The ultrastructure of gallbladder carcinoma.

Nine malignant epithelial tumors of the gallbladder were examined electron microscopically. The tumors included the following subtypes: well-differentiated adenocarcinoma, signet ring cell carcinoma, adenosquamous carcinoma, and oat cell carcinoma. Well-differentiated adenocarcinomas recapitulated the ultrastructure of the normal surface epithelium of the gallbladder. Although columnar cells predominated, the narrow and dark-staining pencil-like cells were also present. The former appeared immature and secreted more mucin than normal columnar cells. Microvilli with long dense core rootlets were seen in one tumor. The most characteristic ultrastructural features of the signet ring cell carcinoma were the multiple mucin vacuoles and the intracytoplasmic lumina some of which appeared cystically dilated. Adenosquamous carcinomas were composed of four different cell types. Well-differentiated adenocarcinomas and adenosquamous carcinomas showed in addition a few endocrine cells similar to these seen in the human gallbladder mucosa in association with intestinal metaplasia. Oat cell carcinomas contained neurosecretory granules but also exhibited glandular markers. Electron microscopy seems the most reliable method for the identification of oat cell carcinomas.