Figure 2. Hypothetical mechanisms driving midline emergence.

During gastrulation, cells emerge from the epiblast layer and go through EMT to form the mesodermal layer. Emanating from the primitive streak, the axial mesodermal cell population (in red) goes through complete or partial EMT and rapidly becomes morphologically distinct from the rest of the mesoderm (in pink). Cells might redefine or invert their polarity and go through complete or partial MET transition to invade the surface of the embryo. As their apical surface is relatively small compare to adjacent endodermal cells, it is likely that apical constriction plays a role in their emergence. The contraction of actin cables by the motor non-muscle myosin II is generally the main mechanism to generate the forces necessary for apical constriction (center). Shown on the right hand side is a wholemount ventral view of the mouse embryo at E7.0–7.25 displaying F-actin staining (in red and monochrome) of axial mesoderm cells emerging at the surface of the embryo together with a green endoderm marker and blue nuclear marker. A/B = Apical/Basal