The hair color , like that of the skin and eyes, is determined by a pigment called melanin. the cells that manufacture melanin are the melanocytes . The characteristics of this melanin, and therefore the pigmentation of each person, are regulated by more than a hundred not at all .
At the root of the hair is a very complex structure known as the hair bulb that contains several types of cells, including keratinocytes and melanocytes. Keratinocytes make the strands of keratin or hair, and melanocytes are responsible for coloring with melanin (red hair, blonde, brown, black...). Some of these melanocytes are stem cells capable of originating new melanocytes throughout life to maintain hair color.
As you age, the melanocyte stem cells in the hair follicle disappear and the hair takes on a silvery, gray or eventually white hue due to a decrease or lack of melanin.
The loss of melanocytes in the hair is considered a physiological fact that usually starts between the ages of 30 and 40, causing the first white hairs. Why it occurs and how it could be prevented is not yet known exactly. Genetic factors play a role: gray hair occurs at a younger age when your parents also had it early, and it is more common in Caucasians than Asians or African-Americans. Environmental factors are also involved: very intense stress and possibly tobacco. In exceptional cases, white hair can appear due to genetic syndromes of premature aging or associated with autoimmune diseases.
Gray hair is also considered a marker of cardiovascular disease, as the two share aging mechanisms such as oxidation and cellular senescence and death. It has recently been observed that the gray hair of patients receiving immunotherapy (a type of treatment that stimulates the defenses) for cancer can repigment itself. This observation opens up new lines of research to treat gray hair.