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.

Led by researchers from the Grossman School of Medicine in New York, the research focused on cells called melanocyte stem cells, or McSCs, found in the skin of mice and also in humans.

The scientists suggested that if their discoveries are found to 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 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.

It is inside these compartments where McSCs are exposed to the signals that influence maturity.

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 published in Nature, as hair ages and 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.”