247 THE IMPORTANCE OF FOLLICULAR DIFFERENTIATION TO OBTAIN FULLY COMPETENT BOVINE OOCYTES

Abstract

Making an oocyte with the capacity to form a Day-7 embryo that will implant and produce a live healthy calf is a long and complex process. Since the beginning of bovine IVF, it has been clear that the follicle from which an oocyte is obtained is the most important factor to predict outcome. The blastocyst rate is above 75% with in vivo-matured oocytes, but success varies greatly following in vitro maturation, depending more on follicular status than size. Indeed, aspirating oocytes from non-preovulatory follicles can result in 30% blastocyst development following fertilization but rarely more, unless the follicle is selected. Follicles leading to improved development are either early atretic or exposed to FSH withdrawal. If no exogenous FSH is given, natural early atresia will occur the day after the dominant follicle is selected, but will be limited to a few follicles per wave. If FSH is given to generate a wave of dominant follicles, and then withdrawn, most of the follicles will continue growth and differentiation under basal LH and a maximal oocyte quality will be reached between Day 2 and 3 after the last FSH injection. Under these conditions, the oocyte has the right cascade of signaling to condense its chromatin and to prepare for fertilization. Oocytes obtained from less-prepared follicles are of lower quality, as demonstrated by lower blastocyst rates, lower blastocyst quality, lower implantation rates, and higher abortion rates. Genomic analysis of embryos generated from slaughterhouse-derived oocytes indicates a progressive dysfunction in direct link with the in vitro exposure time. The best way to minimize in vitro-related stress is to begin with a competent oocyte and the best demonstration of that is when some animals (treated with FSH and then coasted) generate 100% blastocyst rate for a recovery of more than 5 oocytes, indicating that the initial quality of oocytes is not much affected by the in vitro culture. It is surprising that blastocyst rates following coasting and in vitro maturation can surpass that observed with Day-7 embryo collections based on the number of ovulations. Finally, new epigenetic analysis will progressively unravel differences of oocytes obtained from different sources and will become an important research tool to assess follicular conditions that will lead to minimal culture stress and optimal embryo quality. If epigenetic programming of the embryo depends on its capacity to deal with the stress created by culture, as has been shown in other species, the initial quality of the oocyte will potentially impact the health status of the newborn calf and resulting adult.