Influence of hardening of the zona pellucida on in vitro fertilization of bovine oocytes.

The influence of hardening of the zona pellucida of in vivo matured bovine oocytes on fertilizability was investigated. For the study, 163 preovulatory and 73 postovulatory oocytes recovered from superovulated heifers were used. The preovulatory oocytes, before they were used for in vitro fertilization, consisted of: 1) those cultured in vitro for 4 to 6 h to permit final maturation and 2) those incubated in the rabbit oviduct for 4 to 5 h to permit final maturation and induce hardening of the zona pellucida. A few oocytes served as a control of nuclear maturity and the zona pellucida solubility. Preovulatory and postovulatory oocytes were both inseminated in vitro using frozen-thawed, heparin treated and swim-up separated spermatozoa. Significant differences (P<0.01) were established between fertilization rates of cultured preovulatory oocytes (68.8%) and those incubated in the rabbit oviducts (42.9%), or those recovered from bovine oviducts (40.7%). It can be concluded that hardening of the zona pellucida distinctly influences the fertilizability of oocytes. This factor should be taken into account when considering the source of oocytes or the kind of treatment to be used for in vitro fertilization.