Spatial Patterning in Explicitly Cellular Environments: Activity-Regulated Juxtacrine Signalling

Abstract

Pattern formation in multicellular organisms generally occurs within populations of cells that are in close contact. It is thus natural and important to consider models of pattern formation that are constructed using a spatially discrete cellular structure. Here, the particular case of pattern formation in cellular systems that depends on contact-dependent (juxtacrine) signalling between cells is discussed. Spatial and spatio-temporal patterns can emerge in populations of cells coupled by juxtacrine signalling when the degree of activation of the relevant cell-surface receptors regulates both the pathway of differentiation adopted by the cell and the ability of the cell to participate in further juxtacrine signalling. When this latter condition applies, juxtacrine signalling couples all the cells of a population to form a spatially extended signalling network. Due to the essential nonlinearity of the signalling, such juxtacrine networks can exhibit dynamics that are quite different to those in networks of cells coupled by linear diffusion. Two simple cases are discussed here, in which receptor activation either diminishes or enhances the signalling ability of a cell. In the former case, signalling can act to amplify small differences between cells via a feedback-mediated competition, leading to stable spatially periodic patterns (a process known as lateral inhibition). In the latter case, signalling can result in a range of different patterns, including stable spatial gradients, propagating fronts, and periodic and quasi-periodic spatial patterns. These quite simple examples serve to illustrate the potential richness of this important class of biological signalling, and provide guidance for the development of more complex models.