The physiology of pigmented nevi.

Melanocytes are pigment-producing cells derived from the neural crest. These specialized exocrine cells produce melanin, which is packaged and dispersed to neighboring keratinocytes in organelles called melanosomes. Within the melanocyte, tyrosine is converted to dopa, and then dopaquinone via the bifunctional enzyme tyrosinase. Dopaquinone is oxidized further to form the pigment melanin. Each epidermal melanocyte secretes melanosomes to approximately 36 adjacent keratinocytes, forming an epidermal melanin unit. Genetically programmed constitutive skin color is determined by the amount of cutaneous melanin pigmentation. The common mole or acquired melanocytic nevus (AMN) is a collection of nevomelanocytes grouped into nests located in the epidermis (junctional nevus), dermis (dermal nevus), or both (compound nevus). It is hypothesized that nevomelanocytes are derived from either epidermal melanoblasts or dermal Schwann cells. AMN first appear at approximately 1 year of age, peaking in number during the second or third decades of life, and disappearing by the seventh to ninth decades. AMN may appear suddenly or become more prominent in response to sun exposure, cortisone and corticotropin, blistering diseases, chemotherapy, immunosuppression, and other factors that are not well-defined. Reports of AMN increasing in size and darkening in color during puberty and pregnancy have been reported but not quantitated systematically.melanocytes, keratinocytes, acquired melanocytic nevi.