Abstract:

Objective To study the clinical, histopathologic and ultrastructural characteristics of achromic naevus （AN）. Methods Clinical data, including sex, age, age of onset, pattern of lesions, involved sites, shape and number of lesions and associated systemic diseases, were collected from 85 patients with AN. Skin melanin index was detected in 34 lesions of 19 patients with AN, 30 lesions of 12 patients with vitiligo and 64 contralateral normal skin islands of the 31 patients. Reflectance confocal microscopy （RCM） was performed to analyze the lesion, normal skin and junctional area between lesional and normal skin of 62 patients with AN. Tissue samples were obtained from lesions and perilesional normal skin of 17 patients with AN and subjected to pathological examination as well as ultrastructural study with transmission electron microscopy; also, skin biopsy specimens were immunostained for tyrosinase, HMB45, tyrosinase-related protein-1 （TRP-1）, TRP-2 and CD117. Results Of the 85 patients with AN, 23 （27.1%） developed lesions at birth, and 21 （24.7%） after 3 years of age; 72 （84.7%） had irregularly shaped lesions, 54 （63.5%） had only a single lesion. The mean melanin index and relative melanin index of AN lesions were 186.56 ± 52.86 and 80 ± 11, respectively, significantly lower than those in normal skin islands （223.88 ± 63.19 and 100, both P < 0.01）, but higher than those in depigmented lesions from 12 patients with vitiligo （128.57 ± 64.31 and 60 ± 20, both P < 0.01）. RCM revealed a decline in the number of melanocytes and brightness of melanin caps, even distribution of melanin in lesions, as well as obscure demarcation between lesions and normal skin from patients with AN. Fontana-Masson stain showed that the melanin content was lower in lesions than in perilesional skin （1810.12 ± 327.96 vs 2064.24 ± 260.41） from patients with AN. Microscopic examination demonstrated a decrease in melanocyte and melanosome number, presence of immature melanocytes at stage Ⅱ and Ⅲ in cytoplasm and dendrites of melanocytes and keratinocytes, aggregated melanosomes in affected keratinocytes in lesions of AN. In 17 patients with AN, the relative expression levels of tyrosinase and TRP-1 were 1827.35 ± 307.09 and 6102.54 ± 1642.64, respectively, in normal skin specimens, significantly higher than those in lesional skin （1477.35 ± 224.05, 5322.33 ± 1565.26, both P < 0.01）; no statistical difference was observed in the expression levels of HMB45, TRP-2 or CD117 between lesional and normal skin. Conclusions AN is an early-onset, nonfamilial aggregated, stable leukoderma with irregular margins, and in lesions of AN, the number of both melanocytes and melanosomes is decreased with the presence of immature melanosomes. The measurement of relative melanin index and reflectance confocal microscopy may offer a non-invasive approach to the diagnosis of AN.

Key words: Nevus depigmentosus, Hypopigmentary, Melanocyte, Melanin, Reflectance confocal microscopy (RCM)