Secretion of human chorionic gonadotropin in early pregnancy.

Remarkable secretion from the syncytial trophoblast of chorionic villi was observed in early pregnancy. Histology and histochemistry revealed many cytoplasmic protrusions containing numerous secretory granules, which were of basophilic glycoprotein in nature. Also, immunohistochemistry proved that these were human chorionic gonadotropin (hCG). Electron microscopy clarified the overall secretory process, which started with the production of small granules in the Golgi complex; then, the granules migrated toward the cell surface, increasing their size by fusing with each other, and gathered in cytoplasmic protrusions, then liquefied to be liberated by a mode of exocytosis into the maternal bloodstream (intervillous space). This remarkable hCG secretion, morphologically represented by the many cytoplasmic protrusions containing numerous secretory granules, arose synchronously on the surface of almost all chorionic villi. This phenomenon was limited to the eighth and ninth weeks of gestation, concurring with the maximal hCG concentration of maternal blood serum during pregnancy.