It starts with one, maybe two, grey hairs, and most people put it down to ageing without thinking about the process that turns their hair grey.
But a new study suggests stem cells may get stuck as hair ages, and lose their ability to mature and maintain hair colour.
Certain stem cells – cells that are able to develop into many different cell types – have a unique ability to move between growth compartments in hair follicles.
It is these cells that lose the ability to move with age, paving the way for grey hair.
The scientists suggested that if their findings hold true for humans, they could open up a potential way to reverse or prevent the greying of hair.
Hair colour is controlled by whether continually multiplying pools of McSCs within hair follicles (where hair grows from) get the signal to become mature cells that make the protein pigments responsible for colour.
Researchers found that during normal hair growth, such cells continually move back and forth as they transit between compartments of the developing hair follicle.
Specifically, the research team found that McSCs transform between their most primitive stem cell state and the next stage of their maturation, the transit-amplifying state, depending on their location.
According to the findings, as hair ages, sheds, and then repeatedly grows back, increasing numbers of McSCs get stuck in the stem cell compartment called the hair follicle bulge.
They remain there and do not mature into the transit-amplifying state, and do not travel back to their original location in the compartment, where they would have been prodded to regenerate into pigment cells.
Study lead investigator Qi Sun, a postdoctoral fellow at NYU Langone Health in New York, said: “Our study adds to our basic understanding of how melanocyte stem cells work to colour hair.
“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 greying of human hair by helping jammed cells to move again between developing hair follicle compartments.”
In the latest experiments in mice whose hair was physically aged by plucking and forced regrowth, the number of hair follicles with McSCs lodged in the follicle bulge increased from 15% before plucking to nearly half after forced ageing.
These cells remained incapable of regenerating or maturing into pigment-producing melanocytes, the study published in Nature found.
The stuck McSCs, the researchers found, ceased their regenerative behaviour as they were no longer exposed to much of the signalling that allowed them to produce pigment in new hair follicles, which continued to grow.
But other McSCs that continued to move back and forth between the follicle bulge and hair germ retained their ability to regenerate as McSCs, mature into melanocytes, and produce pigment over the study period of two years.
Study senior investigator Mayumi Ito, a professor in the Ronald O Perelman Department of Dermatology and the Department of Cell Biology at NYU Langone Health, said: “It is the loss of chameleon-like function in melanocyte stem cells that may be responsible for greying and loss of hair colour.”
The researchers plan to investigate means of restoring movement of McSCs or of physically moving them back to their germ compartment, where they can produce pigment.