Clinical, pathologic, and ultrastructural studies of progressive macular hypomelanosis

Progressive macular hypomelanosis (PMH), a condition of uncertain etiology, is characterized by asymptomatic hypopigmented macules, predominantly located on the trunk. To date, the study of this disease has been sporadic and there are still no clinical diagnostic criteria. The aim of this study was to investigate the histopathologic and ultrastructural characteristics of PMH, and propose the clinical diagnostic criteria of PMH.

The Wood's lamp and Confocal Laser Scanning Microscopy were used to observe the lesions' features. Skin biopsies were used for hematoxylin and eosin staining, melanin staining, antibodies staining of S-100 protein, tyrosinase-related protein-1(TRP-1) and tyrosinase (T311), and also for ultra-structural study. Melanocytes were isolated and cultured from the lesions.

Under Wood's lamp examination, the lesions of PMH showed punctiform red fluorescence. Confocal Laser Scanning Microscopy observation of the lesion showed that its "pigmented ring" around the dermal papillae was intact, but its melanin content was decreased compared with the surrounding normal skin. Ferrous sulfate staining showed that melanin content in the lesion of PMH was significantly decreased compared with the normal skin (P < 0.05). S-100 staining showed that the number of positive cells in the basal layer had no statistical significance (P > 0.05) between the lesion areas (8.25 ± 0.96) and the surrounding normal skin (8.75 ± 1.71). TRP-1 staining showed no significant difference between lesion areas (4.25 ± 0.96) and the surrounding normal skin (4.50 ± 1.29) (P > 0.05), and T311 staining also showed no difference between lesion areas (4.01 ± 0.87) and the surrounding normal skin (4.30 ± 1.05) (P > 0.05). Ultra-structural studies revealed a large reduction in the number of mature melanosome from PMH lesions. There were many membrane-bound groups in PMH lesions with normal appearance the margin, which contained a number of smaller type II-IV melanosomes, which were distributed in clusters. No degradation of melanosomes was present in the lysosomal compartments of PMH lesions. When melanocytes from the PMH lesions were cultured in vitro, the morphology of those melanocytes showed no difference compared with normal melanocytes.

As a result of the above findings, we discussed and summarized the PMH's clinical diagnostic criteria.