Progesterone synthesis by human placental syncytiotrophoblast in vitro-preferred precursor and effect of human chorionic gonadotropin

Freshly prepared human placental syncytiotrophoblast (ST) initially secreted appreciable amounts of progesterone in vitro. After 6 days of culture progesterone secretion by ST declined to low levels (2–4 ng per 72 h). Addition of pregnenolone, but not of cholesterol, in the culture medium at this stage resulted in up to 62 fold increase of progesterone secretion by ST. Exogenously added human chorionic gonadotropin (HCG) failed to increase progesterone production by ST, whether in the absence or presence of precursor pregnenolone. Antisera against HCG also did not affect progesterone production by ST.