Demystifying our hair’s missing melanin
Daddy, why is some of your hair gray? Didn’t you say your beard used to kind of red like my hair? The first thought that popped into my head when asked this by my kindergartner the other day was “because of you, my darling!” I’m glad I didn’t respond that way though, because aside from being not wanting to be mean to by making my six-year-old feel guilty about what she did as a baby, I’d have been wrong. My children aren’t the reason I started finding gray hairs in around my temples and beard on my 30th birthday— my parents are.
Hair coloring, like skin coloring, comes from the pigment called melanin. Cells called melanocytes sit at the end of the hair follicle as it grows, adding the two flavors of melanin to the shaft: eumelanins for blacks and browns, and pheomelanins for reds and yellows. While your scalp’s hair will have one recipe, your pubic, armpit and facial hair will have slightly different mixes, usually with a bit more pheomelanins for a lighter, redder color. The exact formula for your coloring is determined by genetics, as is the timing of when that formulation will break down.
Hair replacement harries melanocytes
Hair grows in cycles, growing for three to five years at up to one centimeter a month, and then resting, and then dying and being shed. Each hair operates on its own schedule, which is how we avoid a yearly molt and instead have a generally even volume of hair on our head. As the hair grows, melanin is added to it, up until the end when the root is left white. At the end of it’s growth cycle, the melanocytes are sometimes damaged and die off, leaving no way to replace add melanin in future hairs growing at that site.
The rate of melanocyte die-off seems to be primarily tied to genetics, rather than stress, lack of sleep, diet, etc., (although some hormones, drugs and diseases are known to speed this process up). Many people start seeing some gray around age 30, and by age 50 half of us will have lost half the color in our hair. Men usually have more gray than women, and people of European descent usually have more gray than people with African or Asian heritage. So just as your genes created your initial hair color, they’ll be setting up your second hair coloration too.
Source: Why Does Hair Change Colour And Turn Grey? by Rodney Sinclair, IFL Science