Although in vitro culture has been widely adopted as a method for the production of mammalian embryos, these systems can result in embryos with varying morphology compared with those produced in vivo. Ultra-structural characteristics of embryos from different production systems can be compared in an unbiased manner using morphometric methods. However, limited information is available in the literature concerning morphometric evaluation of mammalian embryos. To our knowledge, the data reported in this paper represent the first application of morphometric techniques to quantify the effects of culture on ultrastructural characteristics of preimplantation bovine embryos.
The age of compact morulae produced in vitro for this study was based upon the interval postinsemination. In contrast, bovine embryos produced in vivo reach the compact morula-blastocyst transition between Days 6 and 8 postes-trus with the first appearance of early blastocyst stage embryos occurring around Day 7. Variance in the day of collection of compact morulae produced in vivo may result from several factors. First, because the donor cattle used in this study were observed for estrus at approximately 12-h intervals, detection of first standing estrus may not have been entirely coordinated with the actual time of true estrus initiation. Thus, embryos may have been deviated in development by up to 12 h (0.5 days) relative to their stated day of gestation. Second, although embryos from superovulated donors have been suggested to be representative of in vivo embryos obtained from nonstimulated animals, multiple follicles may ovulate over an extended period of time, contributing to variance in stage of development on the day of embryo collection. Finally, morulae produced in vivo remain in the compacted stage for more than one cleavage division and undergo the transition to blastocyst more slowly than those embryos produced in vitro. Therefore, it is not surprising that compact morulae used in this study were recovered at both Day 6 and Day 8 postestrus.