Fig. 4. Activation of the PMC GRN. Selected components of the maternal (blue) and early specification regulatory layers (yellow) are shown (see also Fig. 3). β-Catenin is stabilized in micromeres and in other vegetal blastomeres by a maternally controlled vegetal stabilization system (MVS), which requires the function of Dishevelled and other maternal Wnt signaling components. β-Catenin, acting with TCF, directly activates (green arrow) pmar1 in the micromeres. Throughout most of the embryo, HesC represses (blue line) early PMC specification genes. This repression is relieved in the micromere territory, where Pmar1 blocks hesC expression (red line), either directly or indirectly. Note that additional mechanisms play a role in restricting pmar1 expression to the micromeres, as β-catenin is stabilized throughout a broader vegetal domain. Similarly, although hesC is repressed throughout the micromere territory, unknown mechanisms restrict the activation of downstream PMC specification genes to the large micromere lineage.