Skin is the body’s largest organ. It is also the most visible. A square inch of skin has millions of cells and many nerve endings for sensing temperature, pain, pressure, and touch. It keeps us sheltered from the elements and protects our insides from drying up. Yet despite its toughness, skin is sensitive and can be damaged if it isn’t cared for. It requires our protection from the sun and from irritants. It needs our help in retaining moisture. It demands a little thought now and then.
No one can deny the importance of skin color within our society. Socially it can affect many things, but scientifically, skin color is a lot less complex.
Most scientists agree that sunlight levels determine whether a population tends to have light skin or dark skin. Sunlight contains ultraviolet radiation (UV). The darker the skin, the less UV penetrates into it. Too much UV is bad, because it destroys an important nutrient, called folate. But some UV on our skin is good, because it helps our cells produce vitamin D, another important nutrient.
Where there is a lot of direct sunlight, like in places close to the equator, people get more than enough sunlight to produce vitamin D, but they are very susceptible to folate damage. Having dark skin that protects against the destruction of folate is advantageous in these parts of the world. As a result, people with genes for dark skin are healthier and produce more offspring, who also have darker skin. After many generations, populations in these areas are mostly dark-skinned.
The skin colors themselves are the result of a pigment called melanin. Melanin is produced by melanocytes, cells that reside in the skin’s second layer or ‘Dermis’. There are two types of melanin: eumelanin, which produces a brown to black color, and pheomelanin, which produces a red to yellow color. Although melanin is the main determining factor in what color a person’s skin will be, there is also another pigment located in the dermis, which contributes to the color. This yellow-colored pigment, called carotene, is coverted to vitamin A. Along with the pigments, there are other factors that contribute to skin color, such as the subcutaneous, or fatty, tissue layer of skin. The way that light filters through this layer, as well as the other two layers of skin, has an effect on the way that skin color is perceived. Sometimes skin color develops unusually.
Most people have moles (also known as birthmarks or nevi). Generally these are just harmless clusters of darkly pigmented cells, but they should be watched because changes can warn of developing skin cancer. Vitiligo is an autoimmune disease that causes the skin to develop milkywhite patches. These develop because the body mistakenly makes antibodies to its own melanin. Hair color is also a result of how much melanin a person has in their body. Individual hair shafts grow from follicles that are rooted in the subcutaneous layer, passing through the dermis and epidermis. Each hair is genetically programmed to produce a certain amount of melanin. The production of melanin occurs in the middle layer of the hair shaft, referred to as the cortex. When hair greys, the cortex has slowed the amount of melanin produced by the hair, causing the hair to turn grey and eventually white.