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

Recent studies with female ICR mice have suggested that oocyte DNAfragmentation is one reason for poor oocyte quality and lower fertilityassociated with ageing. Since it was not determined if this increased'apoptotic' potential in aged oocytes is due to changes within the oocyteitself or within the microenvironment of cumulus cells (CC) surrounding thegerm cell, we sought to clarify if CC were required to affect the rate ofapoptosis 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 groupwere incubated at 37 degrees C in human tubal fluid medium under paraffinoil for 24 h. The other half of the COC in each group were denuded of CCand incubated under the same conditions (denuded oocytes; DO). Followingincubation, COC were stripped of adherent CC by gentle pipetting. All DOwere then fixed and checked by light microscopy for morphological changescharacteristic of apoptosis. In young mice, the presence of CC had nosignificant effect on oocyte death rate (18 +/- 9% and 14 +/- 6% apoptoticoocytes in COC and DO, respectively; P > 0.05). However, in aged micethe percentage of CC-enclosed oocytes that underwent apoptosis wassignificantly greater as compared to the death rate in DO (48 +/- 3% versus19 +/- 8% apoptotic oocytes, respectively; P < 0.05). This increaseddeath potential was due to the presence of CC since the occurrence ofapoptosis 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 apoptosisin oocytes maintained in vitro requires the CC.