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

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.

Comparative spatiotemporal analysis of Hox gene expression in early stages of intermediate mesoderm formation

“…In the Hoxa cluster, Hoxa1 is the first to be detected at stage 3 at the posterior part of the PS (data not shown), consistent with its ortholog expression in early mouse embryos (Scotti and Kmita, 2012). At stage 4, Hoxa1 expression is spread anteriorly to encompass the posterior two-thirds of the PS length, including the region that contributes cells to the prospective IM, which is in agreement with PS cell movements as recorded in Voiculescu et al (2007). It could also be detected in regions immediately lateral to the PS ( Fig.…”

Identification of emergent motion compartments in the amniote embryo

“…We note that differential rates in cell division-within various epiblastic tissue regions -fail to account for the length disparities between the paraxial and midline axial tissue. 5 Hence, our hypothesis evoked forces driving latent morphogenetic movements within the lateral epiblast. These movements were hypothesized to be latent in the intact gastrula in the sense that subtle localized tissue motion within the lateral epiblast is masked by the proximity of relatively high magnitude morphogenetic deformations such as axis elongation and mesendodermal ingression.…”

“…Assessing the role of localized morphogenetic movements in the generation of axial structures A remarkable characteristic of early embryos is morphogenetic self-regulation-an ability to restore the normal course of embryogenesis after tissue injury/insult. 5 Previous results from avian gastrulae have indicated regulation of axial form. 25 The identification of tissue motion domains in lateral tissue versus Hensen's node tissue suggests possible force-based mechanisms for driving morphological changes in whole gastrulae.…”

“…5 Instead, persistence of constant width in the lateral region versus elongation in the nearby medial region suggests that localized physical forces are deforming the embryo. We hypothesized that local or internal tissue forces (churning) drive the observed morphogenetic deformation(s).…”

“…4 In concert with the axially-localized convergent extension motion pattern, there is a bilaterally localized large-scale vortex-like motion pattern driven by intercalation of epiblastic cells. 5 These dynamic studies indicate that spatially localized movements shape the relatively uncomplicated blastodermal plate into an embryo with topologically distinct structural landmarks-a presumptive primitive streak and Hensen's node-aligned along the nascent anterior-posterior axis.…”