Melanocytes are pigment cells that exist in the stratum basale, or basal layer, of the epidermis. These cells produce melanin, a dark pigment that protects the body from ultraviolet (UV) radiation. Melanin's color causes it to absorb harmful ultraviolet-B (UV-B) radiation, thus shielding the underlying cells. Although melanocytes are present in the stratum basale, these cells have long appendages that reach up into the outer layers of the epidermis. There, melanocytes transfer melanin to keratinocytes.
Humans generally have around the same number of melanocytes regardless of the genetic background. Melanocytes comprise between 5 and 10 percent of the cells in the stratum basale. Genetic factors determine the amount of melanin each individual melanocyte produces. Individuals with darker-pigmented skin have more active melanocytes. These melanocytes produce more melanin than melanocytes in individuals with more lightly pigmented skin. In individuals with a condition called albinism, the pathways that produce melanin are disrupted due to genetic alterations. Melanocytes in these individuals produce little to no melanin.
Although individuals have a genetic baseline level of melanin, melanin production can be increased under certain environmental conditions. In particular, increased exposure to UV-B radiation triggers melanin production through the activation of hormonal signals. This increased melanin production after exposure to sunlight is what happens when skin gets tan.
One of the evolutionary issues with which the human body has had to deal is how to produce vitamin D but not destroy the folate needed by the body to produce DNA. Vitamin D aids the body in calcium absorption, and folate (which is obtained by eating fruits and vegetables) has multiple roles in promoting health (including sperm and egg production). Humans are believed to have evolved in Africa, where the dark pigment melanin protects against UV degradation of folate. High levels of UV radiation near the equator provide sufficient photons for the production of vitamin D even with high levels of melanin pigment in the skin. As humans migrated to more northern latitudes, the need for dark skin tones was reduced because those areas receive far less sunlight. People with darkly pigmented skin living in northern latitudes do not receive sufficient quantities of light for vitamin D production, and as a result, may suffer from bone growth disorders such as rickets. Thus over time, the dark melanin pigment was reduced, and skin color became lighter.
Other factors beyond melanin production can also influence skin color. Red blood cells are red because hemoglobin, the protein responsible for transporting oxygen throughout the body, has a red pigment. Because of this red pigment, blood vessels in the dermis give the skin a pink tinge, especially in individuals with lower levels of melanin. Oxygenated hemoglobin appears redder than deoxygenated hemoglobin, so a lack of oxygen saturation can cause skin to appear bluish gray. When blood vessels near the surface of the skin dilate, a greater quantity of oxygen-rich blood flows through, and the skin appears redder. This happens when an individual becomes flushed due to heat, fever, inflammation, or embarrassment.