The mystery of baldness could soon be lifted: researchers have just identified the cell division mechanism of the hair follicles at the origin of hair loss.
- The thinning and then the loss of hair are due to an imbalance in the cell division of the stem cells of the hair follicle.
- In young mice, the divisions are just as symmetrical as asymmetrical, which guarantees regeneration of the hair follicle.
- As we age, these divisions become mostly asymmetrical, which does not allow the follicle to replenish to form new hair.
Why, when they age, do some men see their hair thinning out, while others keep their hair? While in France, an estimated 10 million men and 2 million women suffer from alopecia, that is to say a partial or total loss of hair and / or body hair, researchers from Tokyo Medical and Dental University (TMDU) and University of Tokyo provide an answer to this question.
In a study published in the journal Nature Agingthey explain that they have identified a new mechanism by which hair follicles lose their capacity for regeneration, and eventually disappear.
An imbalance in the cell division of hair follicle stem cells
Hair follicles are tiny cavities in which new hair is constantly growing. In order for new hair to grow there, proper functioning of the hair follicle stem cells (CSFP) is essential. These CSFPs divide cyclically in two ways: symmetrical divisions, which generate two identical cells that will have the same fate, and asymmetrical divisions, which generate a cell in the process of differentiation and a self-renewing stem cell. This combination ensures the sustainability of the stem cell population, but how these contribute to the loss of function of CSFPs due to aging was not yet fully understood by scientists.
“For proper tissue function, symmetric and asymmetric cell divisions must be in balance, explains Emi Nishimura, the researcher who led the work. When stem cells preferentially undergo one or the other or, even worse, deviate from the typical process of one or the other type of cell division, the organ suffers.”
To understand how stem cell division plays into hair growth during aging, the researchers conducted experiments on young and old mice that they subjected to two different types of tests: cell fate tracing and cell division axis analyses. For cell fate tracing, the stem cells of the hair follicle were marked with a fluorescent protein in order to be able to follow them over time, while the second test consisted of measuring the angle of division of the CSFPs.
The researchers then found that in young mice, the cell divisions of the hair follicle are just as symmetrical as asymmetrical. On the other hand, as we age, asymmetric division becomes predominant, which prevents proper regeneration of the hair follicle.
Exhaustion and then disappearance of the hair follicles
But why does the mode of cell division change so drastically during aging?
Researchers have found the answer: it’s due to hemidesmosomes, a class of proteins that connect cells to the extracellular matrix (ECM), the proteins that surround cells. During the aging process, the hemidesmosomes are destabilized, resulting in the aberrant asymmetry during hair follicle stem cell division. This has the consequence of exhausting them, then making them disappear. Result, the hair thins, then falls over time.
“These are striking results that show how hair follicles lose their ability to regenerate hair over time, concludes Hiroyuki Matsumura, co-lead author of the work. Our results may contribute to the development of new approaches to regulate organ aging and diseases associated with aging.”
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