MSH: the melanocyte-stimulating hormone

The melanogenesis and mechanisms of skin-lightening agents--existing and new approaches

Skin-lightening products are commercially available for cosmetic purposes to obtain lighter skin complexion. Clinically, they are also used for treatment of hyperpigmentary disorders such as melasma, café au lait spot and solar lentigo. All of these target naturally melanin production, and many of the commonly used agents are known as competitive inhibitors of tyrosinase, one of the key enzymes in melanogenesis. In this review, we present an overview of commonly used skin-whitening ingredients that are commercialized, but we also hypothesize on other mechanisms that could be important targets to control skin pigmentation such as for example regulation of the adrenergic and glutaminergic signalling and also control of tetrahydrobiopterins in the human skin.

The discovery of the human melanocyte

Around 2200 bc the first written description of a human pigmentation disorder, most likely vitiligo, was recorded, and from that moment the history of research into human pigmentation can be traced. For the following 4000 yr, the origins of human skin colour remained an enigma that was to generate a multitude of misconceptions. Even after European physicians began to dissect and compare dark and light coloured skin to reveal its underlying anatomy, the origins of skin and hair pigmentation were a matter of frequently erroneous speculation. The true source of human pigmentation was only finally revealed with the discovery of the melanocyte in the 19th century. Once tyrosinase was identified to be the key enzyme in pigment formation, attention focused on elucidating the chemical structure of melanin, an enterprise that remains incomplete. The developmental origins of the melanocyte were described from 1940 to 1960, and the concept of the epidermal melanin unit was introduced together with a description of the ultrastructure of the melanosome and melanosome transfer. With these advances came the realization that different skin types exhibit distinct differences at the histological level that relate to varying amounts of eumelanin and pheomelanin produced by the melanocytes. The foundation established over the past 4000 yr is the basis for all current research into this fascinating cell type.

Biology of hypopigmentation

A review of the basics of pigment cell biology is followed by a discussion of the characteristics of several disorders of hypopigmentation. By determining such features as inheritance pattern, time of onset (congenital, childhood, adulthood), natural history (stable vs progressive), type of pigment loss (diffuse or circumscribed), distribution of lesions (generalized vs localized), degree of pigment loss (incomplete or complete), number of melanocytes, if any, in biopsy specimens of affected areas, type of melanocytic dysfunction, and associated inflammation or infection, one can classify the disorders of hypopigmentation. The proposed pathophysiology for each disorder of hypomelanosis is presented.

Activation of nevi in patients with malignant melanoma

Activation of melanocytic nevi from patients with malignant melanoma is documented microscopically. Features of activation include marked junctional activity, lack of lateral margination of junctional melanocytic activity, and--to a lesser degree--increased pigment production, inflammation, mitotic activity, and cytologic atypia. The changes are most pronounced in nevi from the same lymph drainage region as the malignant melanoma, suggesting a local activating factor. The presence of marked junctional activity in these lesions and the occurrence of these changes in nevi of patients with clinical Stage I malignant melanomas provide evidence that these lesions do not represent metastases.