Fig. 6. EGFR regulates cortical myosin accumulation. (A) Cell boundaries with high myosin-GFP accumulation often corresponded to the outer boundary of dp-ERK. b and b' are higher magnification images of a and a', respectively. Arrows indicate the high myosin-GFP accumulation at the interface of cells with high and low dp-ERK levels. (B,C) MHC accumulated in arc-like patterns in Egfr heterozygous embryos, but these patterns were lost in Egfr homozygotes (see text for quantitative assessment). In B', 8 out of 12 class T junctions were judged to be in arcs. Two examples of arcs are highlighted with yellow curved lines 1 and 2. (D) Two classes of tricellular junctions. In class T junctions, `horizontal' cell boundaries are colored pink, and `vertical' ones, blue. Myosin concentration was more than 50% enriched in horizontal cell junctions compared to the vertical one (mean±s.e.m., arbitrary unit, number of measurement indicated in parentheses, P<0.002, Student's t-test). (E) Expression of sSPI (sSpi) by the prd enhancer in even-numbered parasegments induced the massive activation of ERK and accumulation of MHC at the cell boundaries between cells with high and low dp-ERK expression (arrowheads). sSPI also induced the precocious invagination of the segmental furrow (asterisk) that becomes contiguous with the tracheal pit (tr2). An arrow indicates unaffected segment boundary. (F) Enrichment of junctional myosin by sSPI. The intensity of myosin signal was compared between the border along the stripe of elevated dp-ERK (DV border, red lines) and those intersecting them (AP border, black lines; mean±s.e.m., arbitrary unit). sSpi significantly increased the DV boundary signal compared to AP boundary (^*, P<10^-5, Student's t-test: two-sample assuming equal variances, two-tail). No such enrichment was observed in the control segments. (G) A model for the EGFR-dependent coordination of cell movement. Left: Myosin accumulates at the boundary of cells with high and low EGFR activity. The contractile force of myosin (blue arrows) helps shrinkage of cell boundary and cell intercalation (juxtaposed cell boundaries shown in green). In addition, the contractile force smoothens other cell-row boundaries. Right: once the cell-row boundaries form a continuous arc, the net force in the convex contractile supracellular actomyosin cable (pink) is oriented toward the inside of the arc (magenta arrows). Red asterisk indicates the invagination site. Scale bars: 10 µm.