Mammalian mesoderm morphology and migration dynamics are regionally regulated in the gastrulating mouse embryo

In mouse embryo gastrulation, epiblast cells delaminate at the primitive streak through an epithelial-mesenchymal transition, followed by cellular migration. This process leads to mesoderm and definitive endoderm specification. Little is known about mouse mesoderm migration and subsequent fate divergence among its subpopulations.Mosaic expression of a membrane reporter in nascent mesoderm enabled recording cell shape and trajectory through live imaging of embryonic and extra-embryonic populations. Transcriptomes were generated to detect potential differentially enriched gene signatures in embryonic and extra-embryonic mesoderm.We have found that, 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 patterns of mesoderm subpopulations.Doctorat en Sciences biomédicales et pharmaceutiques (Médecine)