Scientists at the University of Bath and the University of Edinburgh have been taking a closer look at developing mice. Specifically, they have been looking at the embryos of piebald mice to see the patterns that determine a mouse’s final pigmentation. In a paper published in Nature Communications, the researchers just admitted and announced that there doesn’t seem to be any set patterns.
This comes as a surprise to many folks. Scientists have always assumed that piebald animals—especially mice, cats, and horses—got their color patterns in utero. In developed skin, pigment is put out by a process called melanocytes, a specialized skin cell. Embryonic animals possess proto-melanocytes, called melanoblasts. These spread slowly through the also-developing skin. Scientists did not assume it that each cat or mouse developed a perfect pattern, but they did assume that there was something guiding the way the melanoblasts themselves moved.
For example, cats have black backs and white bellies tend to have a defective version of a gene which they call “kit.” From a scientist’s point of view, these were black cats whose melanoblasts started developing along their backs, then moved down towards their bellies. Unfortunately this happened only in late development, the melanoblasts moved more slowly than usual, and the they didn’t quite close in over the belly before the cat was fully and truly developed.
When they studied developing mice, however, the researchers found that melanoblasts do not behave anything like that. First of all, they mostly proliferate in early development. Secondly, when they do spread out, they do so randomly. Although melanoblasts do sometimes repel each other when they get too close, when scientists studied how they moved (and took images after in twenty minute intervals) they noted that the repulsion didn’t actually speed dispersal.
In conclusion, the researchers found that skin cells become pigmented through “process of undirected migration, proliferation and tissue expansion.” There’s no director-protein or special chemical allowing maelanoblasts in one cell know they should expand to the next. The pigment simply goes wherever. And while there are larger factors that affect how the cat looks—for example, it turns out the cats with the defective kit gene have melanoblasts which don’t multiply as fast as they do in other cats, which is why they are part-white—there’s no direction at all to where the melanoblasts go.