Compaction of morula-stage mammalian embryos is required for proper blastocyst formation and leads to differentiation of the inner cell mass and trophectoderm. Many structural events are associated with compaction, including blastomere polarization, blastomere flattening, and gap junction formation. In bovine and human embryos, the events of compaction are dependent on embryonic age, blastomere number, and number of cell divisions.
Bovine embryos produced in vitro can be characterized as having altered morphology compared with those produced in vivo. Compared with embryos produced in vivo, embryos produced in vitro have fewer total blastomeres, demonstrate incomplete compaction and cell-to-cell coupling, and have a greater variance in morphological quality and developmental rate. Differences in morphology may contribute to the decrease in agreement among evaluators when determining stage of development for compact morulae produced in vitro. Furthermore, it is interesting to note that pregnancy rates after transfer of morula-stage bovine embryos produced in vitro were lower than pregnancy rates after the transfer of morulae produced in vivo.
Efforts to improve methods for the production of embryos in vitro are dependent on a better understanding of the effects of culture environments on embryo development. It has been suggested that addition of serum to culture medium increases the occurrence of lipid droplets and alters mitochondrial structure in ovine embryos. Elimination of serum from culture medium for the first 72 h postinsemination (hpi) resulted in bovine embryos that were more advanced in development by 168 hpi but were of lower morphological quality than those produced entirely in the presence of serum. Embryos produced in serum-free medium possessed fewer lipid droplets and vacuoles compared with those cultured in the presence of serum.
To date, ultrastructural evaluations of preimplantation-stage embryos from sheep and cattle have been based on subjective observation. Morphometric analysis offers a more objective method of assessing differences in cellular ultrastructure that may occur in embryos as a result of in vitro culture. Therefore, the objective of this study was to use morphometric analysis to quantify the ultrastructure of bovine compact morulae produced either in vivo or in vitro using three embryo culture media.