1. Wellcome Trust and Cancer Research UK Gurdon Institute, University of Cambridge, Tennis Court Road, Cambridge CB2 1QR, UK
2. Department of Genetics, University of Cambridge, Downing Street, Cambridge CB2 3EH, UK
3. Department of Genetics and Development, College of Physicians and Surgeons, Columbia University, 701 West 168th Street, New York, New York 10032, USA
4. Present address: Developmental Biology Program, Sloan-Kettering Institute, 1275 York Avenue, New York, New York 10021, USA

Correspondence to: Magdalena Zernicka-Goetz^1,2 Correspondence and requests for materials should be addressed to M.Z.-G. (Email: mzg@mole.bio.cam.ac.uk).

Abstract

One of the unanswered questions in mammalian development is how the embryonic–abembryonic axis of the blastocyst is first established. It is possible that the first cleavage division contributes to this process, because in most mouse embryos the progeny of one two-cell blastomere primarily populate the embryonic part of the blastocyst and the progeny of its sister populate the abembryonic part^{1, 2, 3, 4}. However, it is not known whether the embryonic–abembryonic axis is set up by the first cleavage itself, by polarity in the oocyte that then sets the first cleavage plane with respect to the animal pole, or indeed whether it can be divorced entirely from the first cleavage and established in relation to the animal pole. Here we test the importance of the orientation of the first cleavage by imposing an elongated shape on the zygote so that the division no longer passes close to the animal pole, marked by the second polar body. Non-invasive lineage tracing shows that even when the first cleavage occurs along the short axis imposed by this experimental treatment, the progeny of the resulting two-cell blastomeres tend to populate the respective embryonic and abembryonic parts of the blastocyst. Thus, the first cleavage contributes to breaking the symmetry of the embryo, generating blastomeres with different developmental characteristics.

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