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.
References:
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
