Figure 8.

Pathways of diffusive and juxtacrine signalling. (a) In diffusive signalling, cells of type R (G) generate morphogens a (b) which diffuse in space and influence other cells through a signal $B_n^{\mathsf{\text{diff}}}$ which biases subsequent differentiation. (b) In juxtacrine signalling, cells of type R (G) force a signal $B_n^{\mathsf{\text{just}}}$ which acts on neighbouring cells.