Rendez-vous in the oviduct: implications for superovulation and embryo transfer

The meeting between the maternal and paternal gametes is dependant upon a number of complicated processes. On the maternal side it involves maturation of the oocytes under the influence on both peripheral and follicular endocrine factors. Deviations in the normal pattern of maturation will lead to ovulation of inferior oocytes. On the paternal side the transport of spermatozoa in the female genital tract following mating is an area of great importance. The establishment of the sperm reservoir in the isthmus is dependant upon a number of factors (intracellular calcium concentrations, oligo-saccharides, change in estradiol: progesterone ratio in the afferent blood supply). Alterations of the normal micro-environment may disturb both binding, release and transport as a whole. The process of fertilization occurs in the ampullar region of the oviduct and it involves several well tuned steps: binding to the zona pellucida where the acrosome reaction takes place, penetration, fusion between the oolemma and the sperm plasma membrane, activation with the release of the cortical granules, decondensation of the sperm chromatin, pronucleus formation and finally syngamy where the two pronuclei fuses. The egg will experience the first cleavage shortly thereafter. Superovulation may disturb a number of these processes including oocyte maturation (arrest at MI) and sperm and zygote transport in the oviduct caused by the deviant endocrine environment, thus leading to a higher incidence of lack of fertilization and poor embryos quality.