Formation of a Morphogen Gradient: Acceleration by Degradation

Abstract

Concentration profiles of signaling molecules, known as morphogen gradients, determine polarity and spatial patterning in the development of all multicellular organisms. A widely used approach to explain the establishment of morphogen concentration gradients assumes that signaling molecules are produced locally, then spread via a free diffusion along the line of developing cells and degraded uniformly. However, recent experiments have produced controversial observations concerning the feasibility of this theoretical description. Some experimentally measured dispersions for morphogens cannot support fast formation of stationary concentration profiles. In addition, the latest theoretical analyses of times to establish the morphogen gradient yield a surprising linear scaling as a function of length from the source that is not expected for the unbiased diffusion process. We propose here a theoretical approach that provides a possible physical–chemical mechanism to explain these observations. It is argued that relaxation times to establish morphogen gradients are mostly determined by first arrival times, and the degradation plays a critical role in this mechanism by effectively accelerating diffusion of signaling molecules via removal of slow moving particles. This coupling between diffusion and degradation is analogous to the action of the effective field that drives particles away from the local source.