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 evaluate the usefulness of a pre-maturation step in improving the coordination between cytoplasmic and nuclear maturation of horse compact cumulus oocytes by the addition of roscovitine (ROSC). Oocytes were collected by scraping and pre-cultured for 18 h in a maturation medium TCM199 supplemented with pyruvate, LH, FSH, insulin growth factor (IGF), epidermal growth factor (EGF), insulin, transferrin and selenium (IVM-ROSC) or in a simple medium (M199-ROSC). After pre-maturation, oocytes from both the groups were in part denuded and fixed-stained and in part in vitro matured to assess the kinetic of in vitro maturation (IVM). The nuclear progression and the cytoskeletal organization of microfilaments and cortical granules (CG) of treated and untreated oocytes were assessed by fluorescent probes. Oocytes immediately fixed after recovery and oocytes pre-cultured in M199-ROSC for 18 h did not show metaphase II (MII) plates, whereas in IVM-ROSC group, 6/69 oocytes (8.7%) showed MII plates. After inhibition, during maturation kinetics at 11, 18 and 29 h, maturation rate of M199-ROSC group progressively increased and at 29 h of IVM, reached the maturation rate of control group (13/66, 19.7% vs 31/125, 24.8%). No statistically significant differences in cytoplasmic maturation were found. The number of MII plates after 29 h of IVM, was significantly higher (p < 0.05) in IVM-ROSC group (34/90) compared with M199-ROSC (13/66) and control groups (31/125) as well as the number of oocytes with microfilaments and CG distributed in cortical region (25/34 vs 3/13 and 7/31 respectively). Our results showed that pre-culturing in the presence of Roscovitine in a fully supplemented maturation medium containing gonadotropins and growth factors partially suppressed the meiotic maturation, but established a more suitable environment for improving cytoplasmic maturation of horse compact cumulus oocytes as defined by microfilaments and CG configuration.