Melanin is present in human's skin, hair, and eyes. It is also important in protecting the skin from UV radiation. Melanoblasts rise from the neural crest in the developing embryo; in particular, the second month of development. The destination of these melanoblasts include the dermis, epidermis, uveal tract of eye, hair follicles, the stria vasculare, the vestibular organ and endolymphatic sac of the ear, and leptomeninges of the brain. Migration and development in the dermis takes 10-12 weeks and 12-14 weeks for the epidermis. Upon reach of destination, the melanoblasts differentiate into melanocytes, which typically occurs at six months in the developing fetus. Melanocytes in the epidermis continue to replicate and produce melanin; whereas, the dermal melanocytes decrease in number during gestation, and eventually disappear at birth.
Melanin synthesis occurs in membrane bound organelles called the melanosomes, which are produced by the melanocytes. The differences in skin color between people is based on cutaneous pigmentation. After production of melanin by melanocytes, the melanosomes must be transferred to surrounding keratinocytes. The abundance of melanocytes relatively constant in all skin types; however, the size and distribution differs within keratinocytes. The difference between dark and light skin color is that darker skinned people have many large and dense melanosomal particles; whereas, light skin color has smaller particles and are less dense. The melanosome type and distribution is present at birth.
The mechanism for how melanin helps to protect the skin from UV radiation is melanin accumulates above the nucleus and absorbs the UV-rays before they reach the nucleus and damage the DNA.
Costin, G.E., Hearing, V.J. 2007. Human skin pigmentation: melanocytes modulate skin color in response to stress. The FASEB Journal. 21(4): 976-994.