- Serotoninergic and melatoninergic systems are fully expressed in human skin. Slominski, A., Pisarchik, A., Semak, I., Sweatman, T., Wortsman, J., Szczesniewski, A., Slugocki, G., McNulty, J., Kauser, S., Tobin, D.J., Jing, C., Johansson, O. FASEB J. (2002)