Effects of single or serial embryo splitting on the development and morphokinetics of in vitro produced bovine embryos

Abstract

Embryo splitting can be used in cattle in vitro production (IVP) to improve embryo availability and to increase selection intensity. Despite this widespread utility, a comparative investigation of the viability of IVP embryos split at Day 2 (2-cell stage), Day 3 (8-cell stage), and blastocyst stage has not been undertaken. Similarly, the suitability of splitting Day 3 embryos with atypical numbers of blastomeres, and the feasibility of serial-splitting cleavage stage embryos, have not been investigated in cattle. Here, we demonstrate that the strategy most likely to produce the greatest output of viable embryos is the splitting of Day 3 embryos into four parts, regardless of whether embryos with exactly eight cells or an atypical number of blastomeres are used. This approach was found to produce 1.8 blastocysts per zygote on average compared to just 0.4 blastocysts per zygote for non-split controls. Single-splitting was also found to be superior to serial-splitting which, whilst feasible, impaired embryo viability as judged by cell number at day 7 post-insemination. Interestingly, zygotes (≥2 cells) split once on either Day 2 or Day 3 post-insemination, whilst resulting in smaller blastocysts than control embryos, displayed higher cell counts than expected at the blastocyst stage, suggesting a compensatory mechanism might be at play. Indeed, time-lapse imagery revealed that zygotes split at 2-cells reached the compact morula and expanded blastocyst stages earlier than either those split at Day 3 or non-split controls. Developmental events between splits originating from the same progenitor appeared well synchronized only up to the third cleavage division.