External Light Activates Hair Follicle Stem Cells Through Eyes Via the ipRGC-SCN-Sympathetic Neural Circuit

The document presents several abstracts from different studies, but the relevant one for hair loss, alopecia, and hair growth is the first abstract. It reports that external light can activate hair follicle stem cells through a neural circuit involving the eyes. Specifically, light detected by intrinsically photosensitive retinal ganglion cells (ipRGCs) signals through the suprachiasmatic nucleus (SCN) and then through the sympathetic nervous system to the hair follicles. This activation requires intact sympathetic innervation to the hair follicles and results in increased norepinephrine release in the skin, which then activates hair follicle stem cells by modulating hedgehog signaling. This suggests that the eyes can serve as a gateway for external light to rapidly regulate tissue stem cells, potentially affecting not just skin but other tissues as well. The study implies a direct link between external light exposure and hair follicle stem cell activation, which could have broader implications for understanding how environmental factors influence tissue regeneration.