Periodic somite segmentation in vertebrate embryos is controlled by the 'segmentation clock', which consists of numerous cellular oscillators. Although the properties of a single oscillator, driven by a hairy negative-feedback loop, have been investigated, the system-level properties of the segmentation clock remain largely unknown. To explore these characteristics, we have examined the response of a normally oscillating clock in zebrafish to experimental stimuli using in vivo mosaic experiments and mathematical simulation. We demonstrate that the segmentation clock behaves as a coupled oscillator, by showing that Notch-dependent intercellular communication, the activity of which is regulated by the internal hairy oscillator, couples neighbouring cells to facilitate synchronized oscillation. Furthermore, the oscillation phase of individual oscillators fluctuates due to developmental noise such as stochastic gene expression and active cell proliferation. The intercellular coupling was found to have a crucial role in minimizing the effects of this noise to maintain coherent oscillation.

The mechanism by which animal markings are formed is an intriguing problem that has remained unsolved for a long time. One of the most important questions is whether the positional information for the pattern formation is derived from a covert prepattern or an autonomous mechanism. In this study, using the zebrafish as the model system, we attempted to answer this classic question. We ablated the pigment cells in limited areas of zebrafish skin by using laser irradiation, and we observed the regeneration of the pigmentation pattern. Depending on the area ablated, different patterns regenerated in a specific time course. The regenerated patterns and the transition of the stripes during the regeneration process suggest that pattern formation is independent of the prepattern; furthermore, pattern formation occurs by an autonomous mechanism that satisfies the condition of ''local self-enhancement and long-range inhibition.'' Because the zebrafish is the only striped animal for which detailed molecular genetic studies have been conducted, our finding will facilitate the identification of the molecular and cellular mechanisms that underlie skin pattern formation.local self-enhancement and long-range inhibition ͉ pigment patterns ͉ reaction-diffusion mechanism A nimals exhibit a wide spectrum of pigment patterns. Although it has been attracting considerable interest for long time (1, 2), the mechanism underlying the emergence of these patterns remains largely unknown.In his pioneering paper in 1952 (3), Alan Turing showed that various spatial patterns arise in a system in which two substances react and diffuse at different rates. This system is now commonly termed as the reaction-diffusion (RD) ¶ system, and it has become the standard for theoretically studying of biological pattern formation (4, 5). To form the stationary patterns, the system needs to satisfy a necessary condition, ''local selfenhancement and long-range inhibition'' (6-9). Recent molecular genetic experiments corroborated the existence of such condition during the events in biological pattern formation (9-11).Using computer simulations, theoretical studies have shown that a variety of characteristic animal skin patterns can be reproduced by RD mechanism (12-15). Nevertheless, the similarity of the patterns made by the computer simulation is not enough to prove that the RD mechanism underlies the pigment pattern (16,17). Therefore, it is desired to identify the molecular network that controls the pigment pattern formation.Recent studies have focused on the stripe pattern of zebrafish as the model system to investigate the mechanism of animal pigmentation (18)(19)(20). Many genes related to the pigmentation of zebrafish have been identified (21-27), and investigations on the functions of these genes are underway (28-33). However, a strong theoretical premise is necessary to develop a complex molecular network that can explain the pattern formation. If it is shown that the RD mechanism underlies the stripes of zebrafish, the mechanism can be used as the ...

Although Turing's reaction -diffusion model (RD model) has been gradually accepted among biologists, application of the model is still limited. Accumulated experimental studies have shown that the morphogen gradient model can explain most patterning phenomena in embryogenesis. These experiments have been performed only in a few model animals. Therefore, it is not clear whether the discovered principle of the mechanism is generally applicable. The wing venation pattern of Drosophila melanogaster is largely determined by the morphogen gradient mechanism. We found that the gradient model cannot be applied to some other species. In the Hemiptera insect Orosanga japonicus, each individual has a unique pattern. Veins of O. japonicus extend radially from the proximal region and bifurcate to add the veins in the distal region. Interestingly, the bifurcation points are almost random and the vein number at the wing edge differs with wing size. However, the spacing between the veins is maintained evenly. Computer simulation of the RD model showed these properties do not fit the morphogen gradient model, but perfectly fit the RD model. This result suggests that the RD model may explain phenomena to which the morphogen gradient mechanism is currently believed to apply.