Mammalian embryos begin as a single fertilised cell and develop into a multicellular organism with dynamically evolving cell fates. A fundamental question in developmental biology is when and how the earliest cell differentiation occurs to establish distinct cell fates. Traditional theory holds that all cells before the 8-cell stage are identical. However, using advanced microinjection and live imaging technologies, our results challenge this view, revealing that the first two cells formed after the first cleavage are already distinct, with this asymmetry established by the second polar body.
At the 1-cell stage after fertilisation, we observed for the first time that RNA transcription occurs in the second polar body, previously regarded as a mere byproduct of embryonic development. A specific subset of mRNAs encoding pluripotency-related factors is enriched in the second polar body, where they are translated into proteins and further transported to the embryo. This leads to the asymmetric allocation of pluripotency factors between the two blastomeres after the first cell division, creating the earliest bias in cell fate.
At the 2-cell stage, the cell attached to the second polar body undergoes earlier zygotic genome activation, resulting in asynchronous RNA transcription between the two cells. This cell also exhibits greater pluripotency, contributing more descendants to the pluripotent inner cell population from the 16-cell stage onward. Removal of the second polar body at the 1-cell stage causes the loss of this cell fate bias, resulting in a failure of the embryo to develop to birth after implantation.
Our findings represent the earliest timepoint at which cell differentiation occurs in mammalian embryos within the first day after fertilisation. Moreover, we demonstrate that the second polar body, which was previously considered insignificant, plays a critical role in establishing the initial fundamental fate bias. Our study pioneered real-time imaging of RNAs and their translation dynamics at different stages of early mammalian embryo development to regulate cell fate, offering new insights into the precise orchestration of distinct cell fates during early development.