The development of bovine embryos following enzymatic removal of the zona pellucida

Abstract

This study to determine the survival of bovine embryos with and without the zona pellucida was divided into two parts. The first part consisted of removing the zona pellucida with a .5% pronase solution from half of the embryos before culturing in Ham's F-10 plus 10% Heat-Treated Fetal Calf serum. The second part consisted of transferring the embryos to recipient cows following removal of the zona pellucida. Embryos were collected at different stages of development: (1) 1-4 cell stage (2-3 days postestrus), (2) 5-16 cell stage (3-5 days postestrus), (3) Morula stage (5-6 days postestrus), (4) early blastocyst stage (6-7 days postestrus), (5) expanded blastocyst stage (7- 9 days postestrus), and (6) unfertilized ova (2 days post onset of estrus). The times required for removal of the zona pellucida from the different stages were 131, 246, 19, 7, 33 and 71 minutes, respectively. Significant negative correlation coefficients of .418 and .720 between time required to remove the zona pellucida and embryo stage and between embryo stage and zona pellucida thickness, respectively, were observed. These two correlation coefficients tend to indicate that the older embryos required a reduced time period for removal of the zona pellucida and that zona pellucida thickness is reduced as aging occurs. There was no significant difference in survival time for embryos cultured with and without their zonae pellucidae within or between any embryo stage. There was a significant effect on survival time between stages as indicated by a positive correlation coefficient of .3943 between embryo stage and survival time. This correlation coefficient indicates that survival time in culture increases as embryo age at the beginning of culture increases. Denuded embryos were transferred starting with blastocyst stage embryos and working to younger stage embryos. The blastocyst and early blastocyst stage denuded embryos produced 2 pregnancies out of 7 transfers and 5 pregnancies out of 11 transfers, respectively. No pregnancies were obtained from any of the earlier stage embryos. These data indicate that the zona pellucida is not needed for implantation of early blastocyst stage embryos, but may be required for morula and earlier stage embryos.