1. Melanocytes in the Hair Bulb: The Cells That Put Color into Hair
The root of a hair strand swells inside the skin into a structure called the "hair bulb." This is where melanocytes, the cells responsible for making pigment, are located. These melanocytes pass a substance called melanin to the hair matrix cells, which are the cells that actually build the hair shaft. It is this handover of melanin that gives the hair its color [1].
If the melanocytes stop working and no longer produce melanin, or if they fail to pass it on to the building cells, the new hair grows out without pigment. This unpigmented hair reflects light, making it appear white [2]. It is important to remember that no white color is being added; rather, the normal color simply fails to enter the hair structure.
2. The Bulge in the Follicle: Where Reserve Pigment Cells Wait
The tube inside which a hair grows is called the follicle. Partway up this tube, there is a slight bulge known as the "bulge." This area is thought to be the storage place for melanocyte stem cells, which are the precursors or "reserves" for the active melanocytes found in the hair bulb [3].
However, scientists have not yet agreed on a single explanation for how gray hair forms. A review article lists three main theories: first, that the active melanocytes in the bulb decrease; second, that the stem cells in the bulge decrease; and third, that oxidative stress during hair growth [3]. This article focuses mainly on research related to the second idea—that the reserves run out—but this does not mean the other theories are wrong.
3. How Reserve Cells Get Used Up: Nishimura's 2005 Study and Cells in the Wrong Place
In 2005, a team led by Emi Nishimura published a paper in the journal *Science*. By studying mice and hair follicles from older people, they reported that gray hair occurs when melanocyte stem cells fail to maintain themselves properly [4].
Later, the same research group at the Institute of Medical Science, The University of Tokyo, found something specific in the whisker follicles of older mice. They observed that pigment stem cells were changing into color-making melanocytes directly inside their waiting place, known as the "niche," instead of moving to where they should go. This process is called "ectopic differentiation" [5].
As a result, the waiting stem cells are used up before they can produce new pigment cells for future hairs, leading to white hair [5]. It is crucial to note that this specific finding about ectopic differentiation was observed in mouse whisker follicles. While studies on human follicles are ongoing, it has not yet been confirmed that this exact mechanism happens in humans.
4. Can Strong Stress Reduce the Reserve Cells? The 2020 Study by Zhang and Hsu
You may have heard stories of someone’s hair turning completely white overnight due to shock. In January 2020, a group led by Zhang and Hsu reported a study in *Nature* using mice. They found that acute stress causes the sympathetic nervous system to become overactive, releasing a substance called noradrenaline. The study reported that this was followed by a rapid loss of melanocyte stem cells, resulting in gray hair [6].
The researchers stated that this was not caused by immune attacks or stress hormones. However, these results were obtained from mice. It is not yet known if human gray hair turns white through this same mechanism when a person experiences stress.
5. Can Gray Hair Help Protect Against Cancer? The 2025 Tokyo Study in Mice
In October 2025, the Institute of Medical Science, The University of Tokyo, announced research published in *Nature Cell Biology*. In mouse experiments, when cells received major damage to their DNA from radiation or other stresses that cause major DNA damage, pigment stem cells changed into color-making cells and were used up, causing white hair. This reaction is called senescence differentiation [7]. It can be seen as a braking mechanism that uses up damaged cells.
However, when the mice were exposed to carcinogenic drugs (DMBA) or ultraviolet light, this aging differentiation was suppressed. The report suggests that in these cases, damaged stem cells remained and could potentially become the source of cancer [7]. This leads to the view that gray hair might be a result of a reaction that helps prevent cancer. Yet, since this is primarily based on mouse experiments, we cannot say that people with gray hair are necessarily less likely to get cancer.
6. Try It Yourself: Draw a Hair Follicle and the Path of New Pigment Cells
You can try drawing a cross-section of a hair follicle on paper. Mark the positions of the bulge and the hair bulb, then draw arrows showing how new pigment cells reach the bulb. Adding where "ectopic differentiation" occurs (the niche) will help you understand the explanation better.
If you can explain the diagram to someone else, you might also read research introductions from the Institute of Medical Science, The University of Tokyo, or their October 2025 announcement in Japanese to compare with this text. You can also look for books about skin and hair at your local library to see where the bulge is drawn in diagrams.
If you have concerns about your hair or scalp, do not judge by yourself. Consult a dermatologist or other specialist for advice.