FIGURE 4.

Multicellular protein network leading to boundary formation. The main interactions between species have been included (see text). In the steady state Notch, X, pathway, i.e., lines departing from X, is active in boundary cells (dark) whereas Wg, Y, pathway is nonactive (light). The opposite occurs in neighboring nonboundary cells. This activity pattern (spatial toggle switch) is driven by the concentration pattern of Notch and Wg as indicated by the color intensity of the letters X and Y dark/light corresponds to high/low concentrations. The mathematical representation of this spatially extended motif can be obtained by using the rules shown in Fig. 3 B.