MATURATION-PROMOTING FACTOR (MPF) AND MITOGEN-ACTIVATED PROTEIN-KINASE (MAPK) EXPRESSION IN RELATION TO OOCYTE COMPETENCE FOR IN-VITRO MATURATION IN THE MARE

In the equine species, a large proportion of oocytes fail to complete meiosis during in-vitro culture. The biochemical and molecular basis o f this failure is unknown. The meiotic cell cycle is controlled in par t by the maturation-promoting factor (MPF) and the mitogen-activated p rotein kinase (MAPK), In this study, we evaluated the oocyte competenc e for in-vitro maturation and the expression of MPF components (p34cdc 2 and cyclin B) and MAPK after in-vitro culture. The maturation rate w as influenced by the culture medium and the physiological stage of the mare at the time of oocyte recovery. We showed that MAPK and the two subunits of MPF were present in equine oocytes whatever the nuclear st age they reached after in-vitro culture and whatever the culture mediu m used. In incompetent oocytes, MAPK remained in its non-phosphorylate d form, supposed to be inactive. In conclusion, the incompetence of eq uine oocytes to resume and complete meiosis is not due to the absence of p34cdc2, cyclin B or MAPK, Our results suggest that it is more prob ably due to a deficiency of regulators of MPF and/or to an inability t o phosphorylate MAPK.