The hair cycle consists of four phases: anagen (growing phase), catagen (transition phase), telogen (resting phase), and exogen (shedding phase).
Hair follicles push out hairs to grow during the anagen stage and shrink, consequently causing hair growth to slow, during the catagen stage. Hair also separates from the substratum of the follicle while in the catagen (transition) phase.
Now detached from the follicles, hair strands become stagnant - neither growing nor falling out - in the telogen phase, making room for new hairs to form. An extension of the telogen phase, the exogen stage is when the hair follicles begin to fall out.
In 2021, Harvard researchers conducting a study on stem cell biology found that corticosterone, a stress hormone in mice and the equivalent of cortisol in humans, places hair follicle stem cells into an extended telogen phase without new hair or hair follicle growth.
What does this mean?
In general terms, stem cells are considered immature, undifferentiated cells that can develop into a variety of cell types. They are vital in renewing the body's tissues and organs, and for the case of hair growth, they are a crucial component in augmenting the catagen (growth) phase of the cycle. These hair growth stem cells rest in a compartment of the hair follicle called the bulge and begin their job when hair falls out, helping generate new follicles and hair.
During the telogen (resting) phase, hair stem cells become dormant, halting any growth. If these cells remain quiescent, new tissue will not be generated, and hair loss can occur.
When environmental factors trigger long-term stress, corticosterone is produced, which has been seen to produce a negative effect on hair stem cells in mice.
In short, corticosterone (or cortisol in humans) can down regulate hair stem cells in regenerating new follicles and hair, ultimately leading to hair loss.
Taking a closer look, the researchers found that corticosterone is not a direct regulator of hair stem cells; rather, the hormone acts on dermal cells underneath the hair follicle known as dermal papilla, which in turn activate stem cells to complete their job. They do so by secreting a protein known as Gas6, which has functions in cell proliferation.
In short, corticosterone halts the function of dermal papilla, which cannot produce the Gas6 molecules needed to activate stem cells. As a result, hair growth is terminated.
Ultimately, this study not only suggests a potential pathway for treatment - utilizing the Gas6 pathway to promote hair stem cells to regenerate hair - but also reveals a larger paradigm for how cross-organ interaction plays a macro role in local areas.
Sources
Lau, J. (2021, March 31).
Researchers discover how chronic stress leads to hair loss. Harvard Gazette. https://news.harvard.edu/gazette/story/2021/03/researchers-discover-how-chronic-stress-leads-to-hair-loss/
Roland, J. (2020, September 25).
Stages of hair growth plus how to maintain hair health in every stage. Healthline. https://www.healthline.com/health/stages-of-hair-growth
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