[Histopathology of testicular germ cell tumors]

The most common type of testicular tumor is the germ cell tumor, which shows peculiar histological and biological features. The histopathology of germ cell tumors of the testis is illustrated here according to the WHO classification except for extremely rare polyembryoma and teratoma with malignant transformation. The tumors are divided roughly into 2 groups, one histological type including seminoma, spermatocytic seminoma, embryonal carcinoma, yolk sac tumor, choriocarcinoma and teratoma, and more than one histological type including many possible combinations of one histological type. Seminoma and spermatocytic seminoma show some similar features to the germ cell line, while embryonal carcinoma, yolk sac tumor, choriocarcinoma, and teratoma show differentiation toward a variety of structures appearing at any stage of embryogenesis instead of the original testicular tissue. The histology of the metastatic disease may or may not be the same as that of the primary lesion. The reasons for the occurrence of histological differences between primary and metastatic tumors is discussed. Two major tumor markers of the germ cell tumor, HCG and AFP, are analyzed using immunohistochemical procedure, and the significance of immunostaining for these markers in clinicopathological study is stressed.