Clues to the Pathogenesis of Melasma from its Histologic Findings

Abstract

Melasma is a common acquired hypermelanosis that affects sun-exposed areas of the skin, especially the face. Its histologic manifestations are evident in the epidermis, extracellular matrix, and dermis. One of the hallmarks of melasma is an increase in the amount of epidermal melanin; however, whether melanocyte numbers increase or not is a topic of debate. Interestingly, basement membrane abnormalities also characterize melasma. Furthermore, solar elastosis is recognized as one of the dermal pathologic findings of melasma. These findings suggest that extracellular matrix abnormalities are consistently found in melasma. In the dermis, increased vascularity and increases in mast cell numbers are observed, indicating that dermal factors have important roles in the pathogenesis of melasma, despite melasma being characterized by epidermal hyperpigmentation. This review discusses these histologic characteristics of melasma, and it considers their implications for the pathogenesis of this skin condition.