Several hypotheses have been proposed to explain the occurrence of GCT in extragonadal sites. During the migration of the germ cells from the yolk sac wall to the hindgut and to the gonadal ridge, some cells may be left behind or stray from the normal path and come to rest at various midline sites in the embryo. Viable cells may later undergo a transformation and give rise to tumors in those locations. Alternatively, GCT may arise from pluripotent embryonal cells that failed to develop along normal differentiation pathways and undergo cancerous transformation.

Histologically, GCT can have benign and malignant elements. Although their locations may vary, their histology remains the same. They are classified based on the malignant potential of the cancer cells as well as the stage at which this development occurs. Multipotential germ cells may differentiate into unipotential primitive germ cells from which the ovaries and testes develop. Germinomas are a cancerous development of the unipotential primitive germs cells. Seminomas refer to
GCT of the testes, whereas similar tumors of the ovary are called dysgerminomas. The multipotential germ cell also may undergo embryonal differentiation. Tumors arising from this early stage are called embryonal carcinoma. Further differentiation of the multipotential germ cells gives rise to embryonal and extraembryonal structures. Tumors from the former are called teratomas and may be mature, immature, or malignant. Extraembryonal structures include the yolk sac and placenta. Tumors arising from the cells forming those structures are called yolk sac tumors or endodermal sinus tumors, and choriocarcinomas, respectively (Fig. 313.1).

Histologic variations exist between and within GCT. Because treatment is determined by the most malignant component, a meticulous histologic assessment of the whole tumor is necessary. Germinomas are composed entirely of malignant germ cell elements. They are the most common pure GCT of the ovary and central nervous system. Embryonal carcinoma rarely occurs as a single histologic subtype and is seen much more commonly as a component of a mixed GCT. Embryonal carcinoma is characterized histologically by anaplasia, necrosis, and frequent mitoses; these tumors are highly malignant. Teratomas contain elements from at least two of the three germ cell layers: endoderm, mesoderm, and ectoderm. In a mature teratoma, elements are fully differentiated, and the tumor may contain teeth, bone, hair, and skin. Immature teratomas contain elements more reminiscent of fetal or embryonal structures. Malignant teratomas most commonly contain yolk sac tumor elements but also may have elements of neuroblastoma or medulloepithelioma. Yolk sac tumors are the most common malignant GCT of the young child and the most common type of malignant GCT of the testes of infant and young boys. The most common histologic patterns of yolk sac tumor contain Schiller-Duval bodies. Choriocarcinoma microscopically resembles the chorion layer of the placenta, with multinucleated syncytiotrophoblasts and cytotrophoblasts. It may arise from the nongestational pluripotent germ cell or from the gestational placenta in a pregnant woman.