The Hidden Markings on Human Skin Revealed.
Humans have invisible skin patterns called Blaschko lines that trace the journeys our cells took as they divided and grew into the skin during embryonic development. These lines are drawn by the paths of keratinocytes and melanocytes. Mutations involving color-producing cells can lead to pigmentary mosaicism presenting as streaks and swirls following Blaschko lines. Another rare but extreme example of mosaicism in humans is chimerism. These lines can also help doctors diagnose skin conditions.