The pigment melanin, which is responsible for skin and hair color in mammals, is produced in specialized cells called melanocytes and then distributed to other cells. But not every cell in the complex layers of skin becomes pigmented.
The question of how melanin is delivered to appropriate locations may have been answered by a study from researchers at the Massachusetts General Hospital (MGH) Cutaneous Biology Research Center (CBRC).
“Pigment recipient cells essentially tell melanocytes where to deposit melanin, and the pattern of those recipients determines pigment patterns,” says Janice Brissette, PhD, who led the study. “Recipient cells act like the outlines in a child's coloring book; as recipient cells develop, they form a ‘picture' that is initially colorless but is then ‘colored in' by the melanocytes.” The report appears in the Sept. 7 issue of Cell.
In humans, melanin is deposited in both the skin and the hair; but in some other mammals such as mice, melanin is primarily deposited in the coat, leaving the skin beneath the coat unpigmented. Melanocytes deposit melanin via cellular extensions called dendrites that reach out to other cells in the epidermis (the outer layer of skin) or the hair follicles. But the mechanism determining whether melanin is delivered to a particular cell has been unknown.
The MGH-CBRC researchers theorized that a mouse gene known as Foxn1 might play a role. Lack of Foxn1 is responsible for so-called ‘nude mice,' which have hair that is so brittle it breaks off, resulting in virtually total hairlessnes, and other defects of the skin. A similar phenomenon exists in humans with inactivation of the corresponding gene.