Melanogenesis — an update

Human skin and hair colour are due to a pigment, melanin, which is produced at the dermal epidermal level by specialized cells, the melanocytes, in a process named melanogenesis.The melanocytes are derived from their precursors, melanoblasts, with migrate to their definitive locations in the course of the final steps of embryogenesis. The melanocytes produce melanosomes, small organelles where melanin synthesis will take place. There are four stages of development of melanosomes, and only from the third stage onward melanosomes can synthesize melanin. Then, the transfer of melanin from the melanocytes to the keratinocytes can take place. According to the individuals, there is a phenotypic diversity of pigmentation, which is described in Fitzpatrick classification, and due to the ratio eumelanin (dark melanin) and phaeomelanin (reddish melanin) produced by the melanocytes. Melanogenesis is quite a complex series of chemical reactions, along which three enzymes, tyrosinase (TYR), tyrosinase-related protein 1 (TRP1) and DOPAchrome tautomerase (DCT) are absolutely required. Other enzymes also play a role in this process.Melanocortin-1 receptor (MC1R) and Microphthalmia-associated transcription factor (MITF) also play a pivotal role in the stimulation (or not) of the melanocytes. Skin pigmentation is regulated by a series of intrinsic factors such as peptides, cytokines, prostaglandins, NO, but also oestrogens. On the other hand, UV radiation (UVR) is a potent extrinsic regulator of melanogenesis. In this paper, it was emphasized on the importance of the paracrine regulation of skin pigmentation. The major role of keratinocytes is well known, but other skin cells like fibroblast, immune cells or endothelial cells are of major importance in this cell-to-cell communication with the melanocytes and will regulate melanogenesis.Finally, a brief glossary was proposed about various existing melanins.