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In the course of mammalian development, the initial state of totipotency must be lost to allow the acquisition of specific cell fates. The first differentiation event results in the formation of trophectoderm (TE) and the inner cell mass (ICM). In the mouse embryo, the cell fate of these two compartments is set quickly after blastocyst formation. However, recent reports suggest that the plasticity of these two lineages might be extended in species other than the mouse. Here, we investigated how the cellular plasticity of early mammalian embryos relates to developmental time scale and changes in gene expression using rabbit isolated ICMs. We studied the dynamics of rabbit blastocyst formation using time-lapse imaging and identified GATA3 as an early marker of rabbit TE and CDX2 as a marker of fully formed TE. We then analysed the developmental potential of rabbit ICMs isolated by immunosurgery and subsequently cultured in vitro. ICMs originating from early- to mid-blastocyst stage embryos are able to re-form a blastocyst-like structure, with a functional TE, and an ICM containing both SOX2-positive epiblast cells and SOX17-positive primitive endoderm cells. We further observed that rabbit ICMs isolated from later blastocyst stages lose the ability for TE specification, instead forming a halo-like cavity with an outer layer of SOX17-positive cells. Our data indicate that in mammalian embryos, the potential for TE differentiation gives way to the formation of a different type of extraembryonic epithelial layer, suggesting a potential common mechanism of pluripotency restriction between eutherian mammals.
Filimonow et al. (Tue,) studied this question.