Fig. 8. Model of fog function in controlling cell shape change. (A) The patterning gene twist (twi), a transcription factor, specifies mesodermal fate of the ventral cells. As a consequence of twi expression, these cells activate transcription of fog (arrow), resulting in the production and secretion of fog protein from the apical side of the cell (blue dots). (B) Reception of fog signal on the apical side of the cell results in localized activation of ROK (red asterisk) which in turn activates the contractility of myosin with actin. This local source of actomyosin contractility drives myosin (pink) to the apical side of the cell (arrows). (C) The actin-myosin cytoskeleton is tethered to the cell surface through adherens junctions (orange). The force generated by apically localized contraction of the actin-myosin cytoskeleton therefore pulls down and flattens the domed apical cell surface and draws the adherens junctions up to the apical edge of the cell (arrows). (D) The continued contraction of apical actin-myosin exerts further force on the adherens junctions, pulling them close together, and resulting in the apical constriction of the cells.