Figure 6. Hypothetical mechanisms driving notochord submergence.

Depicted here are three non-mutually exclusive mechanistic events that could lead to the submergence of midline cells (red) from the endoderm (purple) on the embryo’s surface. The first two mechanisms (epithelium bending and ingression of individual cells) might exert forces emanating directly from the midline (red arrows): apical constriction of midline cells could lead to epithelial bending or ingression of individual cells. In the third (purse-string closure), adjacent endodermal cells could exert forces (purple arrows) to promote internalization of midline cells via a purse-string mechanism, as seen in Drosophila dorsal closure or wound healing events. D = dorsal, V = ventral