To understand why our hair turns gray, I must first explain how hair follicle stem cells — which are present in the skin and produce hair throughout a person’s lifetime — work. Hair follicle stem cells are defined, like all stem cells, as having a low replication rate and a high capacity for differentiation. This means that they proliferate little, to protect their genetic material, but have a great capacity for differentiation. And because of this capacity they are stem cells, because they can give rise to different cells with very different functions.
When proliferation signals from other regions of the hair follicle reach the niche in which these stem cells are located, such as when hair begins to grow, the cells begin to proliferate. They first produce a population of transiently amplified cells, which multiply very rapidly. Stem cells only divide on specific occasions and remain in the niche; a biological mechanism that ensures that they are thus protected from mutations, because mutations can appear with each cell division. And they are also conserved as a kind of repository for when the organism must generate new cells if it needs to regenerate the tissue. Let’s say they are like the hard drive.
In the hair follicle there are several types of stem cells depending on their role. On the one hand, there are the stem cells of the hair follicle, which are the ones that will generate the hair itself, and then there are other types, such as melanocytes. As I explained, when the growth signal reaches them, transiently amplified cells appear that proliferate a lot and make the hair grow. This happens because they acquire the characteristics of a hair cell. At the same time, the same signals also activate the melanocytic stem cells. And these follow the same process I just told you about.
What happens is that melanocyte stem cells age faster than those that give rise to hair. That means that, as you get older, the potential of these stem cells to generate melanocytes is less than that of those that generate hair fiber. And melanocytes are what give hair its color. So there comes a time when the hair stops having its natural color because the hair follicle has no functional melanocytes. This is the reason why gray hairs appear. That is, they appear because the hair lacks something: the cells capable of giving it color. And this is all we knew until now.
But this past April, researchers at New York University published a paper in the journal Nature that revolutionizes one of the dogmas of stem cells. As described so far, stem cells proliferate little and have no function other than to become other cells that are already functional. And there is the dogma that there is a timeline behind all of this: when a stem cell proliferates, it divides into two; one of them does begin to proliferate, transforms into transiently amplified cells and begins to divide much more, while the other stays in the niche as a stem cell and does nothing more until it receives a new signal that restarts this process. This transition would be unidirectional: the stem cell that proliferates and manages to differentiate, that is, to become a cell that already fulfills a specific function in the organism, cannot go backwards. The process cannot be reversed from differentiated cell to stem cell.
This dogma is only broken in two situations. One of them is cancer, in which its cells can indeed dedifferentiate, migrate and become tumors in other tissues of the body, hence metastasis. But what this recently published study — carried out using mice as study subjects — reveals is that melanocyte stem cells also have this ability to return to that dedifferentiated state: that is, they return to that stem cell state. This may explain why we get gray hair. According to these researchers, what may be happening is not that there comes a time when melanocytes stop proliferating, but that as they are continuously in this state of differentiation and dedifferentiation, they become dormant and lose their ability to give color to the hair.
Beyond explaining why we get gray hair, the New York University study opens up new therapeutic options for all pigmentation-related diseases, such as vitiligo or certain types of hair pigmentation disorders.
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