The skin of mammals and birds contain a dark pigment called melanin. The more melanin the skin has the darker the pigmentation. This is what makes humans differ in skin color. Individuals of a common race may also have varying degrees of melanin.
Production of melanin is by special types of cells. These cells are found in the epidermis of the skin. The epidermis is the topmost layer of the skin.
These special melanin producing cells are called Melanocytes.
The process by which Melanocytes produce melanin is known as Melanogenesis. Melanocytes account for about 5% to 10% of all the cells of the epidermis. They have a concentration of about 1000 to 2000 Melanocytes per square millimeter of the epidermis. Albinism results when the melanocytes are unable to produce melanin. Vitiligo, which was rumoured to have lightened the skin of pop star Michael Jackson, is a condition where Melanocytes are totally absent from the skin.
An interesting fact is that all the human beings on earth genetically have the same amounts of Melanocytes. So why the differences in pigmentation?
UV radiation is one of the reasons. Melanin is produced to protect the skin from the harmful effects of UV rays. Exposure to UV radiation triggers the Melanocytes to produce more melanin. This is the science behind tanning. Despite the numbers of Melanocytes being relatively the same across the human species, their sizes vary from person to person.
This affects the amounts of melanin produced by an individual cell resulting in different pigmentations.
Different races also produce different shades of melanin. The amounts produced also vary considerably from race to race.
Melanocytes are affected by a number of diseases. The most lethal form is Melanoma which is the deadliest of all skin cancers. Like other types of cancer, Melanoma can be treated if discovered early. If not, it spreads throughout the body and is almost certainly fatal.