Melanin has biophysical and biochemical properties related to its functions in skin. It protects against the damaging effects of UV radiation on DNA and is also an efficient scavenger of damaging free radicals. However a high concentration of melanin may adversely affect synthesis of vitamin D in darker-skinned individuals living in northern latitudes. Melanin pigmentation is both constitutive and facultative. Constitutive pigmentation is the intrinsic level of pigmentation and is genetically determined, whereas facultative pigmentation represents reversible changes induced by environmental agents, e.g. UV and X-radiation, chemicals, and hormones. Racial variations in pigmentation are due to differences in melanocyte morphology and activity rather than to differences in frequency or distribution. In naturally heavily pigmented skins the cells tend to be larger, more dendritic, and to contain more large, late-stage melanosomes than melanocytes of paler skins. The keratinocytes in turn contain more melanosomes, individually dispersed, whereas in light skins, the majority are contained within secondary lysosomes to form melanosome complexes.
Response to UV light includes immediate tanning, pigment darkening, which can occur within a matter of minutes, probably due to photo-oxidation of pre-existing melanin. Delayed tanning occurs after about 48 hours, and involves stimulation of melanogenesis within the melanocytes, and transfer of additional melanosomes to keratinocytes. There may also be some increase in size of active melanocytes, and in their apparent numbers, mainly through activation of dormant cells. Freckles in the human body of red-haired individuals are usually thought to be induced by UV, though they do not appear until several years after birth, despite exposure. Paradoxically, melanocytes are significantly fewer in freckles than in adjacent paler epidermis, but they are larger and more active. What determines the onset of freckles, or their individual location, is not known.
