 +/- 6% apoptotic oocytes, respectively; P > 0.05). These results demonstrate that the age-dependent acceleration of apoptosis in oocytes maintained in vitro requires the CC.