By Sophia David
What do butterflies, snakes and fish all have in common? One answer could be that they all display colourful and spectacular skin pigmentation patterns. The zebrafish, for example, displays a beautiful and characteristic stripy pattern.
In the last decade, zebrafish, also known as Danio rerio, have emerged as an excellent model organism for studying vertebrate biology and, in particular, vertebrate development. This is due to the ease of maintaining large stocks of zebrafish, their quick development, and the transparent nature of zebrafish embryos and larvae. Luckily then, scientists wishing to study how pigment pattern formations develop already have a great model organism at their fingertips.
The zebrafish stripe pattern consists primarily of two types of pigment cell: melanophores (black pigment cells) and xanthophores (yellow pigment cells). Mutants that lack either of these types of cells do not show the stripy pattern.
Previous work by scientists from Osaka University in Japan previously showed that interactions between these two types of pigment cells are important for the development of the stripy pattern. In particular, they found that direct contact between xanthophores and melanophores causes the membrane potential of melanophore cells to change. This is called membrane depolarization. They hypothesized that the membrane depolarization of melanophores affects the movement of the cells and these movements, in turn, result in the formation of the characteristic pigment patterns.
In the study published this week in the journal PNAS, the same scientists tested and confirmed their hypothesis, and further characterized the interaction between the two types of pigment cell. They showed that the xanthophore cells reach out to touch melanophores by extending a part of their cell. These temporary projections of cells are called pseudopodia. Meanwhile, the melanophores show a repulsive response to the pseudopodia of xanthophores and move away. The xanthophores are not discouraged, however, and continue to chase the running melanophores. The authors called these “run-and-chase” movements. They believe that these movements cause the segregation of xanthophores and melanophores into distinct stripes.
The scientists further demonstrated that these run-and-chase movements are disrupted in mutant zebrafish that do not show the typical stripy patterns. For example, “jaguar” mutants have broader and fuzzier stripes. The scientists showed that the repulsive response of melanophores in jaguar zebrafish is inhibited compared to in wild-type zebrafish so essentially the melanophores cannot “run away” so quickly. This is thought to lead to the incomplete segregation of the two types of cell, resulting in broader and fuzzier stripes.
There is still much left to understand, however. The next steps are to understand the precise molecular mechanisms that occur when the two types of cells interact and how these lead to specific cell movements. Furthermore, the scientists want to understand how those mechanisms differ in the mutant zebrafish.