TLDR Human skin can make serotonin and melatonin.
In a study from 2002, researchers examined the presence of genes and enzymes involved in the conversion of tryptophan to serotonin and melatonin in various human skin samples, including normal skin, skin with basal cell carcinoma and melanoma, and various cultured skin cells. They found that most samples expressed the genes for tryptophan hydroxylase, arylalkylamine N-acetyltransferase (AANAT), and hydroxyindole-O-methyltransferase (HIOMT), which are key in the synthesis of serotonin and melatonin. The activity of AANAT and the presence of its product, N-acetylserotonin (NAS), were confirmed in human skin and cultured cells, while HIOMT activity was detected in human skin, keratinocytes, and melanoma cells. The study concluded that human skin has the intrinsic capability to produce serotonin and melatonin.
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