Secret of zebrafish stripes discovered
25 September 2007
The intricacies of how a zebrafish (Danio rerio) grows one of its stripes have been unravelled by scientists working at the University of Bath. Using time lapse photography, the team tracked the movement of pigment cells as the stripe developed. They also discovered that a molecule called Sdf guides the pigment. Sdf is also known to instruct nerve and sex cell development and the knowledge could have parallels in human development.
The research was funded by the Medical Research Council and the University of Bath, where lead author Dr Robert Kelsh is based. The study results are published in the journal Development.
Dark spots and stripes in fish, amphibians and reptiles are usually caused by a type of cell, known as a melanophore, which contains high quantities of the pigment melanin.
Using time-lapse photography, the team put together movies showing how the melanophores migrate in developing embryos of both the wild-type (naturally occurring) zebrafish and a mutant variety known as choker. One of the four stripes that usually runs along the side of the zebrafish is missing in choker, in its place a dark collar of pigment forms around the fish’s neck area.
“It is as though someone has put up ‘keep off the grass’ signs in the wild-type zebrafish to keep the melanophores in separate paths (stripes),” said Dr Kelsh.
“The melanophores stay on the footpath of the developing stripe in a very orderly fashion, but in choker it’s as though these signs have been knocked down, and the pigment cells run all over the grass between the footpaths.”
Zebrafish show their stripes in the lab.
Dr Kelsh’s team observed that, at first, cells migrate through the neck region in both wild-type and choker mutant fish to generate two stripes. Then, whilst cell migration ceases in the neck region of the wild-type embryo, melanophores in the choker mutant exit from the two stripes and busily cluster around the collar region of the developing fish.
When researchers looked at where the Sdf molecule was found, they could see that it appeared to line up in the same pattern as a third stripe in the wild-type zebrafish, and around the collar in choker mutants.
“The correlation of Sdf expression and melanophore pattern in wild-type and choker mutants embryos was so striking, it immediately suggested that Sdf plays a key role in dictating the pattern of at least this one stripe in zebrafish,” said Dr Kelsh.
To test the theory, researchers positioned a bead soaked in human Sdf onto the skin of zebrafish embryo. Even in the choker mutant, this bead attracted the melanophores to where it was implanted.
The researchers also used a technique known as protein knockdown to reduce the amount of Sdf produced in the embryo, and were able to partially remove the third stripe in wild-types and to reduce colouring in the neck of the choker mutant.
Dr Kelsh explained:
“Sdf is really important, not just for pigmentation but also the development of neurons and germ cells.
The other three stripes must have some other mechanism controlling their development. We are still looking into how these other three stripes might form.
Pigment pattern formation is a classic problem in developmental biology and whilst there have been lots of mathematical theories on how the fish make different patterns, the underlying genetics have been unclear.
Similarities between animals mean that we can take what we learn about the development of these simple models, and begin applying them to more complex systems, such as humans.”
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