The lines indicate the pathways that replicating cells have followed from the development of the fetus until adulthood. So the patterns are just a consequence of how the skin has developed.
We all have these lines but normally they remain invisible, except when a person suffers a skin condition that follows Blaschko’s lines, which either can be inherited or acquired skin diseases. For example: pigmentary disorders, X-linked genetic skin disorders or acquired inflammatory skin rashes.
Blaschko’s lines are believed to be a skin expression of mosaicism: the presence of two (or more) populations of cells with different genotypes in one individual who has developed from a single fertilized egg. When two populations of cells have different DNA, skin cells can get different instructions for how dark to make the skin. As a consequence Blaschko’s lines may become visible.
“When there is a big difference between the two DNA’s instructions on how dark to make the skin, then you get obvious Blaschko’s lines,” explains Geneticist Barry Starr of Stanford University. “If the differences are more subtle, then you might need something extra like UV light to see the pattern.”
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