Skin color differences arise from differences in the number, size and arrangement of melanosomes. In dark skin, there are more melanosomes, which are larger and distributed throughout the cytoplasm rather than just in the perinuclear area as in light skin. There are two kinds of melanin, one found in people with dark hair and the other in people with red and blond hair. The epidermis responds to UV light. Melanin darkens via a photochemical reaction.
Melanocytes increase production of melanin and the numbers of melanosomes.
Inactive melanocytes become active. Melanocytes increase the arborization of their cell processes. Keratinocytes increase uptake of melanosomes form melanocytes. The melanosomes in keratinocytes are degraded as the keratinocytes move upward within the epidermis. 7-dehydrocholesterol is concentrated in the living layers of the epidermis, where it is photochemically converted to vitamin D, which is involved in absorption of dietary calcium.