Figure 5
Representative depictions of embryoid bodies (a–c, e, f) and the early embryo whose structure they recapitulate (d). Well-formed embryoid bodies were present in both testicular cases reported here (a, b). An embryoid body may be a component of metastasis, as seen in a case of a testicular germ cell tumor metastatic to lymph nodes (c). In addition to gonadal locations, polyembryomas and the embryoid body that composes this neoplasm may be seen at extragonadal sites such as the posterior mediastinum (e). These illustrations all demonstrate the distinct components of the embryoid body, which are defined by a well-formed hyperchromatic germ disc (GD) with underlying less conspicuous delicate yolk-sac epithelium (YSE) and distinct embryologic cavities. The space superior to the germ disc recapitulates the amniotic cavity (AC), while the cavity beneath the yolk-sac epithelium recapitulates the yolk-sac cavity (YSC). By immunohistochemistry, the embryoid body demonstrates expression of alpha-fetoprotein (f) in the yolk-sac epithelium with only artifactual faint staining in the germ disc. A in d indicates amniotic cavity and YS, yolk sac cavity.
