Division behaviours and their effects on pre-implantation development of pig embryos

The quality of pre-implantation embryos could affect developmental efficiency after embryo transfer. However, the assessment of pre-implantation embryos was unsatisfactory, especially in pig embryos to date. Therefore, this study was designed to investigate available and applicable parameters that indicate developmental potential and quality of porcine pre-implantation embryos produced by handmade cloning (HMC), and parthenogenetic activation without zona pellucida (PAZF) and with zona pellucida (PAZI). Firstly, a common division behaviour was detected, that is the formation of uneven division with two unequal size blastomeres (UD 2-cell), especially in HMC embryos; then, the proportion of UD 2-cell was found to be significantly higher than that of even division with equal size blastomeres (ED 2-cell) (72.56 ± 4.56 vs. 24.57 ± 1.92). The formation of UD 2-cell might be due to the spindle migration along the long axis in 1-cell stage, and the cleavage furrow was not formed in the centre of cytoplasm. In the two sister blastomeres of UD 2-cell, uneven distribution of organelles (mitochondria and lipid droplet) was observed with lower proportion in the smaller one (p < .05). Although no difference in blastocyst rate was observed between UD and ED 2-cell embryos, the cell number per blastocyst from UD 2-cell embryos was lower than that from ED 2-cell embryos (44.15 ± 2.05 vs. 51.55 ± 1.83). Besides, because of non-synchronized division of each blastomere, the following three cleavage routes were observed in all HMC/PAZF/PAZI embryos: T1 (2-cell → 3-cell → 4-cell → ≥5-cell → morula → blastocyst), T2 (2-cell → 3-cell → 4-cell → morula → blastocyst) and T3 (2-cell → 3-cell/4-cell → morula → blastocyst). Therefore, in pig in vitro-produced embryos, division behaviours of uneven volume of cytoplasm and non-synchronized cell cycles were observed at the early embryonic developmental stage, which might be another potential factor to evaluate embryonic development.