Animal and vegetal poles of the mouse egg predict the polarity of the embryonic axis, yet are nonessential for development

Recent studies suggest early (preimplantation) events might be important in the development of polarity in mammalian embryos. We report here lineage t racing experiments with green fluorescent protein showing that cells locate d either near to or opposite the polar body at the 8-cell stage of the mous e embryo retain their same relative positions in the blastocyst, Thus they come to lie on either end of an axis of symmetry of the blastocyst that has recently been shown to correlate with the anterior-posterior axis of the p ostimplantation embryo (see R. J. Weber, R, A. Pedersen, F, Wianny, M, J, E vans and M, Zernicka-Goetz (1999), Development 126, 5591-5598). The embryon ic axes of the mouse can therefore be related to the position of the polar body at the 8-cell stage, and by implication, to the animal-vegetal axis of the zygote. However, we also show that chimeric embryos constructed from 2 -cell stage blastomeres from which the animal or the vegetal poles have bee n removed can develop into normal blastocysts and become fertile adult mice . This is also true of chimeras composed of animal or vegetal pole cells de rived through normal cleavage to the 8-cell stage. We discuss that although polarity of the postimplantation embryo can be traced back to the 8-cell s tage and in turn to the organisation of the egg, it is not absolutely fixed by this time.