Melanophores in the stripes of adult zebrafish do not have the nature to gather, but disperse when they have the space to move.

Animal skin pattern is one of the good model systems used to study the mechanism of pattern formation. Molecular genetic studies with zebrafish have shown that pigment cells play a major role in the mechanism of stripe formation. Among the variety of cellular events that may be involved in the mechanism, aggregation of melanophores has been suggested as an important factor for pattern formation. However, only a few experimental studies detected the migration ability of melanophores in vivo. Here, we tried to determine whether melanophores really have the ability to aggregate in the skin of zebrafish. Melanophores in the adult stripes are packed densely and they rarely move. However, when the neighboring pigment cells are killed, they move and regenerate the stripe pattern, suggesting that melanophores retain the migration ability. To analyze the migration, we ablated a part of the melanophores by laser to give free space to the remaining cells; we then traced the migration. Contrary to our expectation, we found that melanophores repulsed one another and dispersed from the aggregated condition in the absence of xanthophores. Apparent aggregation may be forced by the stronger repulsive effect against the xanthophores, which excludes melanophores from the yellow stripe region.