Antikeratin antibodies in tumor diagnosis. Distinction between seminoma and embryonal carcinoma

The authors investigated the presence and distribution of keratin in germ cell tumors using a rabbit‐anti‐keratin antiserum and a monoclonal antikeratin antibody—which is specific for keratin classes of 40, 50, and 56.5 kdaltons—by various immunohistochemical methods on forzen sections, alcohol‐fixed, and formalin‐fixed paraffin‐embedded tissues. Thirty‐four germ cell tumors were studied. These were the following: 18 seminomas, 10 embryonal carcinomas, 2 teratocarcinomas, 3 yolk sac tumors and 1 choriocarcinoma. All seminomas, including four poorly differentiated (so‐called anaplastic seminomas), gave negative results, regardless of the method employed. Embryonal carcinoma, the epithelial component of the teratocarcinoma, the yolk sac tumors, and choriocarcinoma were at least focally positive for keratin. The monoclonal antibody provided a cleaner background and stronger staining than the rabbit‐anti‐total‐human‐epidermal‐keratin antibody. Best results were obtained from fresh‐frozen sections or alcohol‐fixed, paraffin‐embedded materials. Formalin‐fixed, nonseminomatous tumors, when predigested with trypsin and incubated overnight with primary antibody, gave no false‐negative results but staining was often focal. The authors' results agree with the reported absence of detectable keratin in primordial germ cells of the normal testis, and with prevailing concepts of the histogenesis of germ cell tumors. These results indicate that the presence or absence of keratin by immunocytochemical methods can be helpful in distinguishing seminoma from embryonal carcinoma.