Let us quote from his blog post: Coloured Cells Chase Each Other To Make A Fish’s Stripes:
[An]... astonishing GIF shows a microscopic chase scene: A black cell flees from the touch of a yellow cell, and the yellow cell goes after it.
On their own, the two cells go round and round. But if there are lots of them, the yellow cells end up corralling the black ones into long bands. And that, according to Hiroaki Yamanaka and Shigeru Kondo from Osaka University, is why zebrafish gets its stripes......
Kondo’s work has a long pedigree that began with the English mathematician Alan Turing. When Turing wasn’t changing the face of computer science or breaking German codes in WWII, he was thinking about animal patterns. In 1952, he proposed a simple mathematical model involving two molecules: an activator that produces a pattern, and an inhibitor that blocks it. Both diffuse through the skin, and react with each other. By evolving small changes in how quickly these molecules spread and how strongly they interact, animals can produce radically different patterns, from cheetah spots to zebra stripes...
Turing’s ideas about “reaction-diffusion systems” were based on abstract maths. But in recent decades, scientists like Kondo have shown that many animal patterns behave exactly as he predicted.
...
To test ...[their ideas], Yamanaka and Kondo harvested two types of pigmented cells from the fins of zebrafish: black melanophores and yellow xanthophores. On their own, both types of cell move in random directions. But when Yamanaka and Kondo mixed the cells, they saw something astonishing.
The yellow cells speed up, and actively extended finger-like projections called pseudopodia towards the black ones. Upon contact, the black cells recoil and run away, only to be pursued by the yellow ones. And since the black cells are still slightly faster, the result is a continuous “run-and-chase movement”. (For context, the black cells move at just over 2 micrometres (millionths of a metre) per hour. They’re around 50 micrometres wide, so it takes them a day to cover their own length.)
If hundreds of these chases play out across a crowded skin, Yamanaka and Kondo believe that the yellow cells would collectively push black ones away, resulting in clearly defined stripes of dark and light. And that, of course, is exactly what you see in a normal zebrafish.
And it’s not what happens in mutant zebrafish with weird skin pattern. In the so-called jaguar mutants, which have fuzzy stripes, the black cells are less attractive to the yellow cells and less strongly repulsed by them. The two groups of cells move in the normal way on their own. It’s just their interactions that are different. Their lazier pursuits mean that they don’t segregate as neatly, which leads to fuzzier stripes.
Meanwhile, in the leopard mutants, which have spots instead of stripes, the black cells don’t flee from the yellow cells at all. Instead, they move towards them. The result is an embrace rather than a chase. Isolated black cells are killed off by the yellow ones, while those that randomly cluster together find safety in numbers and survive. The result: black spots in a sea of yellow.
At first glance, this seems very different to what Turing suggested. Rather than moving molecules that activate or inhibit the production of colour, you have moving cells that are themselves coloured. But there are similarities too. In Turing’s model, the two molecules react with one another, and both diffuse at different speeds. Here, the yellow and black cells certainly interact, and they move at different speeds.
But why exactly are the yellow cells attracted to the black ones, and why are the black ones repelled? How do the mutations behind the jaguar and leopard patterns change these movements? And why do the cell chases almost always go in an anticlockwise spiral?
And perhaps most importantly, how do these chases actually play out in the skin of a fish? ....
Reference: Yamanaka & Kondo. 2013. In vitro analysis suggests that difference in cell movement during direct interaction can generate various pigment patterns in vivo. PNAS http://dx.doi.org/10.1073/pnas.1315416111
Yong judiciously points up the limitations of the current state of this research. Such circumspection is one reason we rely on this science blogger.
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