Yolk sac tumor of the stomach with an adenocarcinomatous component: a case report with immunohistochemical analysis.

A 56-year-old male treated for a gastric yolk sac tumor with an adenocarcinomatous component is described. A mixed area of reticular and glandular neoplastic components was morphologically identified in this tumor. Immunohistochemically, the yolk sac tumor expressed alpha-fetoprotein (AFP), placental alkaline phosphatase (PLAP), and cytokeratin, but was negative for carcinoembryonic antigen (CEA). The adenocarcinoma was positive for CEA and cytokeratin, partially positive for PLAP, and negative for AFP. In the mixed area, the tumor cells were positive for cytokeratin, weakly expressed AFP and PLAP, and sporadically stained for CEA both in the reticular and glandular components. This area was identified as a transitional area of the yolk sac tumor and adenocarcinoma. These findings demonstrate that the yolk sac and adenocarcinomatous components are closely related. It also suggests that the tumor arose from multipotential neoplastic mucosal epithelial cells with both yolk sac and gastric mucosal phenotypes.