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Ten trophoblastic tumors, including seven classical choriocarcinomas, two choriocarcinomas with atypical histology, and one placental-site trophoblastic tumor (PSTT), were studied to compare their fine structural features. Ultrastructurally, the classical choriocarcinomas showed well-defined cytotrophoblasts and syncytiotrophoblasts. The cytotrophoblasts were primitive epithelial cells, while the syncytiotrophoblasts were complex cells with multiple nuclei and dense cytoplasm containing dilated endoplasmic reticulum, lysosomes, vesicles, and tonofilaments. The syncytiotrophoblast cell membranes often contained numerous microvilli. In the choriocarcinomas, scattered intermediate trophoblasts showed features transitional between the cytotrophoblasts and the syncytiotrophoblasts, with moderately complex cytoplasm containing some of the organelles found in the syncytiotrophoblasts. Histologically, the atypical choriocarcinomas showed a predominance of mononucleate and binucleate cells and indistinct syncytiotrophoblasts. Ultrastructurally, these atypical tumors were composed largely of intermediate trophoblasts, yet contained scattered syncytiotrophoblasts with microvilli in compressed aggregates. The PSTT was composed primarily of intermediate trophoblasts that contained prominent paranuclear filaments not seen in the intermediate trophoblasts of the choriocarcinomas. Rare cells resembling syncytiotrophoblasts were found in the PSTT, but no cytotrophoblasts were observed. Immunoreactivity for human chorionic gonadotropin and human placental lactogen was found in the intermediate trophoblasts and syncytiotrophoblasts of both the choriocarcinomas and the PSTT, demonstrating functional homology between these tumors despite some ultrastructural differences. These results demonstrate ultrastructural features of trophoblastic cells that correlate with the morphologic diversity seen in these tumors by light microscopy. Furthermore, the comparisons suggest that the PSTT is composed of a distinct form of intermediate trophoblast that appears to reflect its origin from the extravillous trophoblast.

A carcinoma of the urinary bladder with a distinct choriocarcinomatous component that developed in a postmenopausal woman was associated with high titers of circulating chorionic gonadotropin. The diagnosis was supported by histochemical demonstration of human chorionic gonadotropin in syncytial tumor giant cells and electron microscopic features consistent with syncytiotrophoblastic cell differentiation.