Developmental Potential of Embryos Produced in Vitro by Sperm from Bulls of Contrasting in Vivo Fertility and Ovocytes Retrieved from a Same Donor

Laboratory assessments of sperm traits are poor indicators of fertility. Because the variability in the quality of oocytes collected from different donors drastically influence in vitro embryo production, the aim of this work was to implement an in vitro model to compare the developmental potential of embryos produced by bulls of contrasting in vivo fertility and ovocytes retrieved from the same donor in order to minimize the female related variability. For each trial (n=54), one pair of good quality ovaries of a same donor were split longitudinally and oocytes were recovered by slicing method. Thereafter, matured oocytes were fertilized with sperm of two bulls of low estimated relative conception rates (ERCR +2) contrasting field fertility (6 vs. 6 bulls). Cleavage and blastocyst formation rates were compared. Sperm of high fertility bulls (ERCR+) gave also higher fertilization rates (cleavage) in vitro when compared to low fertility (ERCR-) bulls (odd ratio=1.23). Conversely, the embryonic development to the blastocyst stage was reduced (odd ratio=0.84) in the ERCR+ with respect to the ERCR- bulls. This paradoxical result demonstrates that in vivo bull fertility hardly correlates with in vitro blastocyst yield, but confirms that early events occurring at fertilization are better indicators of the fertility potential. Furthermore, this experimental approach indicates that differences in embryo production rates between bulls of contrasting field fertility may not be outlined in vitro even when bulls are compared using oocytes with variability limited to the same donor.