What could cause hairs to gray at the tips but not the roots?
I have noticed that some of my sporadic gray hairs are gray at the tip side but oddly, not near the roots. Some are even only gray in the middle. I find all of this very counter intuitive, and I assure you nobody is secretly dying my hair.
How does the graying process work, and how is it that this can be happening?
Interesting question, I am not sure if I have a definite answer, but at least some ideas: The pigment in the hair is made by specialized cells, the melanocytes. The make the pigment (eumelanin=dark and pheomelanin=red/yellowish) which is then deposited into the growing hair. They are located at the bottom of hair bulb and usually die at the end of each hair cycle. To have the hair of the next hair cycle colored as well, this melanocyte population in the hair bulb needs to be replenished. This is done by melanocate stem cells, which live in the hair bulge and which start to reproduce and differentiate in melanocyte which then migrate down to the hair bulb. There is an excellent article following this process:
The hair later gets grey (low pigment) and white (no pigment) when these melanocytes are not, or not fulley replenished in later hair cycles. Why this happens, there are a few hypothesis.
First, its possible, that with age the melanocyte stem cell population (which not only need to replenish the melanocytes, but also need to maintain its own population) is getting smaller and disappears over time. This process would explain gradually graying over time. See this reference:
Another possibility is the presence of reactive oxigen species (ROS) which occur naturally in melanocytes. Over time, the enzyme which intercepts this ROS (catalase) is getting less active or present in the cells and thus oxidative damage is getting bigger, damaging the melanocytes and their ability to make pigment. It would actually bleach itself...
- Senile hair graying: H2O2-mediated oxidative stress affects human hair color by blunting methionine sulfoxide repair
Melanin has also an important role in intercepting ROS, so in this process less and less pigment is made (as the damage accumulates) and this also leads to hair, which is less pigmented for exogeneous oxidative stress:
- Towards a "free radical theory of graying": melanocyte apoptosis in the aging human hair follicle is an indicator of oxidative stress induced tissue damage.
It could be possible (and this is a wild guess) that the phenomenon of the tips graying first is situated by this. Hair less protected against exogeneous oxidative stress (smoke, UV etc.) so the melanin is "used up" during this until the hair greys at the tip, since its the oldest and most exposed part of the hair. This article by Tobin gives also a nice overview:
Your hair grows out from the roots, and typically as you age, the pigments (melanin) that would normally keep your hair a certain colour are not produced anymore because the pigment producing cells (melanocytes) start to die.
They're likely different colours in the middle because that will be the result of a gradual dying off of the melanocytes at the base of that strand. As a result the root (newest part of the hair) will typically be gray when they have mostly all died off, the middle will have a bit of melanin because it was produced when there were still some functional melanocytes, and the tip (the oldest part of the hair) will still be full of melanin from when it was produced when the melanocytes were alive.
As for why this happens it's not fully understood why the melanocytes die as we age, but the gene Bcl-2 has been found to be essential for the maintenance of melanocytes. Mutations in this gene could be responsible for premature graying . There is some literature on the role of Bcl2 family genes as regulators of cell death  but how these things are connected seems largely still unknown.
There have been some reports of people indicating that a change in lifestyle and/or diet have led to a decrease in gray hairs, or even that the hair begins to regain its colour leaving a gray section at the tip or in the middle. I've only found one relevant article which makes reference to some research into the restoration of hair growth and colour in mice. An American group are studying the role of the Wnt signaling protein in controlling the activity of melanocytes. I'm just speculating here, but it could be that fluctuations in Wnt protein production (possibly influenced by diet and lifestyle and overall health) may switch melanocyte melanin production on and off. I haven't been able to find any literature to back up this hypothesis though.
Sorry, I realized I mis-typed what actually happens. The counter intuitive aspect is seeing gray tips or middles, not roots, like one would expect.. editing.
I edited my question; if you could please update your answer to reflect my corrected question, that would be great.
The last link is at least questionable. TRP-2 (or usually called DCT) is of course also present in follicular melanocytes in the hair, otherwise there would be not eumelanin (dark) pigmentation. The observation that Wnt may play a role is pretty broad, since Wnt signalling is essential for the melanocyte to function. I can go through my library tomorrow and write some more about it if wanted (the topic is pretty old).
This is a consistent phenomenon with me and I don't live a stressful life and eat pretty consistently, so I'm skeptical that it's environmental. About 20 to 30% of my graying hairs are like this (black at the roots, gray on the oldest half.) @Chris I'd love to hear more.