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The question is pretty simple: what is happening molecularly when hair turns grey or white? I would imagine that it is due to the lack of a particular compound. I'm also interested in why some people's onset of grey hair happens much later than others.

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Each individuals hair colour is determined by the particular pigment they produce (called melanin - the same stuff in your skin that makes you tan).

As the body ages this pigment (produced by the melanonocytes - cells that reside in the hair follicle) is produced less and less, until the hair is no longer coloured, and appears grey.

This is unique to each individual because it is a genetic (and therefore highly heritable) trait. Because it is in no way linked to mortality there is no selection pressure against greying hair.

The reason that the pigment is not longer produced is the gradual depletion of the stem-cell pool with age. This is common to many tissues, hair follicles being just one. As the 'pool' of stem cells is depleted, the melanocytes are no longer replaced as frequently, and thus less pigment is produced as we age.

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Are there ideas as to why the pigment production declines with age? –  Rory M Jun 27 '12 at 10:09
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@RoryM - added reference to interesting review on the subject. –  Luke Jun 27 '12 at 10:58
    
Thanks for the answer and the edit (which is what I was most curious about). –  LanceLafontaine Jun 27 '12 at 16:20

A person’s hair color is the result of pigments known as melanin. Made from two amino acids (tyrosine and phenylalanine), these pigments are produced by a specialized group of cells known as melanocytes. They do this through a process called melanogenesis. Melanocytes are found throughout our body and the melanin they produce is what gives our skin, hair, and eyes their color. The melanocytes responsible for hair color are found in the bulbs of your hair follicles.

Even though there is a wide range of hair and skin color, there are only two main types of melanin- eumelanin is what produces dark browns and blacks, and pheomelanin produces reddish/yellow. How these cells blend together will determine what color your hair will be.

It isn’t fully known what makes the melanocytes blend together in the ways they do, to make the specific hair color for each individual. What is known is that there appears to be clear genetic factors. One gene, and its alleles responsible for red hair, have already been identified; known as MC1R, this is also the same gene responsible for red pigments in several other species, like cows.

Once the specific formulation of melanin is produced, their granules are transferred to adjacent keratinocytes, also found in the bulbs of your hair follicles. Keratinocytes are what produce keratine, the dead protein cells that make up our hair. Once a Keratinocyte undergoes its scheduled death, it retains the melanin making our hair color what it is.

Gray hair is the result of less melanin within the keratin. The less melanin, the more gray your hair will be. White hair has no melanin at all.

There are a few different processes that can make our hair turn gray. The one most people think of is the natural graying that occurs as we grow older. As we age our melanocytes become inactive, but are still present. The older we get, melanocytes decrease in number. The result is less and less melanin, until none are present. Thus, we slowly turn gray, and then our hair turns white in the winter of our lives.

In February of 2005, Harvard scientists proposed a theory as to why melanocytes decrease as we age. They found that a failure of melanocyte stem cells was the cause. These stem cells stop producing adequate numbers of melanocytes to maintain our color.

In 2009, scientists in Europe found another contributing factor. They found that hair follicles produce small amounts of hydrogen peroxide. As any teenager preparing for a concert knows, if you expose your hair to hydrogen peroxide, it will make your hair lighter. Bleached hair anyone?!! Normally this small amount of hydrogen peroxide is broken down by an enzyme called catalase. As we age, catalase production is reduced. The result is a build up of hydrogen peroxide, which blocks melanin production by melanocytes.

There are several other things that can cause our hair to turn gray. The exact mediating factor of them isn’t fully known. Some include: genetic defects; abnormal hormone production like in the case of sudden or chronic stress; abnormal body distribution of melanin; and climate factors can also cause graying, such as pollutants, toxins, and chemical exposure.

Let me touch a little bit on why stress causes gray hair. In 2011, the 2012 Nobel Prize winner in Chemistry, Robert Lefkowitz, led a research group that discovered the mechanism that could explain why stress causes graying. Stress causes a release of numerous neurotransmitters involved in our fight or flight response. Normally the release of these neurotransmitters is short lived and has very beneficial attributes- the ability to run from a chasing lion, or avoid an oncoming car to name a couple. Long term production of these neurotransmitters, however, can cause DNA damage. This damage was shown by this group to promote aging, the growth of tumors, miscarriages, psychiatric conditions, and graying of hair. Yes, I did say psychiatric conditions. It seems that Mr Lefkowitz just proved why old gray-haired people are sometimes crazy . I found this answer on an article on Google here is the link [ [1]: http://www.todayifoundout.com ]

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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Can you add references to the papers you cite in case other want to follow up? –  kmm 17 mins ago
    
You should cite exact papers using doi or pumbed links. –  WYSIWYG 14 mins ago
    
Sir is it OK now ? –  Pi5 7 mins ago
    
@Pi5 No, these kind of references are not preferred (they are not trustworthy). Use links to actual research articles. Plus, you have exactly copy-pasted the article by the original author. This is plagiarism and is not at all tolerated. You'll end up in deep trouble in future, if you cultivate this habit. –  WYSIWYG 1 min ago

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