Abstract
Fetal (cotyledon) placental tissue was collected from 7 pregnant cows and from 9 cows immediately after delivery of the fetus during parturition induced by luteal removal at 250 or 270 days of gestation. These tissues were incubated with [3H]androstenedione (A), [3H]17.alpha.-hydroxyprogesterone (17.alpha.HP) or [3H]-pregnenolone (Ps) to examine potential estrogen precursors in the mid to late pregnant cow. Percentage conversion to unconjugated and conjugated estrone (E1) and estradiol (E2) was measured for each tissue with the various substrates. Unconjugated estrogens predominated over conjugated (P < 0.001) and E1 over E2 (P < 0.001) in all incubations. There was no difference in conversion of A between pre- and post-partum fetal tissues. There was a significant elevation in conversion of Ps in postpartum fetal tissue (P < 0.05). Placental aromatase activity does not appear to increase as pregnancy progresses towards parturition; the bovine placenta is able to convert C21 steroids to estrogens and induction of enzymes for converting pregnenolone or related substrates to estrogens may be involved in the prepartum rise in estrogen levels.