Why Does Hair Turn Gray with Age?
Hair is a biological product produced on the surface on skin in animals belonging to the class mammals. Hairs are made of a hard substance called keratin which is chemically a protein. Production of hairs and all their features such as the texture, composition and color in an individual are genetically determined and hence are inherited from biological parents.
As to how hair turn white with age is also a genetically determined feature and depends upon the characteristics of genes inherited. To understand how hair turn gray with age needs an understanding of the structure of and growth of hair, factors that influence growth and development of hair and the mechanism of formation of gray hair.
Structure and growth cycle of hair:
Hair is a nonliving structure above the living matrix cells located at the base of hair follicle. A hair is a long or short shaft the size and texture of which varies from individual to individual and also from part to part of the body hair is located in the same individual. A full grown hair has the following structure: a root and a shaft. The root lies in the matrix cells at the base of hair follicle while the shaft bulges above the surface of skin. The shaft consists of a cortex and a cuticle layer. The bulk of hair is cortex region, which is comprised of a tough material called keratin; chemically keratin is a protein. The outer layer of hair, the cuticle, is a protective layer and this is made of tiny scales of keratin covering over the cortex in the form of tiny scales. Cuticle protects hair against physical and chemical damage.
Factors that govern color, growth, development and health of hair:
Each hair passes through a cycle of growth and development. A new hair is formed by the cortex cells, and then it pushes the old hair which falls off. Hair cycles are repeated by the hair follicle in phases depending upon the instructions received from genes in cortex cells and the process is also influenced by hormones, in principle thyroid hormone and sex hormones. The color of hair is due to a class of pigments called melanin which is synthesized by cells called melanocytes located in the hair bulb of the hair follicle.
Formation of melanin is switched on in melanocytes during the growth phase of hair, called anagen, and switched off during the catagen phase when the hair growth is shut down; the shutdown continues until the resting stage (telogen). Melanocytes usually produce only one type of melanin at a time but two types can be produced. The melanin is of two types: eumelanin which is the main coloring agent in brown and black hair; pheomelanin is the principal type in blond and red hair. When melanin is diminished it produces gray hair and its complete absence causes white hair.
Each follicle completes 10 to 20 cycles involving anagen, catagen and telogen phases in its life. Each scalp hair can have a life ranging from 2 to 8 years. The other factors that influence healthy growth of hair are nutritional factors, general conditions of health and disease, age of the person and environmental factors such as pollution, smoking and chemotherapy. Age related loss of pigment formation and graying of hair is a normal process. Early onset of graying of hair in some populations is common as compared to others and this is purely linked to inheritance of genes causing early graying of hair. In Caucasians it has been found that 50 per cent of population develops gray hair by the age of 50 years. Premature graying is however indicative of some abnormalities in physiological functions such as thyroid malfunction, nutritional deficiency, environmental factors and stress.
Mechanism for formation of white hair:
During hair growth (anagen), melanin formed by melanocytes migrates into the adjacent cells called keratinocytes located in the outer layer of skin. As the dead keratinocytes with melanin move into the cortex of hair it gives the hair its black color. Melanocytes continue to provide melanin in a growing hair all though its life of 2 to 8 years. As a person advances in age, melanin synthesis is stopped at some point but the hair continues to grow. As a result the cortex is devoid of melanin pigment and the color becomes gray or white depending upon the density of melanin. A white color hair is totally devoid of melanin pigment.
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 Why does hair turn gray or white http://www.hairfoundation.org/hair-library/article-gray-hair.htm#clocks
 Why does hair turn gray? Scientific American, 2001. http://www.scientificamerican.com/article/why-does-hair-turn-gray/