Saturday, July 30, 2016
Note: This program first aired on July 30, 2016.
This week’s show originated as a question from my nephew. He was wondering why hair turns gray. I have thought of that myself, as I sport a head full of white hairs, which started losing color far ahead of the curve early in my 20s. And the answer to the question is of course, complicated and partially unknown.
To start we have to look at where hair, and fur color comes from. Hair is made of several layers of a structural protein called keratin (the same thing your finger nails are made of). It grows from collection of cells, which some sources refer to as a mini-organ, called a follicle. The skin of most mammals is studded with hair follicles, it is a defining characteristic of the group. The color of hair is controlled by cells around the follicle called melanocytes, who’s job unsurprisingly is to generate a pigment called melanin. Melanin gives hair its color, by being injected into the filamentous protein we call hair, as it grows. The variety of hair colors we see in mammalian animals is determined by the mix and balance of different types of melanins, broadly speaking, eumelanins are black and brown, and phaeomelanins are lighter, reds and yellows.
Human hair has three develpmental stages: anagen or growth phase, which lasts for years, catagen or transitional phase, as the hair transitions from active growth to being shed, and telogen, a quiescent phase that ends with the hair fiber being shed. While we look mainly at what is going on with the hair itself, these phases represent dramatic differences in the functioning of the hair follicle. As the hair is growing in anagen, the melanocytes are pumping melanin into the hair fiber. When the follicle enters catagen, the first thing that happens is the melanocytes stop melanin production and undergo apoptosis, which is programmed cell death. Individual melanocytes then only get to produce melanin for a single strand of hair. Hair pigmentation shuts off in the transitional phase of hair growth, and if it doesn’t turn back on when the cycle starts over, the resulting hair that grows will be colorless, or white.
Now I said that not only does the melanocyte stop producing pigment, it actually dies, and in order for the next hair that grows to have pigment a new melanocyte has to form. And that is where the root of all this going gray as a normal part of aging happens. The new melanocytes arise from stem cells, melanocyte stem cells, which can mature into a pigment producing cell when needed. These stem cells continually divide and create more stem cells, so there is always a supply on hand when the hair follicle returns to the anagen active growth phase. The current thinking is that mammals go gray as they age because stem cells start to lose their integrity the older they are. The older you are the more damaged the cells in the melanocyte stem cell reservoir are, so the chances of a functional stem cell being able to mature into a pigment producing cell go down. The more times cells divide, the more chances there are for errors in transcription, the copying of genetic information. That is why age impacts cellular health, in general, the older a cell is, the more times it has divided. Cells only have so many times they can divide, before the telomere mechanisms inhibit further cell division and the cell effectively is put out to pasture. When we get very old, we don’t have any melanocyte stem cells left. Normal aging (which really means the changing of our cellular DNA, or shortening of our telomeres, that occurs during cell division) results in this decrease in stem cells, but cellular stress in the form of mutagens that damage DNA can as well, things like Xrays and UV light.
Many questions remain about this going gray business. The diversity of pattens of human graying is thought to be genetic but we don’t know why, and are there implications for other stem cells in the body, do all your stem cells function the same way? Why do dogs go gray around their muzzles but not other parts of their bodies? Why do some mammals not go gray at all? Many questions, fewer answers, but that is actually a good thing. If we had all the answers there would be nothing left to do. So please, keep your questions coming.
More than you ever wanted to know about hair: https://repositorium.sdum.uminho.pt/bitstream/1822/15299/1/2010%20Biology%20of%20Human%20Hair%20Know%20Your%20Hair.pdf