Embriologia

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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 inCaenorhabdidtis 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 Celldetermination 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 andembryos 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 relatedphenomena 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/earlydifferentiation/mammalian embryos/polarity

Introduction
The regulation of primary embryonic differentiation and the establishment of embryonic axes are fundamental to life. In many species, an initial polarity of the oocyte and early embryo dictates the establishment of antero–posterior (A/P), dorso–ventral (D/V) and left/right (L/R) axes. The three germ layers differentiate, soma and germline form,and inductive systems regulate embryonic development. Comprehensive discussions of early differentiation in the animal kingdom have been presented by Davidson (1986, 1989), Gurdon (1992) and Wilkins (1993). This review focuses on early development in laboratory mammals and humans, members of Phylum Chordata, subclass Eutheria, as distinct from the egg-laying mammals (Prototheria) and marsupials(Metatheria), and attempts to place these analyses in an evolutionary context. Members
© European Society for Human Reproduction and Embryology

of ancient and modern phyla share reproductive features (Figure 1), and striking examples of long-lasting homologies include conserved pathways such as the homeobox genes in Drosophila, amphibia and eutherians, and oocyte polarity in all animal speciesfrom Caenorhabdidtis elegans to marsupials (Ohno, 1976; Selwood, 1994). Oocyte polarity is plainly visible by pigment distribution in Xenopus laevis, and by shape in the fruit fly Drosophila and the nematode C. elegans. It is not obvious in sea urchins and ascidians. Experiments dating from the beginning of this century showed how polarized ooplasmic determinants regulate embryonic differentiation byimposing an ‘animal/ vegetal axis’ in developing embryos (Wilson, 1928; Davenport, 1979; Gerhart, 1980; Davidson, 1986; Keller, 1986; Schroeder, 1986; Wilt, 1987; Gardner, 1996a). Conventionally, the first polar body marks the animal pole, and the vegetal pole lies diametrically opposite in the oocyte. This polarity 863

R.G.Edwards and H.K.Beard

Figure 1. Phylogenetic relationships and...
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