Yolk sac differentiation in germ cell tumors. A morphologic study of 50 cases with emphasis on hepatic, enteric, and parietal yolk sac features.

We assessed 50 germ cell tumors with areas of yolk sac tumor (YST) for a variety of features including histologic patterns; hyaline droplets; syncytiotrophoblastic elements; hepatic, enteric, and parietal yolk sac differentiation; and granulomatous reaction. Of prime interest was the fact that many YSTs formed hepatic-like foci (22%), enteric-like glands (34%), and parietal yolk sac structures (92%). Hepatoid areas were characterized by nests and cords of polygonal, acidophilic cells with prominent nucleoli and intense cytoplasmic staining for alpha-fetoprotein. Enteric differentiation occurred as well-defined glands with a sharp, striated border and relatively bland nuclear features. Ultrastructurally these glands had apical microvilli with associated glycocalyx and long anchoring rootlets. The apical cytoplasm and luminal contents stained for carcinoembryonic antigen. Parietal yolk sac differentiation was characterized by the intercellular accumulation of basement membrane substance as generally thick and longitudinally arranged bands of eosinophilic material. Such material, by electron microscopy, was both intra- and extracellular, and had irregular outlines and inhomogeneous electron density. It contrasted with the strictly intracellular, round, homogeneous, hyaline globules that, we believe, represent visceral yolk sac differentiation. This intercellular material stained positively for laminin, a basement membrane component. Assessment of 22 embryonal carcinomas and 24 germinomas failed to show hepatic, enteric, and parietal yolk sac features, with one possible exception. We believe these features, especially parietal yolk sac differentiation, are helpful in differentiating YSTs from embryonal carcinomas and germinomas.