Many mitotic figures are present. Anaplastic and solid undifferentiated areas may present as sheets of cells with large, hyperchromatic nuclei, prominent nucleoli and poorly-defines cell borders. Poorly differentiated area in embryonal carcinoma. When large numbers of the embryo-like structures are present the tumor is referred to as a polyembryoma. These structures contain a disc and two cavities surrounded by loose mesenchyme simulating a 2 week embryo.
Embryonal carcinoma metastasizes early and widely via both lymphatic and hematogenous routes.
Radiation is not as effective as with seminoma, but newer chemotherapeutic agents have greatly improved prognosis. Unlike seminoma, however, many embryonal carcinomas are high stage tumors at the time of presentation. It is noted for its resemblance to rat fetal yolk sac and the presence of microscopically distinctive structures known as Schiller-Duval bodies. Pure YST of the adult testis is rare. In this form, it presents as a fairly rapid testicular enlargement.