Summary Background Melasma is a common acquired symmetrical hypermelanosis characterized by irregular light to dark brown macules and patches on sun-exposed areas of the skin. Its histopathological characteristics are not fully understood.
Objectives To characterize the histopathological features of facial melasma skin in comparison with adjacent normal skin.
Methods Biopsies were taken from both melasma lesional skin and adjacent perilesional normal skin in 56 Korean women with melasma. The sections were stained using haematoxylin and eosin, Fontana–Masson, diastase-resistant periodic acid-Schiff, Masson trichrome and Verhoeff–van Gieson stains, and immunostaining for melanocytes. Data on the changes in number of melanocytes and melanin contents of the epidermis were analysed by a computer-assisted image analysis program. The ultrastructure of the skin was also examined.
Results The amount of melanin was significantly increased in all epidermal layers in melasma skin. The staining intensity and number of epidermal melanocytes increased in melasma lesions. Lesional skin showed more prominent solar elastosis compared with normal skin. Melanosomes increased in number and were more widely dispersed in the keratinocytes of the lesional skin. Lesional melanocytes had many more mitochondria, Golgi apparatus, rough endoplasmic reticulum and ribosomes in their cytoplasm. A dihydroxyphenylalanine reaction was apparent in the cisternae and vesicles of the trans-Golgi network in melanocytes from lesional skin.
Conclusions Melasma is characterized by epidermal hyperpigmentation, possibly caused both by an increased number of melanocytes and by an increased activity of melanogenic enzymes overlying dermal changes caused by solar radiation.