Aspects of placental estrogen synthesis in the pig.

Conceptus estrogen synthesis in the pig begins with the d 11 blastocyst and continues throughout pregnancy. Estrogens have been implicated as regulators of numerous in utero events related to conceptus survival and development. Studies conducted in our laboratory indicate that progesterone (P4) production by the porcine placenta increases steadily throughout gestation. Estrone (E1) production is triphasic with peaks between d 14-18, around d 30, and a sustained increase from d 70 until parturition. Addition of pregnenolone (P5) augments in vitro P4 and E1 production by both the placenta and the endometrium. Both estrogen concentrations and the ratio between conjugated and free estrogens change drastically during gestation. Evidence from our laboratory suggests that enhanced and prolonged sulfatase activity by d 90 of gestation, coupled with an increase in sulfatase activity-not a change in aromatase activity-contributes to the rise in free estrogens as parturition approaches. We assessed the relative importance of ovarian versus placental production of P4 as a mediator of prenatal survival, conceptus development, and steroidogenesis by the placenta and endometrium. Among other findings, exogenous administration of the non-aromatizable progestagen medroxyprogesterone acetate (MPA) to ovariectomized (OVX) gilts between either d 20-30 or d 60-70 of gestation did not adversely affect concepts survival or development, in vivo estrogen measurements, or in vitro placental steroidogenesis. We also demonstrated that pregnancy may also be maintained during d 20-30 and d 60-70 in OVX gilts administered large quantities of P5 exogenously, presumably due to placental P4 production. Recent studies have examined variables of region of the placenta and intrauterine position on conceptus development, and placental and endometrial steroidogenesis under normal and crowded intrauterine conditions. Results indicated 1) a differential release of P4 and E1 by different regions of the placenta at certain days of gestation, 2) no compensatory increase in steroidogenic activity of the pig placenta when total placental mass was reduced, and 3) a less pronounced effect of intrauterine position on steroidogenic activity in the pig compared with other litter-bearing species.