Adamantinoma of bone. An electron microscopic and immunohistochemical study

Adamantinoma of bone is a rare tumor, and fine structural analysis has been done in only a few cases. We report four cases studied by electron microscopy and immunohistochemical methods. Ultrastructural evaluation revealed a characteristic constellation of features, including intracellular bundles of type I microfilaments, moderate numbers of evenly dispersed mitochondria, scattered profiles of rough endoplasmic reticulum, occasional Golgi bodies and lysosomes, and scattered glycogen particles. Microvillous processes and desmosomes were identified in all tumors. Well-formed basement membranes enveloped cell clusters but did not surround individual cells. Intercellular basement membrane-like material also was found focally in pools. Ultrastructural features of endothelial differentiation, including Weibel-Palade bodies, micropinocytotic vesicles, and tight junctions, were not identified. Immunoperoxidase stains for coagulation factor VIII (von Willebrand factor) and blood group antigens were negative, whereas similar stains for keratin were positive. Our findings strongly suggest that adamantinoma is a neoplasm expressing definite epithelial, rather than endothelial, characteristics.