Ultrastructural diagnosis of melanoma from an endoscopic biopsy

A 56-year-old male with abdominal symptoms and hypoalbuminemia was found to have multiple polypoid lesions on an upper gastrointestinal series, suggesting intestinal involvement by lymphoma. An endoscopic biopsy was interpreted by light microscopy as poorly differentiated adenocarcinoma. Study of the endoscopic biopsy by electron microscopy revealed epithelial features including intracytoplasmic acini, but the presence of premelanosomes in the tumor cells established the diagnosis of malignant melanoma.