The color of your hair is largely determined by the genes that you inherited from your parents and their ancestors. There are a few exceptions, mostly associated with disease or congenital anomalies that induce change in hair color.
Knowing that hair color is under genetic control is only the beginning of the hair color story. Scientists are still unraveling the mechanisms by which genes and regulatory molecules send the signals that tell hair follicles to produce hair of a given color. Some of the same molecular mechanisms are involved in the genetic control of skin pigmentation. It is not surprising, then, to find blond hair in fair-skinned people and black hair in dark-skinned people. All colors of hair can lose their pigmentation over time and become the gray and white hair associated with older age.
Melanocytes are the targets for genetic signals controlling skin and hair pigmentation. Melanocytes are specialized cells located in the skin and in hair follicles. As the name "melanocyte" implies, a function of melanocytes is to synthesize the pigment melanin-the pigment that gives color to skin and hair.
Melanocytes contain smaller sub-cellular bodies (organelles) called melanosomes. Under control of genetic and molecular signals that include hormone stimulation and enzyme catalysis, the melanosomes synthesize melanin. The melanosomes are of two types: elliptical (oval-shaped) melanosomes are specialized to synthesize brown and black eumelanin; spheroidal (round) melanosomes are specialized to synthesize red and yellow pheomelanin.
Melanocytes in the skin are under other forms of control, also. Keratinocytes, the tough cells that make up outer layers of skin and hair fibers, produce biochemical factors that regulate melanocyte growth and melanization.
In the hair follicle, melanocytes are located near the base of the follicle where a new hair is produced at the start of the anagen (growth) phase of the hair growth cycle. (See About Hair Loss and Hair Loss and the Hair Growth Cycle.) The melanocytes at the base of the hair follicle receive signals that tell them when to start producing melanin. Melanocytes synthesize melanin only during the anagen phase. Melanin synthesis is turned off during the catagen (degradation) and telogen (rest) phases of the hair growth cycle.
The melanin synthesized by melanocytes does not begin to color the newly formed hair in the earliest stages of hair formation: the "tip" of the newly formed hair is completely or nearly colorless. As the hair shaft continues to develop in the follicle, pigment enters the cortex of the shaft-the "core" of the hair shaft that lies under the protective shield of the cuticle or tough outer layer of keratin.
Both the eumelanin and pheomelanin pigments can be synthesized, but only one type of melanin is usually synthesized at any one time. Both types can be synthesized in varying amounts to produce variations in hair color. The amount of melanin in the hair fiber is usually proportional to the intensity of hair color.
When melanocytes begin to become inactive, newly formed hair becomes gray as melanin synthesis declines. Complete inactivity of melanocytes results in white hair, the absence of color in the hair shaft. Why hair follicles become inactive with age is still a topic for research.
Scientists have begun to define more specifically the genes that may be involved in hair pigmentation, and the molecular signaling mechanisms by which genes and regulatory molecules communicate with hair follicle melanocytes. Cells in the follicular papilla at the base of the follicle have been found, in mice, to express a gene that signals follicular melanocytes to synthesize melanin. Another gene, bcl-2, present in both mice and humans, has been found to be associated with melanocyte survival in mice. When this gene is made to stop working in mice, the hair of the mice becomes gray during the second week of the hair growth cycle.
Stem cell factors have also been found to be implicated in hair pigmentation. The presence of a stem cell factor and its receptor has been shown to be required for melanocyte development and activation in a mouse model of hair pigmentation.
Research to date has confirmed that (1) hair pigmentation is under genetic control, (2) to date, there is no known way to change hair color at the genetic level, and (3) inactivation of hair follicle melanocytes is likely to give most of us gray and white hair as we age.