Histopathological Features of Riehl Melanosis

Histological features of Riehl melanosis have rarely been compared between lesional and perilesional normal skin and have not been precisely described using quantitative or immunohistochemical analysis or electron microscopic findings. To investigate the histopathological and immunohistochemical features of lesional and perilesional normal skin of patients with Riehl melanosis, we retrospectively evaluated the electronic medical records and skin biopsy specimens of 48 patients with Riehl melanosis. In addition, electron microscopy was performed on 1 case. Fontana-Masson staining for melanin and immunohistochemical staining for Melan-A, NKI/beteb, tyrosinase, and microphthalmia-associated transcription factor were performed. Although the difference was statistically insignificant, melanin pigment was increased in the epidermis of lesional skin compared with that of perilesional normal skin in patients. The number of melanocytes and their activity were significantly increased in lesional epidermal skin. Melanin pigment was also significantly increased in the lesional dermis. Pigmentary incontinence, basal cell liquefaction, dilated vessels, epidermal spongiosis, and colloid bodies were found in the lesional skin as well as in the perilesional normal skin to a lesser extent. Under electron microscopy of 1 randomly selected subject, many fibrocytes contained numerous melanosome particles in the cytoplasm of the lesional dermis. In perilesional normal skin, fibroblasts also contained melanosome particles; however, the number of melanosome-containing cells was less than that in lesional skin. Riehl melanosis is characterized by increased epidermal melanocytes and pigmentation, primarily involving the dermis, with histologically typical changes at the interface. Unlike that in other pigmentary diseases, most perilesional normal-appearing skin in Riehl melanosis also shows typical histopathological changes, although to a lesser extent.