Hair color is mainly due to melanocytes, the cells that produce pigment for the skin and hair. These melanocytes are formed from melanocyte stem cells (McSC), which age faster than the stem cells responsible for hair growth. As we age, McSCs lose their ability to pigment, causing hair to turn gray. However, the exact causes of this process are still unknown.

To understand how this happens, a team of researchers led by Dr. Mayumi Ito from New York University, funded by the U.S. National Institutes of Health, studied the fluorescent dye-labeled McSCs of mice. Observing these cells under a microscope, the researchers tracked their behavior at different stages of hair growth.

It turned out that McSCs have two possible pathways: some turn into mature pigment cells that color the growing hair, while others move up into the bulge region — where they continue to function as stem cells. By analyzing McSCs at different stages of hair growth, the team saw that all McSCs begin to partially mature when they reach the hair germinal zone (the so-called germinal matrix — the place at the bottom of the hair follicle where cells produce hair fibers). However, unlike other stem cells, McSCs do not have a separate pool that remains exclusively stem cells.

A unique feature of McSCs is their ability to dedifferentiate — that is, to revert to the stem cell state, even after they have begun to produce pigment. This chameleon-like ability allows them to switch between states depending on the signals they receive from different follicle regions. For example, in the germinal area of the hair, the cells receive signals to mature, whereas in the bulge area, these signals are absent and remain in stem cells.

However, the researchers found that more McSCs become “stuck” between the bulge region and the germinal zone in older mice. These stuck cells can no longer turn into pigment cells or return to the stem cell state, leading to a gradual loss of hair color.

Dr. Ito's team believes that losing this flexibility in McSCs — their ability to switch between stem and mature states — may cause hair graying. If scientists can find a way to restore this movement, it may be possible to prevent or even reverse the graying process.

Reference

Sun Q., Lee W., Hu H. et al. Dedifferentiation maintains melanocyte stem cells in a dynamic niche. Nature 2023; 616(7958): 774–782.