Ultrastuctural localization of HCG and subunits in the human placenta and choriocarcinoma cell lines-morphological approach to secretory pathway.

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Abstract
The aim of this study is to examine secretory mechanisms of the human chorionic gonadotropin (HCG) with immunocytochemical staining techniques. In 10 weeks and term placentae, HCG and its subunits were localized in the perinuclear space (PNS) and rough endoplasmic reticulata (RER) of syncytial trophoblasts, but microvilli, on the maternal cell surface were negative. In addition, 10 week old placenta demonstrated the presence of small vesicles and large lysosome-like granules in the cytoplasm facing the maternal circulations. They were stained by using anti-HCG .beta. subunit antibody. The large lysosome-like granules were predominantly demonstrated to contain free .beta. subunits. Term placentae did not show these large lysosome-like granules. These data supported the theory that HCG secretion mechanism is mediated by transportation via small vesicles in the first trimester. The lysosome-like granules may be the site for digestion of excess free subunit. In term placenta, HCG and its subunits were localized in PNS and RER indicating their actual production, in spite of low HCG .beta. level in the maternal blood. These PNS and RER were more prominent in the cytoplasm facing fetal circulation and suggested the possibility of their secretion toward fetal circulation.