[Patterns on the skin. New aspects of their embryologic and genetic causes]

The concept of cutaneous mosaicism has now been proven by numerous studies. In this way, the idea that all nevi represent mosaics has so far been confirmed. We can distinguish five different patterns of mosaicism in human skin in the form of lines of Blaschko, checkerboard pattern, phylloid pattern, patchy pattern without midline separation, and lateralization. Two different etiological categories are epigenetic and genomic mosaicism. According to present knowledge all epigenetic mosaics are caused by the activity of retrotransposons. In addition to the well-known examples of functional X-chrosomosome mosaicism, the concept of autosomal functional mosaicism, as observed in mice and dogs, has recently been established. Most likely, this mechanism may also explain the exceptional familial occurrence of pigmentary mosaicism in man. Phylloid hypomelanosis and speckled lentiginous nevus syndrome are recently described phenotypes that can be added to the list of lethal mutations surviving by mosaicism. The concept of type 2 segmental manifestation of autosomal dominant skin diseases has recently been confrirmed at the molecular level in a case of Hailey-Hailey disease. The concept of didymosis may explain why two different nevi can occur in spatial and temporal proximity, e.g., cutis tricolor or phacomatosis pigmentovascularis. The mechanism of paradominant inheritance may explain why some nevi that usually occur sporadically may affect, by way of exception, several members of a family. Examples are sebaceous nevus and Becker nevus. In autosomal recessive skin disorders, an event of loss of homozygosity may occur at an early developmental stage, giving rise to healthy skin areas arranged in a nevoid distribution. For example, when dermatologists examine a patient with xeroderma pigmentosum they should pay particular attention to such areas of revertant mosaicism.