Germ cell tumors of the testis often secrete enzymes or polypeptide hormones, examples of which include a fetoprotein (AFP) and human chorionic gonadotropin (hCG). AFP is synthesized by the fetal gut, liver, and yolk sac. It may be secreted by either yolk sac tumors (endodermal sinus tumors) or embryonal carcinomas. AFP may also be secreted by liver cell carcinomas. b-hCG is a glycoprotein that is normally synthesized by placental syncytiotrophoblasts. Markedly elevated serum levels are most often associated with choriocarcinomas, which are characterized histologically by a mixture of malignant cytotrophoblasts and syncytiotrophoblasts. Mildly elevated serum levels of b-hCG may be found in patients with other types of germ cell tumors if they contain syncytiotrophoblast-like giant cells. This is found in about 10% of classic seminomas, which are characterized histologically by large cells with distinct cell membranes and clear cytoplasm. An impoant, distinct variant of seminoma is the spermatocytic seminoma. It is characterized by being found in older individuals and by the fact that it does not metastasize. Histologically, a spermatocytic seminoma is characterized by maturation of the tumor cells, some of which histologically resemble secondary spermatocytes. To summarize: markedly elevated levels of hCG are associated with choriocarcinomas, while elevated levels of AFP are most characteristic of yolk sac tumors and embryonal carcinomas. But there are many areas of overlap between tumors, and many tumors are composed of multiple types of germ cell cancers. The only definitive statement that can be made is that elevated serum levels of AFP cannot be seen in a tumor that is a pure seminoma. Reference: Robbins & Cotran Pathologic Basis of Disease, 9edition