Melanogenesis - click to enlarge
When skin is exposed to sunlight, ultraviolet (UV) radiation stimulates the production and release of a hormone, alpha melanocyte stimulating hormone, or α-MSH. Alpha-MSH is secreted by keratinocytes, a specific type of skin cell, then binds to receptors on the surface of another type of cell, called a melanocyte. When α-MSH binds to the receptors, which are known as MC1R, it activates the melanocytes to produce the brown/black pigment melanin. The melanin is subsequently transferred back the keratinocytes, increasing the level of melanin within the epidermis, the outer layer of the skin.
Melanin is a photoprotective to skin cells (protecting them from light/UV radiation) and the greater its density in the epidermis, the more protection is given to the layers of skin below. It does this by absorbing, reflecting and refracting light (particularly UV radiation) and preventing it from penetrating to the nucleus of keratinocytes or to lower levels of the skin. Melanin is also believed to play a role in scavenging free radicals, which can injure skin cells, and in facilitating UV induced apoptosis (programmed cell death), which removes damaged cells. Thus, melanin’s protective role goes beyond providing a physical barrier. It has been suggested that the 2-3 fold melanin levels seen in darker skin types, compared to lighter skin types, convey up to 100-fold difference in sensitivity to ultraviolet radiation due to these protective functions (see Rees 2004).