Melanin and Its Uses
A Human skin color is controlled by melanin, a pigment present in most animals with the possible exception of arachnids1. It is found in not just the skin but also in hair and eyes, and has a wide range of functions. Perhaps the most curious place that scientists have discovered melanin is the brain, since it would not seem necessary to have pigment in a place that cannot be seen. Only recently has research has begun to elucidate some of melanin’s functions in that region of the body, separate from the better established purposes of more visible pigment, of which there are two main types.
B Eumelanin, the most common type, is either black or brown in color, and is primarily responsible for the various shades of skin. Those individuals whose background reaches to the equator generally have higher concentrations of eumelanin and consequently darker skin. Eumelanin is also responsible for hair color, with higher concentrations responsible for black and brown shades, and smaller quantities resulting in blonde hair. Pheomelanin, which is slightly less abundant in nature, is present in all humans but is reddish in color, responsible for red hair and freckles. Differing levels of eumelanin and pheomelanin are one of the primary factors giving humans their myriad expressions of eye color.
C Both eumelanin and pheomelanin act as protection against the broad spectrum of ultraviolet rays from the sun and will be produced in greater quantities when skin is exposed to sunlight, as ultraviolet rays are a primary risk factor in certain forms of skin cancer. Melanin in the eye similarly seems to ensure against eye cancer and vision loss, partially explaining the evolution of different eye colors in humans. However, direct sun exposure is important to humans’ survival because the body needs ultraviolet B rays in order to produce vitamin D, a substance crucial to the absorption of calcium, iron, magnesium, zinc, and phosphates. Various skin tones may have evolved to ensure a balance between vitamin D production and cancer protection. Where sunlight is more direct, more melanin would have been produced to protect against various skin cancers, but production of vitamin D would also have been ensured. But further away from the equator, light enters the atmosphere at an angle. Since ultraviolet rays are refracted when sunlight is not direct, exposure to ultraviolet B rays would have been decreased, and the production of critical vitamin D also thus curbed. As a result, humans living further north and south evolved a system to produce lower amounts of melanin.
D Meanwhile, the type of melanin that is primarily found in the brain is called neuromelanin and is similarly dark in color to eumelanin but is structurally distinct. Until fairly recently, scientists did not understand its usage, and many thought it an inert substance. However, more recently, neuromelanin has been linked to Parkinson’s disease, leading scientists to discover that neuromelanin may provide some unique protective functions. Parkinson’s disease causes individuals to experience loss of motor control, caused by the death of neural cells in the substantia nigra, a region of the brain whose name, translated from Latin, means “black substance,” due to its abundance of dark-colored neuromelanin. The discovery that patients with Parkinson’s disease have fifty percent less neuromelanin than individuals of similar age has led scientists to believe that neuromelanin plays a crucial role in the prevention brain cell death. In fact, neuromelanin concentrations increase with age, which in turn correlates with brain cell degeneration and, therefore, increased need for protection. Further studies have shown that neuromelanin may also be involved in removing toxic metals throughout the body.
E All melanin is produced by cells called melanocytes. In the brain, these pigment factories will produce the melanin, and the melanin will then stay in these cells, a process similar to how melanin accumulates in the eyes. However, in the skin and hair, melanin is transferred to other cells. In the skin, melanin is transferred to the primary skin cells as pigmented granules that are then grouped around the DNA of their new home to protect it from harmful ultraviolet light. All humans have, in general, the same proportion of melanocytes in the skin, but the amount of melanin produced varies, conferring the range of skin color.
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1Arachnid: A class of 8-legged animal including spiders and scorpions.