Placental Estrogen Biosynthesis During Human Pregnancy1

Abstract

Overwhelming evidence has been accumulated which indicates that the placenta is the principal source of the greatly increased amounts of estrogen produced by pregnant women. This view received strong support from the elegant in vitro studies of Ryan, who first demonstrated net estrogen synthesis when testosterone or Δ4-androstenedione3 (1) and androst-5-ene-3β,16α,17β-triol (2) are incubated with placental preparations. Until recently, however, the exact sequence of the in vivo events which precede these final reactions has been unclear. The possible biochemical pathways and some of the important intermediates leading to the three classic estrogens, estrone, estradiol and estriol, are outlined in Fig. 1.