Placental yolk sac tumor with divergent endodermal differentiation.

We report an intraplacental germ cell tumor that was an incidental finding in the placenta of a 34-year-old woman. On pathologic examination, the 4.0-cm tumor was well circumscribed with a thin fibrous capsule. It exhibited three major histologic patterns within a mesenchyme-like myxoid background: a primitive endodermal component, a well-differentiated enteric glandular endodermal component, and a component of fetal hepatic tissue. The primitive endodermal component, the hepatic component, and many parts of the enteric endodermal glandular component were immunoreactive for alpha-fetoprotein. We considered the lesion to be a unique example of a placental yolk sac tumor and only the second reported case of a placental yolk sac tumor.