Early Cleaved Bovine Embryos Show Reduced Incidence of Chromosomal Aberrations and Higher Developmental Potential on Day 4.5 Post-Insemination

The present study aimed to investigate whether the timing of the first zygotic cleavage (FZC) influences the speed of embryo development expressed by the total cell count and the rate of chromosomally aberrant embryos. Bovine embryos were produced in vitro and divided into two categories according to the timing of FZC: early cleavers (at 30 hpi; EC) and non-early cleavers (at 48 hpi; NEC). On day 4.5 pi, embryos were grouped into three classes depending on the number of blastomeres: delayed (<8 BL), normal (816 BL) and advanced (>16 BL). We applied fluorescence in situ hybridization (FISH) with probes for bovine chromosomes 6 and X. The only form of chromosomal imbalance observed was mixoploidy [(2n/3n; 2n/4n); 19.9%, 54/271]. Early cleavers were less often chromosomally unbalanced (13.9%, 20/144) than their NEC counterparts (26.7%, 34/127). Among embryos developing at a normal speed, the NEC embryos were more often abnormal (NEC 20/80; EC 10/79; p < 0.05). The advanced embryos were not observed among the NEC category, whereas such embryos from EC category displayed no chromosomal aberrations. The majority of embryos arrested at the 8 BL stage were of NEC category and were carriers of chromosomally abnormal blastomeres. With regard to embryonic sex, we demonstrated that although males dominate among bovine embryos developing in vitro, the incidence of mixoploidy was equal for both sexes. It can be suggested that a good-quality bovine embryo is usually an early cleaver that develops at higher speed and contains less aberrant cells. The present study also confirmed the usefulness of the FZC as a marker of embryo quality by demonstrating a significantly lower incidence of aberrations in early embryos.