Cells that got stuck could not regenerate pigment
At New York University, scientists have traced the quiet disappearance of hair color to a failure of movement — stem cells that once traveled freely through the hair follicle become stranded with age, cut off from the signals that would renew their purpose. Published in Nature, the research offers not merely an explanation for graying but a map of a cellular journey interrupted, one that may eventually guide medicine toward restoration rather than concealment. The discovery, made in mice, reminds us that aging is often less a destruction than a gradual loss of flexibility — and that understanding what stops moving may be the first step toward setting it in motion again.
- Melanocyte stem cells — the architects of hair color — depend on constant movement between follicle compartments to stay functional, and age quietly robs them of that freedom.
- Forced aging experiments in mice revealed a troubling escalation: trapped stem cells went from 15% of follicles to nearly 50% after repeated hair cycles, showing how quickly the system degrades.
- The cells that remained mobile kept producing pigment for the full two-year study, drawing a sharp line between movement and function — and between stillness and gray.
- Researchers are now pursuing two potential interventions: coaxing trapped cells to move again, or physically relocating them to the germinal compartment where pigment production can restart.
- The mechanism is confirmed in mice but unverified in humans, meaning the path from discovery to treatment remains open — promising, but not yet guaranteed.
Scientists at New York University have identified the cellular reason hair turns gray — a finding published in Nature that could one day allow people to restore their natural color rather than simply covering it up.
At the heart of the discovery are melanocyte stem cells, which produce the pigment that gives hair its color. Under normal conditions, these cells are unusually flexible: they travel back and forth between compartments inside the hair follicle, cycling between a primitive stem state and a more mature form. This movement exposes them to protein signals — including WNT — that keep them active and productive.
With age, that mobility breaks down. As hair falls out and regrows over time, increasing numbers of these cells become trapped in a region called the hair follicle bulge. Stranded there, they never return to the germinal compartment where WNT signals would prompt them to regenerate into pigment-producing cells. Hair continues to grow — but colorlessly.
Led by Mayumi Ito and postdoctoral researcher Qi Sun, the NYU team simulated aging in mice through repeated cycles of plucking and regrowth. Before the process, about 15 percent of follicles contained trapped cells. Afterward, that figure rose to nearly 50 percent. Cells that kept moving retained their pigment-producing ability throughout the two-year study; those that became stuck did not.
What distinguishes melanocyte stem cells is precisely their plasticity — unlike the structural cells of the follicle, which mature in one direction only, these cells can shift back and forth between states. It is this flexibility that sustains color, and its loss that ends it.
The team now plans to test whether trapped cells can be made mobile again, or whether relocating them to the germinal compartment could restart pigment production. Whether the same mechanism operates in human hair remains to be confirmed — but the target is identified, and the question has shifted from why hair grays to whether the process can be undone.
Scientists at New York University have identified the cellular mechanism behind graying hair, a discovery made in mice that could eventually lead to treatments allowing people to restore their hair's natural color without reaching for dye bottles.
The research, published in Nature, focuses on melanocyte stem cells—specialized cells responsible for producing the pigment that colors hair. These cells normally possess a remarkable flexibility: as hair grows and cycles through its natural phases, they move back and forth between different compartments within the hair follicle, shifting between their most primitive stem cell state and a more mature stage called transit amplification. This movement exposes them to varying levels of protein signals that keep them functioning properly.
But something changes with age. As hair falls out and regrows repeatedly over time, more and more of these melanocyte stem cells become trapped in a region called the hair follicle bulge. Once stuck there, they stop maturing and never make the return journey to the germinal compartment—the area where protein signals called WNT would normally push them to regenerate into pigment-producing cells. The result is hair that continues to grow but has lost its color.
Researchers led by Mayumi Ito at NYU Langone Health demonstrated this process in mice by forcing their hair through repeated cycles of plucking and regrowth to simulate aging. Before this artificial aging, about 15 percent of hair follicles contained trapped stem cells. After the forced aging process, that number jumped to nearly 50 percent. The cells that remained mobile—those still moving between the bulge and the germinal compartment—retained their ability to produce pigment throughout the two-year study period. Those that got stuck could not.
What makes this discovery significant is that melanocyte stem cells possess a flexibility that other hair follicle cells do not. The cells that form the follicle structure itself move in only one direction along a fixed timeline as they mature, never returning to their original state. This one-way street helps explain why hair can keep growing even when its pigmentation fails. Melanocyte stem cells, by contrast, are remarkably plastic—they can shift back and forth between states, which is precisely what keeps them functional.
Postdoctoral researcher Qi Sun, the study's lead investigator, explained that the findings open a concrete pathway forward. If the same cellular process occurs in human hair—something that remains to be confirmed—it suggests a potential treatment strategy: helping trapped cells regain their mobility or physically moving them back to the germinal compartment where they can resume producing pigment.
The team is now planning experiments to test these approaches. The work builds on earlier research from the same group showing that WNT signaling is necessary to stimulate melanocyte stem cells to mature and produce pigment. The next phase will focus on whether that signaling can be restored or whether the cells themselves can be relocated to restart the color-production cycle.
For now, the research remains in the mouse model, and the leap to human treatment is not guaranteed. But the mechanism is clear, and the target is identified. The question is no longer why hair grays—it is whether science can reverse the process.
Notable Quotes
The loss of this chameleon-like function in melanocyte stem cells may be responsible for hair aging and color loss, suggesting that cell mobility and reversible differentiation are key to maintaining healthy, pigmented hair.— Mayumi Ito, NYU Langone Health
These newly discovered mechanisms raise the possibility that this same process of melanocyte stem cells could exist in humans, presenting a potential pathway to reverse or prevent human hair aging by helping stuck cells move again between developing follicle compartments.— Qi Sun, NYU Langone Health
The Hearth Conversation Another angle on the story
So these cells get stuck. But why? What's actually trapping them?
It's not a physical trap—it's more like they lose their passport. The cells need to move between compartments, and that movement depends on exposure to certain protein signals. Once they're stuck in the bulge, they're not getting the signals that would tell them to move back to where the action is.
And if we could restore those signals, the cells would move again?
That's the hypothesis. Or we could physically move them back. Either way, the goal is to get them back into circulation, back into the environment where they can mature into pigment-producing cells.
How long would a treatment like that last? Would it be permanent?
That's unknown. The study tracked mice for two years and the mobile cells stayed functional the whole time. But whether a human treatment would be permanent, or need to be repeated—that's a question for future research.
Why did it take this long to figure this out?
Melanocyte stem cells are remarkably plastic—they shift between states. Most other stem cells are more straightforward. It required detailed observation of how these cells actually move and change as hair cycles through growth and rest.
And this only happens in hair?
The study focused on hair, but melanocyte stem cells exist in skin too. Whether the same trapping mechanism affects skin pigmentation is an open question.