a-Fetoprotein (AFP) is a sialic acid glycoprotein with a molecular weight of 70kD that consists of 590aa residues. It has a single polypeptide chain with three repeating domains and it is structurally related to albumin. In normal tissue, AFP antibody intensely stains fetal liver, gastrointestinal tract, and yolk sac. The amount of AFP in fetal liver decreases with advancing gestational stage. In the abnormal tissue, AFP is expressed in hepatocellular carcinoma and gonadal and extragonadal germ cell tumors, including yolk sac tumors. AFP is considered a marker for hepatocellular histogenesis, and AFP immunostaining is found in 12% to 50% of hepatomas. The patchy staining pattern suggests that the AFP expression is distributed heterogeneously. AFP stains 55% to 100% of yolk sac tumors of various morphologic patterns. AFP staining is found in areas of yolk sac or endodermal sinus differentiation. These facts are helpful in distinguishing solid patterns of yolk sac tumors and seminoma since the latter lacks AFP immunoreactivity. AFP can be used in conjunction with PLAP since PLAP stains the majority of seminomas, showing immunoreactivity in up to 90% of the tumors. Nevertheless, weak or focal PLAP staining can be found in other germ cell elements of all histologic types such as yolk sac tumor, embryonal carcinoma, and choriocarcinoma. AFP together with hCG may help to identify poorly differentiated carcinoma of gonadal and germ cell differentiation. In one study, all 12 extragonadal undifferentiated tumors were either stained with AFP or hCG to indicate germ cell differentiation.
Intended for research use only. Not for use in human, therapeutic, or diagnostic applications.