Blaschko's lines, also called the lines of Blaschko, are lines of normal cell development in the skin. These lines are only visible in those with a mosaic skin condition or in chimeras where different cell lines contain different genes. These lines may express different amounts of melanin, or become visible due to a differing susceptibility to disease. In such individuals, they can become apparent as whorls, patches, streaks or lines in a linear or segmental distribution over the skin. They follow a V shape over the back, S-shaped whirls over the chest and sides, and wavy shapes on the head. Not all mosaic skin conditions follow Blaschko's lines. The lines are believed to trace the migration of embryonic cells. They do not correspond to nervous, muscular, or lymphatic systems. The lines are not unique to humans and can be observed in other non-human animals with mosaicism. Alfred Blaschko is credited with the first demonstration of these lines in 1901. Signs and symptoms The skin lesions that follow Blaschko's lines are varied. They include genetic, congenital and acquired (i.e., non-genetic) conditions. Examples include: Pigmentary disorders Nevus achromicus (including incontinentia pigmenti achromians, also known as hypomelanosis of Ito) Epidermal nevus Nevus sebaceus Inflammatory linear verrucous epidermal nevus X-linked genetic skin disorder Incontinentia pigmenti CHILD syndrome X-linked reticulate pigmentary disorder Acquired inflammatory skin rashes Lichen striatus Lichen planus Lupus erythematosus Chimerism McCune-Albright syndrome History Alfred Blaschko, a private practice dermatologist from Berlin, first described and drew the patterns of the lines of Blaschko in 1901. He obtained his data by studying over 140 patients with various nevoid and acquired skin diseases and transposed the visible patterns the diseases followed onto dolls and statues, then compiled the patterns onto a composite schematic of the human body. He described a system of lines across the surface of the human body that nevi and dermatological diseases tended to follow, unrelated to the dermatomes of the body or any other cutaneous or subcutaneous structure. In the same month of 1901, an American dermatologist named Douglas Montgomery presented his own research paper before the American Dermatological Society, based on his studies of extensive linear nevi in human patients. Similar to Blaschko, he suggested that the linear patterns of nevi followed "streams" of tissue growth that occurred during embryogenesis rather than being related to an epidermal structure. In 1945, the Journal of Heredity published Russian scientist Moisey Davidovich Zlotnikov's