Unraveling Melasma: From Epidermal Pigmentation to Microenvironmental Dysregulation.

Melasma is a chronic, acquired hyperpigmentation disease that occurs on light-exposed skin, especially in women of childbearing age. This common dyschromic disorder significantly impairs quality of life, yet treatments are unsatisfactory due to an incomplete understanding of its etiology. Its pathogenesis is multifactorial: ultraviolet (UV) radiation exposure, sex hormone fluctuations, and familial genetics are known triggers. Meanwhile, the persistence of focal hyperpigmentation suggests additional mechanisms beyond enhanced melanocyte activity. Emerging evidence highlights that melasma skin exhibits features of chronic photoaging: solar elastosis, basement membrane (BM) disruption and increased vascularity can be seen in the skin lesions. Senescent dermal fibroblasts under UV stress secrete melanogenic cytokines (e.g., SCF, HGF) that further stimulate melanocytes. In addition, melasma lesions harbor subclinical inflammation: infiltrates of CD4+ T cells, macrophages, and mast cells are visible, accompanied by elevated IL-17 and COX-2, implying an immune-driven component sustains pigment production. Collectively, these observations suggest that melasma behaves as a chronic inflammatory disorder of the skin microenvironment, rather than an isolated pigmentary defect. Concurrently, epidermal alterations such as barrier dysfunction and abnormal melanosome transport exacerbate melanin retention. In this review, by integrating these emerging insights into a unified pathogenic framework, we recognize melasma as a disorder of epidermal-dermal crosstalk and immune modulation, offering novel therapeutic perspectives for this recalcitrant condition.