Melanocyes and Melanin

Most people's skin darkens when exposed to UV light, giving them more protection when it is needed. This is the physiological purpose of sun tanning.
Freckles and moles are formed where there is a localized concentration of melanin in the skin.
So what is Melanin?
Melanin has a photoprotective role.
It is produced by melanocytes, through a process called melanogenesis. Melanocytes are cells located in the bottom layer of the skin's epidermis. Although human beings generally possess a similar concentration of melanocytes in their skin, the melanocytes in some individuals and ethnic groups more frequently or less frequently express the melanin-producing genes, thereby conferring a greater or lesser concentration of skin melanin.
Once made, melanin is moved along arm-like structures called dendrites in a special container called a melanosome which is shipped to the keratinocytes. Melanosomes are vesicles or packages of the chemical inside a plasma membrane. The melanin is in organelles called "melanosomes", that are organized as a cap protecting the nucleus of the keratinocyte.
Because melanin is an aggregate of smaller component molecules, there are a number of different types of melanin with differing proportions and bonding patterns of these component molecules. Both pheomelanin and eumelanin are found in human skin and hair, but eumelanin is the most abundant melanin in humans.
Eumelanin is found in hair and skin, and colors hair grey, black, yellow, and brown. In humans, it is more abundant in peoples with dark skin. There are two different types of eumelanin. The two types are black eumelanin and brown eumelanin. Black eumelanin is the darkest, brown eumelanin is lighter than black eumelanin. black eumelanin is in mostly non-Europeans and aged Europeans, while brown eumelanin is in mostly young Europeans. A small amount of black eumelanin in the absence of other pigments causes grey hair. A small amount of brown eumelanin in the absence of other pigments causes yellow (blond) color hair.
Pheomelanin is also found in hair and skin and is both in lighter skinned humans and darker skinned humans. But in general women have more pheomelanin than men, and thus women's skin is generally redder than men's. Pheomelanin imparts a pink to red hue and, thus, is found in particularly large quantities in red hair. Pheomelanin is particularly concentrated in the lips, nipples, glans, and vagina. Pheomelanin also may become carcinogenic when exposed to the ultraviolet rays of the sun.
Melanocytes insert granules of melanin into specialized cellular vesicles called melanosomes. These are then transferred into the other skin cells of the human epidermis. The melanosomes in each recipient cell accumulate atop the cell nucleus, where they protect the nuclear DNA from mutations caused by the ionizing radiation of the sun's ultraviolet rays. When ultraviolet rays penetrate the skin and damage DNA; fragments from damaged DNA will trigger melanogenesis and cause the melanocyte to produce melanosomes, which are then transferred by dendrite to the top layer of keratinocytes.
People whose ancestors lived for long periods in the regions of the globe near the equator generally have larger quantities of eumelanin in their skins. This makes their skins brown or black and protects them against high levels of exposure to the sun, which more frequently results in melanomas in lighter skinned people.