You no longer need to envy the noble zebra. Humans also have stripes. In fact, all people have them, and everyone’s stripes form the same general patterns. These stripes, called Lines of Blaschko, are invisible most of the time, but certain conditions bring them out.
Image via itsokaytobesmart.com
Just around the turn of the last century, Doctor Alfred Blaschko was studying skin conditions. He noticed that people with many different skin conditions had the same patterns on their skin. This was the same whether the skin disease was inherited or acquired. It was the same in young and old, and across all races. He made a chart of the lines and named them, with a modesty characteristic of the scientific profession, Blaschko Lines.
Blaschko Lines follow the same pattern on all people. They run down the arms and legs. They curve around the sides, like tiger stripes. On the chest and upper back they rise in a swirl before dipping down to meet in a deep “v” along the spine and the middle of the chest. They also run along the face above and below the eyes and over the ears, looking a little like painted-on glasses. These lines don’t correspond with any other system in the body. They don’t follow the lines of nerves, arteries, veins, muscles, or correspond with the endocrine system.
So what are they? Most scientists think that it’s a remnant of the way the epidermal cells move in embryo development. As the fetus develops, skin cells proliferate in groups along these lines. It could be that these different waves of development respond to stresses differently. When the skin is stressed by inherited or acquired skin problems, they move along these lines.
While Blaschko Lines sound like a cheat – “We have stripes! But they’re invisible.” – but there are some dramatic examples of them.Humans with chimerism can show them dramatically. Some people (and animals), happen to have been made with two different sets of DNA. Sometimes different fertilized cells will get mixed up with each other and build one human between them. In rare cases, they can send out different waves of epidermal cells, one alternating with another. Often the skin color of the two types is indistinguishable, or subtle enough that it can only show up under black light. Sometimes, however, the two different sets of DNA code for skin types that are dramatically different, which leaves people with literal stripes on their skin in the pattern of Blaschko Lines.