Time course of In vitro maturation of compact cumulus horse oocytes after roscovitine-induced meiotic inhibition : effects on the coordination between nuclear and cytoplasmic maturation

This study was carried out to determine if meiotic suppression with 66 μM Roscovitine (ROS) for 18h in a fully supplemented maturation medium (IVM-ROS) or in a basic medium (M199-ROS) improves coordination of nuclear and cytoplasmic maturation of horse compact cumulus oocytes. The nuclear maturation and the cytoskeletal organization of microfilaments and cortical granules (CG) were assessed by fluorescent dyes. The statistical analysis was performed by Z Test. The proportion of oocytes (85,51%) resuming meiosis after 18h of culture in IVM-ROS was significantly higher (p<0.01) in comparison to M199-ROS and CTR oocytes, but only a few oocytes showed microfilaments and CG with peripheral distribution. The 24.92% of untreated oocytes (IVM-control) matured in vitro after 29h of IVM but only 3.94% of matured oocytes were characterized by the presence of a microfilaments thickening at the cortical region and 8.66% had the CG migrated to the periphery of the cytoplasm. The maturation rates of oocytes held in M199-ROS before maturation and investigated during maturation kinetics at 11h, 18h and 29h, increased to that of IVM-control at 29h and there were no difference in cytoplasmic maturation in respect to these oocytes. The treatment with IVM-ROS, instead, increased significantly (p<0.05) the attainment of metaphase II stage compared to the M199-ROS and the IVM-control (35.55% vs. 19.7% and 24.92% respectively) and the rate of oocytes with microfilaments and CG distributed to the cortical region. Our results show that pre-culture in maturation medium containing gonadotropins and growth factors, during meiotic suppression, establishes a more suitable environment for partially improving the nuclear and cytoplasmic maturation of horse Cp oocytes.