Biology of testicular tumors.

Testicular tumors arise from the germ cell line and therefore exhibit characteristics of both neoplastic and normal growth and differentiation. Experimental model systems of animal and human tumors have been reviewed with emphasis on the biologic characteristics of these tumors. The embryonal carcinoma cell is the totipotential stem cell that resembles normal germ cells in many ways and is capable of differentiating along the somatic pathways to form endoderm, mesoderm and ectoderm cell types (teratoma) or along extraembryonic pathways to form trophoblast (choriocarcinoma) or yolk sac (endodermal sinus tumor). Markers of the extraembryonic cell types have been defined, and the cell surface characteristics of embryonal carcinoma cells are being intensively studied. Clarification of the biology of testicular tumors will provide the basis for future rational therapy.