Researchers at National Taiwan University shaved patches of fur from mice and exposed some of the animals to extra light, then tested whether the eyes were needed, to see whether a signal from a completely different organ could reach the follicles.
Sabrina Fan's team found that light detected by specialized cells in the mouse retina sent signals through the brain's suprachiasmatic nucleus and out through sympathetic nerves to the skin, where they triggered the release of norepinephrine. That signal activated hair follicle stem cells and sped up visible hair regrowth, an effect that depended on the eyes, not on light reaching the skin.
The pathway showed that an organ with no direct connection to the skin, the eye, could still set off a local growth process there through a relay running through the brain and nervous system. The experiments were done in mice with shaved patches, and whether the same eye-to-skin pathway meaningfully affects human hair growth has not been tested.
Read the original study
Light in the eyes reached the fur.
Source
- Fan, S. M.-Y., Chang, Y.-T., Chen, C.-L., Wang, W.-H., Pan, M.-K., Chen, W.-P., Huang, W.-Y., Xu, Z., Huang, H.-E., Chen, T., Plikus, M. V., Chen, S.-K., & Lin, S.-J. (2018). External light activates hair follicle stem cells through eyes via an ipRGC-SCN-sympathetic neural pathway. Proceedings of the National Academy of Sciences of the United States of America, 115(29), E6880–E6889. https://doi.org/10.1073/pnas.1719548115