Blaschko's lines: strange stripes all over the body

“One of the subjects developmental biologists are interested in is the development of pattern. There are the obvious externally visible patterns , the stripes of a zebra, leopard spots, the ordered ranks of your teeth, etc., etc., etc. And in fact, just about everything about most multicellular organisms is about pattern. Without it, you’d be an amorphous blob.

But there are also invisible patterns that you don’t normally see that are aspects of the process of assembly, the little seams and welds where disparate pieces of the organism are stitched together during development. The best known ones are compartment boundaries in insects. A fly’s wing, for instance, has a normally undetectable line running across the middle of it, a line that cells respect. A cell born on the front half of the wing will multiply and expand its progeny to cover a patch on the surface, but none of its offspring cells will cross over the invisible line into the back half. Similarly, cells born on the back half will never wander into the front.

So you might be wondering if we have anything similar in humans and no, we don’t have strict compartment boundaries like a fly. However, we do have normally invisible lines and stripes of subtle molecular differences running across our bodies, which are occasionally exposed by human mosaicism. These are marks called the lines of Blaschko, after the investigator who first reported a common set of patterns in patients with dermatological disorders in 1901.

Don’t rip off your shirt and start looking for the Blaschko lines, they’re almost always invisible, remember! What happens is that sometimes people with visible dermatological problems — rashes, peculiar pigmentation, swathes of moles, that sort of thing, express the problems in a stereotypically patterned way. On the back, there are V-shaped patterns; on the abdomen and chest, S-shaped swirls; and on the limbs, longitudinal streaks.”