Embriologia

Molecular Human Reproduction vol.3 no.10 pp. 863–905, 1997

Oocyte polarity and cell determination in early mammalian embryos

Robert G.Edwards1 and Helen K.Beard
Moors Barn Farmhouse, Madingley Road, Coton, Cambridge CB3 7PG, UK
1To

whom correspondence should be addressed

Table of Contents
Introduction Polarity and axis formation in various animal phyla Differentiation in Caenorhabdidtis elegans Differentiation in Xenopus laevis Differentiation in Paracentrosus lividus Conclusions from studies on lower animals Early determination and differentiation in mammals Polarity and axis formation Gene expression in mammalian eggs and early embryos Totipotency and cloning

Differentiation of inner cell mass, germline and trophectoderm Cell lineages in the mammalian embryo Cell determination and differentiation in early cleavage stages Lineage map of mammalian regulation and differentiation Addendum. Investigations needed on human embryos Acknowledgements References

Knowledge on determination and differentiation in the mammalian embryo has not kept pace with discoveries in other phyla. Current concepts overlook well-established pathways leading to polarity in oocytes and embryos of other phyla, modern principles of totipotency in plants and animals, and axis formation in lower vertebrates. Various models derived from invertebrates and frogs could be relevant to the situation in eutherian mammals, and we explore the nature of strict genetic controls in these species and its implications for early mammalian differentiation. Concepts on totipotency and related phenomena in animal and human embryos are examined and the possibility raised that two cell lines are formed in early human embryos from the 2–4 cell stage. Clinical consequences are assessed, including causes of the high incidence of chromosomal mosaicism in human embryos. Our interpretations are obviously speculative, and must be clarified by experimentation. Key words: axes/determination/early