Zinc supplementation during in vitro culture of bovine growing oocytes

Background: In cattle, early antral ovarian follicles (0.5-2 mm in diameter) typically contain growing oocytes. Most of these oocytes have filamentous chromatin within the germinal vesicle, are still transcriptionally active and have not yet acquired the meiotic competence, as demonstrated by the inability of resuming meiosis once isolated from the follicular compartment. Consequently, these oocytes cannot currently be used in standard protocols for in vitro embryo production (IVP) limiting the exploitation of the ovarian population for assisted reproduction purposes. Aims: In this view, aim of our studies is to improve the outcome of in vitro culture of growing oocytes (IVCO), in order to increase the amount of oocytes per ovary that can be used for embryo production. Method: By mining transcriptomic databases (http://emb-bioinfo.fsaa.ulaval.ca/IMAGE/) we observed that zinc (Zn) transporters are differentially expressed in different cell types throughout several steps of oogenesis in cattle. This observation, supported by studies in mice that revealed the importance of Zn in regulating oogenesis and early embryogenesis, seems to suggest the supplementation of culture medium with this element as a possible strategy. Hence we tested whether Zn might be beneficial to the acquisition of meiotic competence in bovine growing oocytes. Results: Zn supplementation during 24 hours of IVCO significantly improved the proportion of oocytes reaching the metaphase-II stage of meiosis after subsequent standard in vitro maturation compared to non-supplemented control. Furthermore the global transcriptional activity was significantly increased in growing oocytes cultured with Zn, while short time treatment with a Zn chelator (TPEN) had a negative effect on transcription. From a mechanistic point of view, these findings likely suggest that Zn support the synthesis of molecules needed for the successful completion of the subsequent steps of oogenesis in growing oocytes. Further studies are in progress to test the effect of Zn supplementation on embryo development of growing oocytes.