Hair Growth Found Induced by Light through Eyes via a Neural Pathway

A research team, led by Prof. Sung-Jan Lin (林頌然) of the Department of Biomedical Engineering and Associate Prof. Shih-Kuo Chen (陳示國) of the Department of Life Science at NTU, has found that external light can activate hair follicle stem cells and induce new hair growth after stimulating the sympathetic nerves. This study, titled “External Light Activates Hair follicle Stem Cells through Eyes via an ipRGC–SCN–Sympathetic Neural Pathway,” was published in the Proceedings of the National Academy of Sciences on June 29, 2018.

For many animals, hair is the first line of defense for the skin. When the amount of daily sunlight changes with the seasons, animal hair growth is affected. Hair follicle stem cells have no contact with light but can feel changes in the meganiche through a certain mechanism. Stem cells in the body are subject to partial micro-environment and whole-body control, and then either stay static or are activated. How the cells and the external environment, separate and without contact, interact with one another is the main concern of this study.

This study found that if mice are exposed to intense light (especially blue light) for a few minutes every day, their hair follicles are activated and new hair grows. This physiological reaction uses the intrinsically photosensitive retinal ganglion cells (ipRGCs) as the receptors and sends signals to the suprachiasmatic nucleus (SCN). Afterwards, the whole-body sympathetic nerves are activated, leading to an increase in the release of norepinephrine in the skin, facilitating the activation of the hedgehog signaling pathway, and then activating the hair follicle stem cells. This reaction does not affect the existing circadian physiological cycle.

Combining expertise of different fields and experimentation methods to illustrate this particular physiological mechanism, the study reveals the function of the ipRGCs regulating the autonomic nervous system. It also presents the possibility that the eyes and the brain region controlling the physiological clock have multiple nerve circuits to control various physiological functions. This breakthrough will provide new directions for research related to the use of external light to control internal physiological phenomena and stem cell activity.

The study was supported by NTU, Ministry of Science and Technology, and the Taiwan Foundation for the Development of Biomedical Technology. The first and second authors of the paper are Sabrina Mai-Yi Fan (范邁儀), a postdoctoral researcher at NTU, and Yi-Ting Chang (張宜婷), a graduate of the Department of Life Science who is currently pursuing a doctorate at Johns Hopkins University.

This article will be featured in No. 67 of NTU Highlights (August, 2018).