1. Department of Anatomy & Developmental Biology, University College London, Gower Street, London WC1E 6BT, UK
2. Department of Biology, Gilmer Hall, University of Virginia, Charlottesville, Virginia 22903, USA

Correspondence to: Octavian Voiculescu¹Claudio D. Stern¹ Correspondence and requests for materials should be addressed to O.V. (Email: o.voiculescu@ucl.ac.uk) or C.D.S. (Email: c.stern@ucl.ac.uk).

Abstract

During gastrulation, a single epithelial cell layer, the ectoderm, generates two others: the mesoderm and the endoderm. In amniotes (birds and mammals), mesendoderm formation occurs through an axial midline structure, the primitive streak¹, the formation of which is preceded by massive 'polonaise' movements^{2, 3} of ectoderm cells. The mechanisms controlling these processes are unknown. Here, using multi-photon time-lapse microscopy of chick (Gallus gallus) embryos, we reveal a medio-lateral cell intercalation confined to the ectodermal subdomain where the streak will later form. This intercalation event differs from the convergent extension movements of the mesoderm described in fish and amphibians (anamniotes)^{4, 5, 6, 7, 8}: it occurs before gastrulation and within a tight columnar epithelium. Fibroblast growth factor from the extraembryonic endoderm (hypoblast, a cell layer unique to amniotes) directs the expression of Wnt planar-cell-polarity pathway components to the intercalation domain. Disruption of this Wnt pathway causes the mesendoderm to form peripherally, as in anamniotes^{1, 9}. We propose that the amniote primitive streak evolved from the ancestral blastopore by acquisition of an additional medio-lateral intercalation event, preceding gastrulation and acting independently of mesendoderm formation to position the primitive streak at the midline.

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