Images from videorecordings of wild-type (A, B, D, F, H) and NMY-2::GFP transgenic (C, E, G) embryos at comparable stages. Lateral view; anterior is to the left; ventral is down; focal plane is through the center of the embryo; embryo length is about 50μm. Panels B, D, F show an enlargement of the boxed area from panel A, at successive time points. The ‘inner’ surfaces of the endoderm precursor cells (EPCs) are outlined with dashed white lines (A-H). White arrows (C, E, G) indicate the apical, or ‘outer’ surfaces of the EPCs. Thick white spots (C, E, G) are midbodies (cell division remnants). At the 24-cell stage (A), the EPCs lie on the surface of the embryo. At the 26-cell stage (B, C), the EPCs have begun to ingress and myosin has begun to accumulate at the constricting apical surfaces of the EPCs. By the 28-cell stage (D, E), the EPCs have ingressed further. Early in the 44-cell stage (F, G), the EPCs are almost completely covered, and myosin is highly concentrated at the remaining apical surface. By the late 44-cell stage (H), the EPCs are completely internalized, and the two EPCs divide into four EPCs, less than 30 minutes after initiating ingression.

Anterior is to the left and ventral is down. The germline precursor cell, P₂, is marked with an asterisk; the localization of PAC-1 and CDC-42 is not shown in this cell, which does not display the inner-outer asymmetry of PAR-6 seen in somatic cells. Early in the 4-cell stage, PAC-1 (yellow) localizes to the inner cortex, and PAR-6 (red) and CDC-42 (bright blue) localize to both the inner and outer cortex of the somatic cells. By the late 4-cell stage, in response to an unknown cue, CDC-42 is inactivated (light blue) by PAC-1 at the inner cortex, and active CDC-42 (bright blue) recruits PAR-6 (red) to the outer cortex of the somatic cells.

Several signaling pathways act in and upon the EMS cell at the 4-cell stage and in the EPCs to control ingression. Wnt signals originate from the germline precursor cell P₂ and/or its descendants, which lie adjacent to EMS. Cell contacts polarize EMS (and all other somatic cells), ultimately leading to apical enrichment of myosin in the ingressing EPCs. SKN-1 and Wnt/Fz/Dsh signaling combine to activate endoderm specification regulators END-1 and END-3, resulting in normal E cell cycle expansion. Wnt/Fz/Dsh signaling is also required for apical myosin activation. The role of END-1 and END-3 in apical enrichment and activation of myosin is not yet known. This network of genes combines to drive apical constriction and control the normal ingression of the EPCs.