Placental histology and the production of human choriogonadotrophin and its subunits in pregnancy

The relation between placental histology and the levels of human chorionic gonadotrophin (hCG) and its alpha-hCG and beta-hCG subunits was studied using monoclonal antibodies in immunohistochemical and immunoassay procedures. Between 4 and 9 weeks gestation an increasing ratio of cytotrophoblasts to syncytiotrophoblasts in placental villi corresponded to rising hCG levels, with beta hCG levels declining from 4% to less than 1% of hCG levels. After 8-10 weeks gestation, declining numbers of cytotrophoblasts in villi and the formation of the definitive placenta coincided with falling hCG, rising alpha hCG and insignificant beta hCG levels. Between 18 and 35 weeks, a secondary twofold rise in hCG levels coincided with a similar increase in average placental mass. It is proposed that hCG synthesis in syncytiotrophoblasts depends on the rate of differentiation of cytotrophoblasts into syncytiotrophoblasts.