Yolk sac tumor only occurs as a pure tumor in young males (at about 3 years of age). In adults, yolk sac tumors are pa of mixed germ cell tumors. One characteristic feature of yolk sac tumor is Schiller-Duval bodies, which resemble endodermal sinuses. Schiller-Duval bodies have a capillary at the core and are surrounded by a visceral and a parietal layer (i.e., resemble primitive glomerulus). Ref: Wyatt C., Butterwoh IV J.F., Moos P.J., Mackey D.C., Brown T.G. (2008). Chapter 17. Pathology of the Male and Female Reproductive Tract and Breast. In C. Wyatt, J.F. Butterwoh IV, P.J. Moos, D.C. Mackey, T.G. Brown (Eds), Pathology: The Big Picture.