Restricted-range gradients and travelling fronts in a model of juxtacrine cell relay.

Patterning events in development often depend on the transmission over a range of several cell diameters of signals emanating from a localized source. Experimental studies of such long-range signalling by members of the TGF-beta family of growth factors suggests that a cell-relay mechanism in which cells signal only with their immediate neighbours (i.e., juxtacrine signalling) may be operating in some tissues. Here, this possibility is investigated through the analysis of a model of juxtacrine signalling. Depending on the strength of the signal relay between cells, a localized signal source can generate either stable gradients or travelling fronts of cell activation. Both of these behaviors could in principle be involved in the long-range transmission of signals and patterning of cell fates by cell relays. There are significant and surprising differences between the gradients generated by the mechanism studied here, and those generated by the diffusion of a morphogen. In particular, there is an upper limit on the distance over which any given level of cell activation can be attained in a relay-mediated gradient. irrespective of the strength of signal source.