Did you know you have stripes?

 

Before you go looking, they're invisible - if they were visible, we wouldn't need to tell you about them! The stripes (or Blaschko's lines, after the discoverer Alfred Blaschko) typically follow the same basic pattern - V-shapes on the back, S-shapes on the abdomen and an inverted U-shape around the breast and upper arm. They seem to be a clue to embryological development, though they do not correspond to structures such as nervous, endocrine, lymphatic or vascular.

Though we still don't know where they come from, the current idea is that each tissue patch is made of cells all originating from a single cell during development. The clone cells then follow a set pattern, resulting in Blaschko's lines. The biologist PZ Myers uses the analogy of a clay figurine to describe this idea: "Imagine taking a piece of yellow clay and sandwiching it between two pieces of green clay into a block, and then pushing and stretching the clay block to make a human figurine. The yellow would make a band somewhere in the middle, all right, but it wouldn’t be a simple rectilinear slice anymore — it would express a more complex border that reflected the overall flow of the medium."

Though these stripes are invisible for the majority of people, some conditions cause them to become very visible. The best examples generally come from people who are mosaics/chimeras (mosaics have two or more genetically-distinct cell populations but come from a singly zygote, Chimeras also have geneticially distinct cell groups but come from two zygotes). The stripes betray the different genotypes. Additionally, if one of the genotypes is predisposed toward dermatological conditions, these conditions can develop according to the lines.

They can also become visible in women as a result of X-inactivation, where an X chromosome is shut down in each cell (you may have two, but you only need one to function). Some cells will have the father's X chromosome active, other cells have the mother's. Technically, this means all women are mosaics, because different X chromosomes are silenced in different cells. Visible Blaschko's lines can result from an X-linked skin condition.

So what do they mean? Well, we like Myers' take. "The cool thing about [Blaschko's lines] is that there is a hidden map of your secret history as an individual embedded in silent patterns in your skin - you were not defined as a single, simple, discrete genetic entity at fertilization, but are the product of complicated, subtle changes and errors and shufflings and sortings of cells. We’re all beautiful pointillist masterpieces."