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.
We can see these invisible lines by taking advantage of mosaicism: generate a fly wing with two genetically distinct cell types, for instance by making one type express a pigment marker and the other not, and the boundaries become apparent. There are many ways we can generate mosaics, but in Drosophila we can use somatic recombination — with low frequency, chromosomes in the fly can undergo crossing over in mitosis, not just meiosis, so sometimes the swapping of chromosome segments will turn a daughter cell that should have been heterozygous for an allele into one that is homozygous, allowing a marker allele to express itself.
It's like a secret code written in molecules hidden to the eye until you illuminate it in just the right way to expose it. And these lines aren't just arbitrary, they're significant. The wing boundary defines the expression of important molecules that define the identity of specific structures. The posterior half of the wing is the domain of expression of a molecule called engrailed, which is part of the machinery that makes the back half a back half. We can also stain a wing for just that gene product, and also expose the hidden lines.
We can also mutate the pathway of which engrailed is part, and do interesting things to the fly wing, like turn the back half into a mirror image of the front half. So these lines actually matter for the proper development of a fly.
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.
Here is the standard arrangement:
And here are a few examples:
Note that usually there isn't a whole-body arrangement of tiger stripes everywhere — there may be a single band of peculiar skin that represents one part of the whole.
Where do these come from? The current hypothesis is that a patch of tissue that follows a Blaschko line represents a clone of cells derived from a single cell in the early embryo. These clones follow stereotypical expansion and migration patterns depending on their position in the embryo; this would suggest that a cell in the middle of the back of a tiny embryo, as it grows larger with the growing embryo, would tend to expand first upwards towards the head and then sweep backwards and around to the front. One way to think of it: 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.
What makes the lines visible in some people? The likeliest example is mosaicism, a difference between two adjacent cells in the early embryo that then appears as a genetic difference in the expanded tissues. There are a couple of ways human beings can be mosaic.
The most common example is X-chromosome inactivation in women. Women have two X-chromosomes, but men only have one; to maintain parity in the regulation of expression of X-linked genes, women completely shut down one X. Which one is shut down is entirely random. That means, of course, that all women are mosaic, with different X-chromosomes shut down in different cells. This normally makes no difference, since equivalent alleles are present on each, but occasionally an X-linked skin disorder can manifest itself in a splotchy pattern. Another familiar example is the calico fur color in female cats, caused by the random expression of a pigment gene on the feline X chromosome.
A more spectacular example is tetragametic chimerism. This rare event is the result of the fusion of two non-identical twins at an early stage of development, producing an embryo that is a kind of salt-and-pepper mix of two individuals. After the fusion, the embryo develops normally as a single individual, but genetic or molecular tests can detect the patches of different genotypes. (No scientific tests can tell whether the individual has two souls, however.)
Another way differences can arise is by somatic mutation. Mutations occur all the time, not just in the germ line; we're all a mixture of cells with slightly different mitotic histories and some of them contain novel mutations, usually not of a malign sort, or you wouldn't be reading this right now. But what can happen is that you acquire a mutation in one cell that may predispose its clone of progeny to form moles, or acquire a skin disease, or even tilt it towards going cancerous. It's a fine thing to undergo genetic screening to find that you may not carry certain alleles associated with cancer, but you aren't entirely off the hook: you may have patches of tissue in your body that are perfectly normal and functional except that they carry an enabling mutation that occurred when you were an embryo.
One final likely mechanism is epigenetic. Throughout development, genes are switched on and off by epigenetic modification of the DNA. This process can vary: epigenetic silencing doesn't have to be 0 or 100% absolute, but can differ in degree from cell to cell. It can also vary by chromosome — you're all diploid, and epigenetic modification may affect one chromosome of a pair to a different degree than the other. Since epigenetic modifications are inherited by the progeny of a cell, that means these differences can be propagated into a clonal patch…that on the skin, will likely follow the lines of Blaschko.
Don't fret over these lines; they aren't a disease or a problem or even, in most cases, at all visible. The cool thing about them 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.