Skin, Hair, and Nails
Why does hair turn gray as part of the aging process?
The pigment in hair, as well as in the skin, is called melanin. There are two types of melanin: eumelanin, which is dark brown or black, and pheomelanin, which is reddish yellow. Both are made by a type of cell called a melanocyte that resides in the hair bulb and along the bottom of the outer layer of skin, or epidermis. The melanocytes pass this pigment to adjoining epidermal cells called keratinocytes, which produce the protein keratin—hair’s chief component. When the keratinocytes undergo their scheduled death, they retain the melanin. Thus, the pigment that is visible in the hair and in the skin lies in these dead keratinocyte bodies. Gray hair is simply hair with less melanin, and white hair has no melanin at all. It remains unclear as to how hair loses its pigment. In the early stages of graying, the melanocytes are still present but inactive. Later they seem to decrease in number. Genes control this lack of deposition of melanin. In some families, many members’ hair turns white while they are still in their twenties. Generally speaking, among Caucasians 50 percent are gray by age 50. There is, however, wide variation.
Why is some hair curly while some hair is straight?
The shape of the hair follicle determines how wavy a hair will be. Round follicles produce straight hair. Oval follicles produce wavy hair. Flat follies produce curly hair.