The amniote primitive streak is defined by epithelial cell intercalation before gastrulation

The amniote primitive streak is defined by epithelial cell intercalation before gastrulation

Octavian Voiculescu, Federica Bertocchini, Lewis Wolpert, Ray E. Keller & Claudio D. Stern

Easy reading, clearly illustrated, great supplementary information (including videos), makes coffee taste much better. GEISHA have new images to grab.
Some gastrulation models need modifications. Other go down the drain or to the trash. Well, those using pure fluidics probably to the drain and flush.

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 streak, the formation of which is preceded by massive ‘polonaise’ movements 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): 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. 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.

Last paragraph: We propose that local intercalation in the epiblast is responsible for positioning and shaping the primitive streak and can also explain the polonaise movements (see Fig. 4a–e) without the need for long-range gradients. Convergent extension of the axial mesoderm and neural plate in anamniotes is almost certainly conserved in amniotes, but our study reveals an additional, much earlier (pre-gastrula) cell intercalation, required for morphogenesis of the primitive streak independently of mesendoderm specification. This is apparently unique to amniotes (Fig. 4f–i) and provides a possible answer to the classical question of how evolution converted the equatorial blastopore or shield of Anamnia into the radially oriented primitive streak of amniotes.