MHB gene regulatory network and model. (A) Gene regulatory network ofthe six IsO genes considered (adapted from ). The two morphogens, which are encoded by Fgf8 andWnt1 are shown as blue circles. Arrows represent activation/productionand bars represent inhibition. (B) Boolean update functions for each node . ∧ is the Boolean AND, ∨ is the Boolean OR. (C)Partial differential equation model derived from the Boolean update functions [10, 12, 13]. State variables of the macroscopic model of the two-scale process are:u(t,x) = (Otx2(t,x),Gbx2(t,x),Fgf8(t,x),Wnt1(t,x),En(t,x),Pax(t,x))T(cell-specific) andv(t,x) = (Fgf8(t,x),Wnt1(t,x))T(diffusive).