Co-culture of cumulus-enclosed bovine oocytes with theca cells induces the meiotic arrest but does not inhibit germinal vesicle development

The effects of maturation inhibitors and follicular cells on germinal vesicle (GV) development and meiotic resumption in bovine oocytes were examined. Evaluation of GV development of immature oocytes revealed that oocytes from larger diameter follicles were at more advanced stages. Although culture of cumulus-oocyte complexes (COCs) in the presence of dibutyryl-cAMP, 8-bromo-cAMP or N6-monobutyryl-cAMP did not inhibit meiotic maturation, culture in the presence of cycloheximide (CX) did. Cycloheximide also completely inhibited development of the GV as assessed by changes in morphology. Co-culture of COCs with theca cells (TH) or theca and granulosa cells (TG) also inhibited meiotic resumption but allowed GV development to the most advanced stage, GV-V. Thus co-culture with TH was able to synchronize oocytes at the stage just prior to GV breakdown. When removed from co-culture, the majority of oocytes were able to resume meiosis if co-cultured in a sufficient large volume of medium. The ability of TH to synchronize GV development could be used in a two-step protocol for oocyte maturation, the first step promoting GV development and the second inducing resumption of meiosis. Such an approach might improve the developmental potential of embryos obtained following in vitro fertilization of in vitro matured oocytes.