Abstract
In mouse embryo gastrulation, epiblast cells delaminate at the primitive streak to form mesoderm and definitive endoderm, through an epithelial-mesenchymal transition. Mosaic expression of a membrane reporter in nascent mesoderm enabled recording cell shape and trajectory through live imaging. Upon leaving the streak, cells changed shape and extended protrusions of distinct size and abundance depending on the neighboring germ layer, as well as the region of the embryo. Embryonic trajectories were meandrous but directional, while extra-embryonic mesoderm cells showed little net displacement. Embryonic and extra-embryonic mesoderm transcriptomes highlighted distinct guidance, cytoskeleton, adhesion, and extracellular matrix signatures. Specifically, intermediate filaments were highly expressed in extra-embryonic mesoderm, while live imaging for F-actin showed abundance of actin filaments in embryonic mesoderm only. Accordingly, Rhoa or Rac1 conditional deletion in mesoderm inhibited embryonic, but not extra-embryonic mesoderm migration. Overall, this indicates separate cytoskeleton regulation coordinating the morphology and migration of mesoderm subpopulations.
Highlights
In mice, a first separation of embryonic and extra-embryonic lineages begins in the blastocyst at embryonic day (E) 3.5 when the trophectoderm is set aside from the inner cell mass
The T box transcription factor Brachyury is expressed in posterior epiblast cells that form the primitive streak, maintained in cells that delaminate through the streak, downregulated once cells progress anteriorly in the mesodermal wings
In view of the differences in cell shape and migration, we focused on the cytoskeleton, in particular actin, and intermediate filaments proteins (Vimentin and Keratins)
Summary
A first separation of embryonic and extra-embryonic lineages begins in the blastocyst at embryonic day (E) 3.5 when the trophectoderm is set aside from the inner cell mass. A second step is the segregation of the inner cell mass into the epiblast, the precursor of most fetal cell lineages, and the extra-embryonic primitive endoderm (Chazaud & Yamanaka, 2016). At E6, the embryo is cup-shaped and its anteriorposterior axis is defined. It comprises three cell types, arranged in two layers: the inner layer is formed by epiblast, distally, and extra-embryonic ectoderm, proximally; the outer layer, visceral endoderm, covers the entire embryo surface. The primitive streak, site of gastrulation, is formed at E6.25 in the posterior epiblast, at the junction between embryonic and extra-embryonic regions, and subsequently elongates to the distal tip of the embryo. The primitive streak is the region of the embryo where epiblast cells delaminate through epithelial-mesenchymal transition to generate a new population of mesenchymal cells that form the mesoderm and definitive endoderm layers
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