Melanin is a brown pigment that occurs in the form of intracytoplasmic granules inside melanocytes.
Inside melanocytes, the granules are contained within specialized vesicles called melanosomes. These melanosomes get transferred into epidermal skin cells where they gather over the top of the nucleus and protect the DNA from damage caused by the sun's ultraviolet radiation.
The cutaneous melanin therefore acts as a natural sunscreen and without it, the sun's ultraviolet rays can cause mutations in the DNA that can lead to skin cancer. In general, populations that have originated near the equator and are exposed to more intense sunlight, have more melanin present in their skin to protect it from UV damage, which makes their skin appear brown or black.
By contrast, those populations originating from areas further from the equator, have less cutaneous melanin and are more prone to skin cancers than darker individuals.
However, exposure to the sun stimulates the skin to produce Vitamin D and the sun protection conferred in a darker versus lighter skin type can increase the likelihood of vitamin D deficiency. Vitamin D deficiencies are seen within the cool temperate zones of the Earth or above 36 degrees latitude in the Northern hemisphere and below 36 degrees in the Southern hemisphere.
Therefore, people with darker complexions who live in Canada and the USA have been advised by health authorities to take 1000-2000 International Units of vitamin D per day from autumn through to spring to prevent deficiency.
Melanin is also present in the vascular layer and the iris of the eye, where it protects the eyes from ultraviolet light. Sun-related eye damage is more likely among light-eyed people who may have blue, green or grey eyes.
Populations that have originated in areas with high exposure to sunlight are typically characterized by brown eyes owing to the increased deposits of melanin in the eye.