Bull effect as a source of variability in in vitro embryo production

Variation between bulls in in vitro fertility is a major problem in the in vitro production of bovine embryos. To achieve a better understanding of the bull effect, we conducted three studies where the potential sources of variability between bulls were analysed before, during and after in vitro fertilization (IVF). We first investigated the relation between sperm quality parameters analyzed before and during IVF and embryo development. The selection of thawed sperm on a Percoll density gradient allowed abolishing differences between bulls in the proportions of motile and progressive sperm cells. An important but variable decrease in sperm viability was observed for all bulls during the incubation in IVF medium, which was partly due to the capacitation agents. In contrast, the proportion of sperm cells showing premature acrosome reaction (AR) was in general low. None of those parameters could clearly explain the variations in blastocyst rates with the different bulls. We then evaluated the kinetics of pronucleus formation (PN), embryo development, and the sex ratio of the resulting embryos. The kinetics of PN formation varied between bulls. Bull influences the kinetics of development both before (1st cleavage) and after (transition morula/blastocyst) the major onset of the embryonic genome. A significant faster PN evolution was observed with one bull that also showed a fast kinetics of first cleavage and a shift of sex ratio towards male embryos. Those three parameters could thus be related. In the last study, we tested the hypothesis that generation of hydrogen peroxide (H2O2) by the sperm, or sensitivity of the sperm to oxidative stress could affect IVF outcome. A significant variation in H2O2 generation from both live and dead sperm was noted between bulls. The bulls also showed significant differences in lipid peroxidation in response to an oxidative stress. But those parameters did not seem to be correlated with in vitro fertility. In conclusion, differences were observed between bulls during and after IVF that included sperm viability, AR, production of H2O2, sensitivity to oxidative stress, kinetics of PN formation and embryo development and sex ratio. This suggests a multifactorial origin of the bull effect on in vitro embryo development.Doctorat en sciences (sciences biologiques) (BIOL 3) -- UCL, 200