Grey hair? Stem cells are to blame.

The Problem

Grey hair.

It happens because your hair-related stem cells fail earlier than normal in life. They’re called melanocyte stem cells, which I’ll just call hair stem cells for the rest of this.

We’ve known for a long time that hair goes grey because of hair stem cells failing. What we haven’t known is why. This week’s paper goes a long way to figuring it out.

The Solution

Stem cells often sit in a specific spot and don’t move much. They hang out in a safe area and divide over and over, producing many other cell types that do the heavy lifting. The stem cells stay in one place and don’t change.

At least, that’s what normally happens.

Stem cells have 2 main options in life. Stay a stem cell, or change into a more specialized cell. We call the process of stem cells becoming a more specialized cell type “cell differentiation”.

We think of cell differentiation as a linear process.

Hair Stem Cell → Specific Cell 1 → Even More Specific Cell 2

Once a cell goes down the line, it doesn’t come back. However, like most things in biology, that’s not always true. And importantly for today, hair stem cells ignore this rule like it’s their job.

Hair stem cells basically ignore everything I just said. They:

1. Move around a lot.

2. Become specialized, then turn back into stem cells, then become specialized again, then back into stem cells… and so on.

So much for stem cells hanging out and not changing much.

To prove these points, the authors decided to look at the most specialized cell type that a hair stem cell can turn into. This would be Even More Specific Cell 2 in my sophisticated flow chart above. They made some of these cells produce a red fluorescent protein (called tdTomato) so that they could track them over time.

These cells express the late-pigmentation gene Oca2 and are responsible for giving your hair color. The gene only turns on when the hair stem cells are very specialized.

In early hair growth stages (anagen) the hair stem cells are fully differentiated into the late-pigmentation gene cells. You can see this on the left image. The highlighted boxes show that the Even More Specific Cell 2s have a spread-out shape and are expressing the Oca2 gene.

In the late hair growth stage (telogen) the Even More Specific Cell 2s turned back into hair stem cells. They become more round and no longer express the Oca2 gene (hence the empty black box in the bottom right).

The hair stem cells (and what they become) move around a lot during this whole process. Remember, stem cells typically stay in one place and let their specialized descendants do the moving. It’s super unusual for them to be so mobile.

The authors go on to show that hair stem cells stop moving as much with age, leading to grey hair.

All this movement leaves some hair stem cells stranded in no man’s land. They’re stuck in a location that doesn’t help their activities as a stem cell or as a specialized cell. Without the right cues from their environment, the hair stem cells start just hanging out.

Except, they don’t do their job as a stem cell or as a specialized cell. They just literally sit and don’t do anything. Because of this “laziness”, we get grey hair. It’s a bad deal if you ask me.

This info might seem trivial to you, but it could lead to new ways to prevent/reverse grey hair. Hair coloring is a huge market, bringing in approximately $24 billion in the U.S. alone. Any treatment or prevention of greying will bring the $$$ down the line.

See you next week for more science,

Neil