Tailoring and coupling of reaction-diffusion systems to obtain reproducible complex pattern formation during development of the higher organisms

摘要

Specific adaptations of reaction-diffusion mechanisms are discussed that accounts for the formation of complex patterns in a reproducible way during ontogenetic development. In an activator-inhibitor system, polar (i.e. monotonically graded) distributions appropriate to supply positional information can be stabilized and the transition into symmetric or periodic patterns can be suppressed either by a baseline inhibitor production or by a feedback of activator distribution on the source density. Saturation of autocatalysis enables the formation of stripes, of gradients across the shorter extension of a field, and of size-regulated steplike distributions. Superposition and coupling of several reaction-diffusion mechanisms allow the formation of bipolar and orthogonal coordinate systems. The graded geometry-sensitive patterns generated by reaction-diffusion mechanisms can be converted into more stable pattern of gene activation. The necessary switching behavior of the gene activation can be achieved by positive autoregulation and competitions of the genes and their gene products. Although the signals (gradients) are smooth, sharply confined regions emerge in which particular genes are active. The borders between such regions play a decisive role in the organization of substructures, such as legs and wings. Reproducible pattern formation during development is thought to result from the iterative process; generation of positional information, space-dependent gene activation, generation of new positional informations at the borders, etc. The models discussed are compared with experimental observations.