There’s more to us all than we can see.
Expose the human body to concentrated ultraviolet rays, and stripes emerge. They cannot be seen with the naked eye, except in those with certain, rare skin conditions. But they’re on every human (and exist in certain other species), and they tell the story of how our bodies first took shape, in the womb.
The stripes are called Blaschko’s lines, after the German dermatologist Dr Alfred Blaschko, who first described them in 1901. He noticed that about 150 of his patients all had rashes and moles that seemed to follow a certain pattern. These were arcs on the chest, S shapes on the abdomen, and V shapes down the back, he noted.
Research into what his findings might indicate intensified in the 1970s. German physician Rudolf Happle, founding president of the European Society for Paediatric Dermatology, eventually traced the lines using intense ultraviolet light, and discovered more of them, including spirals on the scalp and squiggles on the neck.
Happle, now in his 80s, studied these lines for decades, and concluded — across various research papers and a 1994 book titled Mosaicism in Human Skin — that the lines and squiggles trace the migration of embryonic cells through the body, as the foetus develops from a fertilised egg.
“The basis for this pattern is that some cells are changed genetically or epigenetically during development,” says Cheng Ming Choung, professor of pathology at University of Southern California, in an essay on Blaschko’s lines published in the journal Experimental Dermatology in 2006.
There is still research underway to determine exactly why the skin registers these striations as it does. One theory suggests that they are a sort of Milky Way of the original cells, spread out as the embryo grew, amid the cellular mutations of later stages.
Some skin rashes and conditions of pigment, such as linear psoriasis, band-like scleroderma, linear lichen planus and linear eczema, tend to follow these lines. (Researchers are trying to understand why). But for the most part, in most humans, the stripes appear to play no role.
Which other species have them? Dogs and cats don’t seem to (because of how their skin and fur develop in relation to pigment). But Choung studied another species, in 1998, as part of his research into the lines in humans, and found that, of all things, chickens have similar stripes on the skin, invisible to the naked eye. It is likely that all this will keep researchers riveted for decades more.
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