Maturation environment and impact on subsequent developmental competence of bovine oocytes.

In vitro production (IVP) of bovine embryos has been improved immensely throughout the last decades. Nevertheless, embryos generated in vitro still differ from their in vivo-produced counterparts. It is possible to achieve blastocyst rates of up to 70% if in vivo-matured oocytes are used. In contrast, if oocytes are matured in vitro, blastocyst rates are only half that of those matured in vivo. This rather limited success may be attributed to the heterogeneous population of oocytes which are normally retrieved from follicles of 3-8 mm rather than from preovulatory follicles. In contrast to the in vivo-ovulated oocyte, these oocytes lack development up to the preovulatory stage and are matured in vitro. Therefore, much effort has been devoted to the establishment of non-invasive and non-perturbing means for selecting the most competent oocytes, for example the extensiveness and compactness of the cumulus-corona investment and the granulation of the ooplasm. In vitro culture (IVC) conditions have been enhanced in the last few years, mainly by adjustment of media formulations, whereas the in vitro maturation (IVM) protocols stay invariable. Consequently, maintaining or mimicking the in vivo situation in vitro will aid to improve the quality and developmental competence of the resulting matured oocyte. The scope of this review is to give an overview of the current situation of in vitro maturation of mammalian oocytes with emphasis on the bovine species. Special attention has been paid to the in vivo situation in the follicle and how a better understanding of these intrafollicular factors will aid to improve the in vitro maturation conditions.