Shield formation at the onset of zebrafish gastrulation

Abstract

During vertebrate gastrulation, the three germ layers, ectoderm, mesoderm and endoderm are formed, and the resulting progenitor cells are brought into the positions from which they will later contribute more complex tissues and organs. A core element in this process is the internalization of mesodermal and endodermal progenitors at the onset of gastrulation. Although many of the molecules that induce mesendoderm have been identified, much less is known about the cellular mechanisms underlying mesendodermal cell internalization and germ layer formation. Here we show that at the onset of zebrafish gastrulation, mesendodermal progenitors in dorsal/axial regions of the germ ring internalize by single cell delamination. Once internalized, mesendodermal progenitors upregulate E-Cadherin (Cadherin 1) expression, become increasingly motile and eventually migrate along the overlying epiblast (ectodermal) cell layer towards the animal pole of the gastrula. When E-Cadherin function is compromised,mesendodermal progenitors still internalize, but, with gastrulation proceeding, fail to elongate and efficiently migrate along the epiblast,whereas epiblast cells themselves exhibit reduced radial cell intercalation movements. This indicates that cadherin-mediated cell-cell adhesion is needed within the forming shield for both epiblast cell intercalation, and mesendodermal progenitor cell elongation and migration during zebrafish gastrulation. Our data provide insight into the cellular mechanisms underlying mesendodermal progenitor cell internalization and subsequent migration during zebrafish gastrulation, and the role of cadherin-mediated cell-cell adhesion in these processes.

Initiation of convergence and extension movements of lateral mesoderm during zebrafish gastrulation

“…As the fish swims, the current of the river carries it passively downstream, so its path is diagonally across and downstream in the river. A similar situation is documented in the zebrafish embryonic shield, where the ectoderm moves vegetalward while the adherent mesoderm travels animalward over it (Montero et al, 2005). If the fish can move between fast, deep and slow, shallow currents (the analog of mesoderm switching adhesion between the overlying ectoderm and the underlying YSL), then the fish's path is intermittently dependent on the ectoderm.…”

Collective mesendoderm migration relies on an intrinsic directionality signal transmitted through cell contacts

“…Previous analyses have shown that E-cadherin is required for process formation, but its potential role on process orientation has not been analyzed (15). By using a morpholino directed against E-cadherin, we similarly found that morphant cells transplanted to a WT host form fewer cytoplasmic extensions (Fig.…”

“…The prechordal plate has been described as a cohesive group, the cohesion of which is thought to be required for effective migration (15,16). However, most studies have focused on the anterior-most cells of the plate, and the architecture of the moving group has thus not been analyzed.…”

“…Looking for the molecular bases of these cell-cell interactions, E-cadherin appeared as an obvious candidate, as it is expressed in migrating prechordal plate cells (15) and cadherin was recently involved in CIL in Xenopus neural crest cells (26). Previous analyses have shown that E-cadherin is required for process formation, but its potential role on process orientation has not been analyzed (15).…”

Quantitative differences in tissue surface tension influence zebrafish germ layer positioning

“…In zebrafish, gastrulation starts at around 50% epiboly, a point at which the blastoderm has spread and covered half of the large yolk cell, and with the internalization of hypoblast cells near the blastoderm margin at the dorsal side of the gastrula. These earliest internalizing cells form anterior axial mesendodermal (prechordal plate) tissue, expressing the homeobox gene goosecoid (gsc), while cells later internalizing at the dorsal blastoderm margin become chordamesoderm (notochord precursors), expressing the homeobox gene floating head (flh) (Dougan et al, 2003;Gritsman et al, 2000;Montero et al, 2005;Warga and Kimmel, 1990;Warga and Nusslein-Volhard, 1999). Progressive single cell ingression and convergence movements, causing cell compaction at the dorsal blastoderm margin, lead to the formation of the embryonic organizer (shield), the analog of the amphibian Spemann organizer (Dougan et al, 2003;Feldman et al, 1998;Gritsman et al, 2000;Montero et al, 2005;Schier and Talbot, 1998;Solnica-Krezel, 2006).…”