A strand of hair can tell a story about stress, time, and survival. Sometimes it turns gray, quietly marking age. Sometimes the same kind of cell that makes that color can turn dangerous.
Scientists from The University of Tokyo have found that one stem cell type can age safely or turn cancerous, depending on the stress it faces. The same decision that fades color can also feed disease.
Cancer link to gray hair
Melanocyte stem cells sit deep inside the hair follicle, waiting for their signal. Their job is simple: create pigment cells that give hair and skin their color.
These stem cells live in a small zone called the bulge-sub-bulge region. When hair sheds and grows again, these cells restart the color cycle.
Over time, tiny cracks form in their DNA – caused by sunlight, chemical exposure, or simply the everyday wear and tear of life.
Once that damage builds up, the cell must choose what to do next. It can protect the body by aging and stepping aside – or ignore the signal and risk turning rogue.
DNA stress and graying
The new study, led by Professor Emi Nishimura and Professor Yasuaki Mohri, tracked how these stem cells react under pressure. The team studied mice using long-term lineage tracing and gene expression tools.
When the cells suffered double-strand breaks in their DNA, they stopped renewing. Instead, they matured too soon in a process called senescence-coupled differentiation, or seno-differentiation.
That decision ends their role as stem cells. The pigment-making line shuts down, and the hair loses color. The process runs under the p53-p21 pathway, a molecular signal that acts like a switch for damage control.
The body sacrifices the damaged cells for the sake of safety, leaving behind gray strands as evidence.
When protection fails
The story shifts when the stress comes from carcinogens such as ultraviolet B radiation. The same cells act differently.
Even with DNA damage, they skip the protective path. Nearby tissue sends KIT ligand signals that block the p53-p21 response. Instead of aging, the cells start dividing again, expanding in clusters.
What once ensured color now turns risky. Those expanding cells can set the stage for melanoma, a dangerous form of skin cancer. The contrast is striking: one type of damage triggers safe aging, another drives dangerous growth.
Gray hair or cancer?
“These findings reveal that the same stem cell population can follow antagonistic fates – exhaustion or expansion – depending on the type of stress and microenvironmental signals,” said Professor Nishimura.
“It reframes hair graying and melanoma not as unrelated events, but as divergent outcomes of stem cell stress responses.”
Her words capture the core of the discovery. Graying hair is not a random sign of age; it is a result of the body choosing caution. Cancer appears when that caution disappears.
The same stem cell, under different signals, becomes either a quiet survivor or a silent threat.
The body’s signal system
Seno-differentiation looks like a built-in safety plan. When a cell senses danger, it stops multiplying. That act protects the tissue from turning malignant.
But when outside factors block that response, damaged cells survive longer than they should. Those survivors can mutate, form tumors, and spread.
The researchers make one point clear: having gray hair does not prevent cancer. Both graying and melanoma grow from the same cellular stress but follow opposite directions. It is the body’s internal signal system that decides who wins.
Between repair and risk
By revealing how stem cells choose between shutting down or expanding, the study bridges the biology of aging and cancer.
The research shows that aging is not only decay but a strategy – one that removes damaged cells before they harm the body. Cancer begins when that cleanup fails.
This idea also supports the importance of “senolysis,” the natural removal of worn-out cells. When the body clears out its tired or broken parts, it lowers the chance of disease. The balance between regeneration and restraint keeps tissues healthy over time.
Every gray hair may reflect a moment of cellular decision-making. The same logic that paints our hair can, under stress, turn into a threat.
The research was supported by the Japan Society for the Promotion of Science and the Japan Agency for Medical Research.
The study is published in the journal Nature Cell Biology.
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