Fig. 9. Model of a Snail-dependent pathway regulating PMC ingression in the PMC-micromere GRN. Pmar1 de-repression system initiates the entire PMC specification program, and activates Alx1, Ets1 and other PMC regulatory genes (light blue oval), which in turn regulate skeletogenic differentiation genes (dark blue box). Alx1 regulates Snail (and other unknown EMT genes, denoted as X), and Snail represses Cadherin to attenuate the cell-cell adhesion, which allows PMCs to ingress into the blastocoele. Ets1 also regulates PMC ingression via unidentified EMT genes (denoted as Y). A subnetwork of EMT genes (light blue square box) regulates the EMT process of PMCs. The developmental stages shown on the left correspond to the chronological sequences of PMC developmental processes shown on the right.