Improving oocyte quality in cows and heifers - What have we learned so far?

The major challenge in assisted reproduction technologies remains producing oocytes with optimal developmental competence. Such developmental competence can be defined as the ability of the oocyte to fulfill maturation, undergo successful fertilization, reach the blastocyst stage and yield a viable and healthy progeny. The main follicular parameters linked to oocyte competence are presented in this review: follicle size, health/atresia status, effects of superstimulation and level of differentiation. In a commercial environment, exogenous FSH superstimulation combined with FSH starvation (referred to as the coasting period) and ovumpick up followed by in vitro maturation and fertilization, leads to maximal blastocyst rates in Bos taurus heifers and cows. This coasting period increases the proportion of medium-to-large size follicles and may induce slight atresia in the follicular cells, which improves developmental competence. The optimal period for oocyte retrieval in this context has been recently characterized in cows and is related to follicular size and state of differentiation. Apoptosis related signals have been identified in various studies as implicated in antral folliculogenesis. Follicular somatic cell transcriptome studies are currently used to identify competence related markers. Finally, oocyte competence acquisition is a complex and dynamic process. The best oocytes are obtained from follicles of the optimal size and differentiation status, which can be manipulated with the right hormonal regimen.