The purpose of the present study is to examine the influence of melatonin on the structure and calcification of both dentin and odontoblasts. The size of calcospherites in predentin increased in proportion to the concentration of melatonin administered. On the melatonin treated groups, the von Korff's fibers were clear and long. The number of odontoblasts tended to increase. The expression of melatonin receptor MT2 was observed in the odontoblasts of the control group. In the melatonin treated groups, a strong expression of MT2 was observed in the odontoblasts. By MELDI TOF-MS analysis, a peak of around 850 m/z was observed in dentin. This peak increased in proportion to the amount of melatonin. It is considered that melatonin activates the odontoblasts and promotes the formation of von Korff's fibers, collagen fiber secretion and calcification of dentin.

Phototoxicity indicates undesirable light effects on the living body. In the present study, we define it as undesirable effects on the living body, due to interaction between light and a chemical substance administered to animals, including humans. Photoirritation is phototoxicity limited to skin reactions. An immune response upon re-exposure to light and a test substance in 1-2 weeks, unlike that upon initial exposure, is referred to as photoallergy. In addition, phototoxicity includes photogenotoxicity and photomutagenicity. Photocarcinogenicity due to exposure to light and a test substance is also known. We describe chemical substances that induce various kinds of phototoxicity and their evaluation methods, and propose relationships between phototoxicity and tissue regeneration.