The earliest stages of development in most animals, including the few mammalian species that have been investigated, are regulated by maternally inherited information. Dependence on expression of the embryonic genome cannot be detected until the mid two-cell stage in the mouse, the four-cell stage in the pig (J. Osborn & C. Polge, personal communication), and the eight-cell stage in the sheep. Information about the timing of activation of the embryonic genome in the human is of relevance not only to the therapeutic practice of in vitro fertilization and embryo transfer (IVF), but more importantly for the successful development of techniques for the preimplantation diagnosis of certain inherited genetic diseases. We describe here changes in the pattern of polypeptides synthesized during the pre-implantation stages of human development, and demonstrate that some of the major qualitative changes which occur between the four- and eight-cell stages are dependent on transcription. In addition, it appears that cleavage is not sensitive to transcriptional inhibition until after the four-cell stage.

Remodelling of the human embryo at implantation is indispensable for successful pregnancy. Yet it has remained mysterious because of the experimental hurdles that beset the study of this developmental phase. Here, we establish an in vitro system to culture human embryos through implantation stages in the absence of maternal tissues and reveal the key events of early human morphogenesis. These include segregation of the pluripotent embryonic and extra-embryonic lineages and morphogenetic re-arrangements leading to: generation of a bi-laminar disc, formation of a pro-amniotic cavity within the embryonic lineage, appearance of the prospective yolk sac, and trophoblast differentiation. Using human embryos and human pluripotent stem cells, we show that the reorganisation of the embryonic lineage is mediated by cellular polarisation leading to cavity formation. Together, our results indicate that the critical remodelling events at this stage of human development are embryo-autonomous highlighting the remarkable and unanticipated self-organising properties of human embryos.

The development of the early 2‐cell mouse embryo to the late 2‐cell stage is marked by the appearance between 23 and 26 h post‐insemination of a complex of polypeptides of mol. wt. approximately 67 K. Addition of alpha‐amanitin between 18 and 21 h post‐insemination prevents or reduces the subsequent appearance of these polypeptides. Addition of alpha‐amanitin after 21 h does not obviously affect the appearance of the approximately 67 K polypeptides. A major change in synthetic profile occurs between 29 and 32 h post‐insemination involving many polypeptides. Addition of alpha‐amanitin to 2‐cell embryos prior to 29 h post‐insemination prevents the appearance of the new polypeptides observed during this major change but does not prevent the disappearance of the old polypeptides. In contrast, addition of alpha‐amanitin after this time does not affect the appearance of the new polypeptides. This result, together with other evidence presented, suggests that during the 2‐cell stage the embryonic genome shows transcriptional activity in two phases at 18‐21 and 26‐29 h post‐insemination, that these transcripts are utilized soon after their synthesis, and that most maternal transcripts used before the second phase of embryonic transcription become ineffective soon afterwards.