Early transcriptional divergence underlies cell fate bias in bovine embryos

Developmental plasticity, or the ability of early embryonic cells to contribute to multiple lineages, is traditionally considered equal among sister blastomeres during early cleavage. However, divergence may occur earlier than expected. We performed single-cell RNA sequencing of bovine embryos from the 2- to 8-cell stages to examine transcriptional asymmetry. While gene expression was uniform at the 2-cell stage, variability increased at the 4-cell stage and became pronounced by the 8-cell stage. At this stage, blastomeres showed heterogeneity in MAPK pathway genes (e.g., RAC1, MAPK14) and the trophectoderm marker CDX2. These differences were associated with blastomere size; as larger blastomeres exhibited molecular and functional features associated with an increased propensity to generate trophectoderm cells. Thus, sister blastomeres exhibited progressive transcriptional divergence prior to compaction in bovine embryos, and these early differences may influence subsequent lineage trajectories.