From weaker limbs to weaker eyesight, our body changes as we age. And along with it comes changes in our hair — and possibly many more grey strands. New findings published in the journal Nature on April 19 suggest that grey hairs appear as we age due to melanocyte stem cells losing their capacity.
The research team from NYU Grossman School of Medicine suggests some stem cells become trapped in the growth compartments of hair follicles as we age, which impairs their potential to develop and conserve hair color.
The team focused on melanocyte stem cells, or McSCs, which are cells found in both mouse and human skin. Hair color determines whether McSC pools within hair follicles receive the signal to develop into cells that make the protein pigments accountable for color. Recent studies reveal that during typical hair development, such cells regularly move back and forth between compartments of the developing hair follicle.
The amount of protein signals that McSCs are exposed to has an effect on how these compartments evolve. The study team discovered that depending on their location, McSCs switch between their most basic stem cell state and the subsequent stage of their maturation, the transit-amplifying state.
What did the research find?
The team found that a number of McSCs become trapped in the stem cell niche known as the hair follicle bulge as hair ages, falls, and then continually regrows. Instead of developing into the transit-amplifying state or restoring to their primary place in the germ compartment, where WNT proteins would have stimulated them to regenerate into pigment cells, they stay where they are.
“Our study adds to our basic understanding of how melanocyte stem cells work to color hair,” says Qi Sun, Ph.D., the study lead investigator and a postdoctoral fellow at NYU Langone Health. “The newfound mechanisms raise the possibility that the same fixed-positioning of melanocyte stem cells may exist in humans. If so, it presents a potential pathway for reversing or preventing the graying of human hair by helping jammed cells to move again between developing hair follicle compartments.”
The team found that other self-regenerating stem cells, such as those comprising the hair follicle itself, are known to move in only one direction, follow a predefined timetable as they mature, and do not display McSC flexibility. For example, transit-amplifying hair follicle cells never regress to their stem cell state. Sun claims that this largely explains why hair may continue to grow even after losing its color.
The same research team at NYU earlier established that WNT signaling was necessary to promote the McSCs’ development and pigment production. Additionally, that research showed that McSCs were significantly less exposed to WNT signaling in the hair follicle bulge than in the hair germ compartment, which is right beneath the hair follicles.
The most recent studies on mice whose hair was physically aged by plucking and forced regeneration revealed that the frequency of hair follicles with McSCs lodged in the follicle bulge jumped from 15% prior to plucking to almost 50% after forced aging. These cells were still unable to proliferate or transform into pigment-producing melanocytes.
The researchers found that because the confined McSCs were no longer exposed to significant WNT signaling, they lost their ability to produce color in developing hair follicles.
However, additional McSCs that switched between the hair germ and follicular bulge over the two-year study period maintained the potential to renew as McSCs, grow into melanocytes, and produce pigment.
“These findings suggest that melanocyte stem cell motility and reversible differentiation are key to keeping hair healthy and colored,” concludes Mayumi Ito, Ph.D., a professor in the Department of Cell Biology at NYU Langone. According to Ito, the team plans to look at ways to get McSCs back to moving freely or physically move them back to their germ compartment, where they can create pigment.