Pigmentation of the skin because of synthesis and dispersion of melanin in the epidermis has a great significance in the cosmetic industry and society in general. It is the key physiological defense against sun-induced damages such as sunburn, photoaging and photocarcinogenesis. Human skin color varies around the world. It ranges from a very dark brown in some African, Australian and Asian-Indian skin to a near pinkish-yellow among some northwest Europeans. Complexion coloration in humans primarily is regulated by the amount and type of melanin synthesized by the epidermal melanocyte. Melanocytes have specialized organelles termed melanosomes that contain several enzymes such as tyrosinase, tyrosinase-related protein-1 and tyrosinase-related protein-2, which catalyze the production of melanin.
Tyrosinase is the first and a rate-limiting step in melanogenesis. However, additional and equally contributing factors consist of efficient transfer of melanin from melanocytes to the neighboring keratinocytes and distribution and degradation of the transferred melanosomes by the recipient keratinocytes. The protease-activated receptor-2 (PAR-2) and unidentified surface lectins and glycoproteins facilitate this transfer process. Skin-lightening agents (melanogenesis inhibitors) have been used widely to either lighten or even-toning the skin. Preparations in the European market are often used to treat age spots and freckles or to obtain even-toning effects, whereas the Asian market uses them to change or modify skin color.
