Abstract
Patterns form with the break of homogeneity, leading to the emergence of new structure or arrangement. There are different physiological and pathological mechanisms which lead to the formation of patterns. Here we first introduce the basics of pattern formation and their possible biological basis. We then discuss different categories of skin patterns and their potential underlying molecular mechanisms. Some patterns, such as the lines of Blaschko and naevus, are based on cell lineage and genetic mosaicism. Other patterns, such as regional specific skin appendages, can be set by distinct combinatorial molecular codes, which in turn may be set by morphogenetic gradients. There are also some patterns, such as the arrangement of hair follicles (hair whorls) and fingerprints, which involve genetics as well as stochastic epigenetic events based on physical-chemical principles. Many appendage primordia are laid out in developmental waves. In the adult, some patterns, such as those involving cycling hair follicles, may appear as traveling waves in mutant mice. Since skin appendages can renew themselves in regeneration, their size and shape can still change in the adult via regulation by hormones and the environment. Some lesion patterns are based on pathological changes involving the above processes and can be used as diagnostic criteria in medicine. Understanding the different mechanisms which lead to patterns on the skin will help us appreciate their full significance in morphogenesis and medical research. Much remains to be learned about complex pattern formation, a level between molecular biology and organism phenotypes.