Human chorionic gonadotropin in the thymus. An immunocytochemical study on discordant expression of subunits

To demonstrate discordant expression of human chorionic gonadotropin (hCG) subunits in the human thymus and thymic tumors, immunocytochemical studies were performed with immunoelectron microscopy. In the fetus, hCG beta-positive cells were identified in excess of hCG alpha-positive cells (hCG alpha: 1.38 +/- 0.18/mm2, hCG beta: 4.50 +/- 1.13/mm2). They were usually different populations in serial sections. There were 0.18 +/- 0.03/mm2 synchronously positive cells using the double immunostaining method, comprising approximately 3% of the total positive cells. The immunoreactive material for both subunits was present at the rough endoplasmic reticulum (RER), perinuclear space, and vesicular structures with protruding microvilli in the lumens. hCG alpha was expressed preferentially in the endocrine tumors of the thymus and non-germ-cell tumor, with rare positivity for beta-subunits of glycoprotein hormones. The hCG alpha-immunoreactive material also was present in the granules as well as in RER of the neoplastic cells. In four teratomas of the thymus, hCG alpha-positivity only was present in endocrinelike cells. The subunit profile of hCG was unbalanced in the thymus and isolated hCG alpha-expression in the fetal thymus may represent the endocrine cell with a primitive storage mechanism. The discordant expression of hCG subunits may be common in nontrophoblastic tissues.