Primary malignant melanoma of the gallbladder in dysplastic naevus syndrome

A case of gallbladder involvement by malignant melanoma in a 57-year-old wo man is reported. The gallbladder, resected for cholelithiasis, harboured a pedunculated polypoid dark mass, which histologically revealed sheets and n ests of epithelioid cells with hyperchromatic nuclei in the lamina propria and at the junctional level. These cells were pigmented (with positive reac tion with Schmorl's stain and bleaching with peroxide) and showed immunohis tochemical positivity for S-100, gp 100 antigen (HMB-45 antibody) and vimen tin. The patient, affected by dysplastic naevus syndrome, had a melanoma in situ excised from the scalp 8 years earlier. The features of the investiga ted lesion address towards a diagnosis of primary gallbladder melanoma. Fur thermore, this is the first time that the existence of such a controversial entity is sustained by the ultrastructural investigation of melanosomes, d emonstrating the presence of two melanocitary populations, a typical one ex clusively junctional and an atypical one both at the junctional level and i n the lamina propria.