Numerous classification systems have been proposed for germ cell tumors of the testis. Classification by histologic type proves to be the most useful with respect to treatment. The 2 major divisions are seminoma and nonseminomatous germ cell tumors (NSGCT), which include embryonal, teratoma, choriocarcinoma, and mixed tumors.

Tumorigenic Hypothesis for Germ Cell Tumor Development

During embryonal development, the totipotential germ cells can travel down normal differentiation pathways and become spermatocytes. However, if these totipotential germ cells travel down abnormal developmental pathways, seminoma or embryonal carcinomas (totipotential tumor cells) develop. If the embryonal cells undergo further differentiation along intraembryonic pathways, teratoma will result.

If the embryonal cells undergo further differentiation along extraembryonic pathways, either choriocarcinoma or yolk sac tumors are formed (

Figure 23-1). This model helps to explain why specific histologic patterns of testicular tumors produce certain tumor markers. Note that yolk sac tumors produce alpha-fetoprotein (AFP) just as the yolk sac produces AFP in normal development. Likewise, choriocarcinoma produces human chorionic gonadotropin (hCG) just as the normal placenta produces hCG.