Pattern Forming Reactions and the Generation of Primary Embryonic Axes

In his pioneering paper Turing [37] discovered a mechanism that allows the generation of patterns even when starting from more or less homogeneous itial situations. To account for essential steps in the early patterning of higher organisms, several extensions have to be made to overcome problems inherent in simple Turing reaction-diffusion type mechanisms: 1. The wavelength problem: Patterns generated by the Turing reaction diffusion mechanism have a characteristic wavelength. Upon growth a transition from a polar pattern into a symmetric and ultimately into a periodic pattern is expected. However, in many developing systems substantial growth is possible without losing the polar character. It is proposed that the maintenance of a polar pattern is accomplished by a feedback of the pattern on the ability of the cells to perform the patterning reaction, i.e., on their competence. Cells distant to a once formed organizing region lose the competence to form additional maxima, making the first formed maximum dominate. 2. The midline problem: The formation of a coordinate system for a bilaterally symmetric organism requires the formation of a midline, i.e., a reference line and not a reference point for the mediolateral patterning. The formation of a single straight line requires the cooperation of a spot like and a stripe-like system. In vertebrates the midline is formed by a local elongation under the influence of the organizer. ‘In contrast, in insects an inhibitory influence from a local dorsal organizer allows midline formation only ventrally. 3 The left-right pattern: It is proposed that the midline signal induces the ‘left’ signal but higher levels of the midline signal repress the ‘left’ signal. Therefore, the ‘left’-signal has to escape from the midline to a lateral position. This mechanism needs only a minute asymmetry for a reproducible shift to the left. it accou if the systematic asymmetry L. lost.