Immunoelectron microscope observations on secretion of human placental lactogen (hPL) in the human chorionic villi.

Immunoelectron microscope localization of human placental lactogen (hPL) was investigated in the chorionic villi from week 7 to full-term gestation with the protein A-gold technique. With specific antiserum against hPL, immuno-reactive gold particles were found to be preferentially located in Golgi-derived, electron-dense small granules of 80-180 nm in the syncytiotrophoblast. Our electron micrographs indicate that these small granules increase in number in the course of gestation and are released by exocytosis from the apical cell surface. The present study reveals that hPL is segregated from the Golgi apparatus, stored in the syncytiotrophoblast as secretory granules, and released into the maternal blood.