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Stem cells
A stress hormone has been found to signal through skin cells to repress the activation of hair-follicle stem cells in mice. When this signalling is blocked, hair growth is stimulated. Stressed humans, watch out.
When American football quarterback Aaron Rodgers told his fans to relax after his team's poor start one season, little did he know that he was also giving a hair-care tip. His advice is particularly helpful now, after a long pandemic year. About one-quarter of people who contract COVID-19 experience hair loss six months after the onset of symptoms1, probably because of the systemic shock caused by the ordeal of infection and recovery. Chronic stress has long been associated with hair loss, but the underlying mechanism that links stress to the dysfunction of hair-follicle stem cells has been elusive. On page 428, Choi et al.2 uncover the connection in mice.
Throughout a person's lifespan, hair growth cycles through three stages: growth (anagen), degeneration (catagen) and rest (telogen). During anagen, a hair follicle continuously pushes out a growing hair shaft. During catagen, hair growth stops and the lower portion of the hair follicle shrinks, but the hair (now known as a club hair) remains in place. During telogen, the club hair remains dormant for some time, eventually falling out. Under severe stress, many hair follicles enter telogen prematurely and the hair quickly falls out.
Hair-follicle stem cells (HFSCs) are located in a region of the hair follicle called the bulge. These cells have a crucial role in governing hair growth by interpreting both internal and external signals. For example, during telogen, HFSCs are kept in a quiescent state and so do not divide3,4. When hair growth is initiated in the next anagen phase, HFSCs are instructed to divide and produce progenitor cells. These progenitors then begin ajourney of differentiation, generating several layers of hair follicles and, ultimately, the hair shaft.
Since HFSCs were identified in the bulge region more than 30 years ago5-7, many regulatory molecules - such as gene-transcription factors and signalling proteins - have been shown to control the cells' quiescence and activation3,4. Nearly all of these regulators are...