Differences in Early Cleavage Rates of In Vitro Produced Bovine Embryos Classified According to the Completion of Their First Cleavage.

This study was undertaken to determine the relationship between the first cleavage division of in vitro produced (IVP) bovine embryos and the following few cleavage divisions in culture. The “early-”, “intermediate-“ and “late-” cleaving embryos presented at 22, 26 and 30 h post insemination (hpi) were cultured separately until they were fixed and stained with Hoechst 33342 at the end of each culture period (30, 48, 72, 96 and 120 hpi). Distributions of embryos that underwent the first cleavage at 22, 26 and 30 hpi were 21.5, 34.7 and 10.0%, respectively. No significant difference among the mean number of cells in the 3 groups was observed until 48 hpi. At 72 hpi, the number of cells in the early-cleaving embryos (10.8 ± 0.5) was higher than those in the intermediate- and late-cleaving embryos (8.7 ± 0.4 and 6.5 ± 0.6, respectively). Higher numbers of cells were also observed in the early-cleaving embryos (18.0 ± 1.5) at 120 hpi, compared with those in the intermediate- and late-cleaving embryos (12.5 ± 0.8 and 11.4 ± 1.5, respectively). More than 80% of the early- and intermediate-cleaving embryos had completed the second cleavage division at 48 hpi, whereas that of the late-cleaving embryos was lower (56.5%). No difference in the proportion of >8-cell embryos among the 3 groups was observed at 48 hpi (22.9 vs 24.7 vs 26.1%). However, the completion of the third cleavage in the early-cleaving embryos was higher than in the intermediate- and late-cleaving embryos at 72 hpi (65.4 vs 49.4 vs 25.0%), and from there onwards (96 hpi; 81.1 vs 63.2 vs 45.5%, 120 hpi; 83.6 vs 62.2 vs 65.0%). The proportion of >16-cell embryos in the early-cleaving embryos was also higher than that of the intermediate- and late-cleaving embryos between 96 hpi (18.9 vs 2.9 vs 0.0%) and 120 hpi (49.1 vs 22.0 vs 25.0%). Our results suggest that the development of IVP bovine embryos is possibly controlled as early as at the 2-cell stage, and the differences between the early and late cleaving embryos are associated with 1) the developmental delay or arrest of embryos during the second cleavage, and 2) the lengthening of the third cleavage, which seems to be related to the subsequent developmental ability of the embryos.