Segmental Vitiligo and Somatic Mosaicism: From Pathogenesis to Therapeutics.

Segmental vitiligo (SV) is a distinct clinical subtype of vitiligo characterized by unilateral, sharply demarcated depigmentation, rapid progression and early stabilization, and frequent leukotrichia. Compared with non-segmental vitiligo (NSV), SV is generally less strongly associated with systemic autoimmunity, suggesting potential differences in pathogenic mechanisms, although overlap phenotypes and mixed presentations have been reported. Accumulating clinical, epidemiological, and mechanistic evidence supports the concept that SV arises from somatic mosaicism, in which post-zygotic genetic or epigenetic alterations occur during embryogenesis and give rise to a clonally distinct melanocyte population distributed along developmental territories. Embryologic studies indicate that melanoblast migration from the neural crest, coupled with high proliferative activity during early development, creates a permissive context for mosaic mutation accumulation. The resulting melanocyte clones align with Blaschko's lines or other embryonic patterning units, accounting for the segmental distribution of SV. Beyond melanocytes, segment-restricted abnormalities in neural and vascular structures have also been reported, raising the possibility of broader neurocutaneous mosaicism, although whether these changes are primary or secondary to localized immune injury remains unresolved. Recognizing SV as a mosaic disorder has important clinical implications, particularly for early intervention, recurrence risk assessment, and the prominent role of autologous surgical therapies. Future advances will depend on multi-omics approaches to identify causal mosaic mutations, refined disease models, and long-term registries to translate developmental insights into precision management strategies.