and Cellular Localization of Ovine Placental Lactogen Messenger Ribonucleic Acid Expression during Mid- and Late Gestation*

Ovine placental lactogen (oPL) is structurally similar to PRL, is a product of the chorionic epithelium, and has been implicated in playing a supportive role in fetal growth. This study examined the concentration and cellular location of oPL mRNA at five stages of pregnancy (days 60,90, 105, 120, and 135) in 21 cross-bred ewes, and results were compared to maternal and fetal serum oPL concentrations, cotyledonary DNA and actin mRNA concentrations, and total fetal weight. The concentration of oPL mRNA in fetal cotyledonary tissue increased (P 5 0.05) from day 60 (15.4 pg/pg total cellular RNA) to day 120 (73.7 pg/pg total cellular RNA) of gestation and then plateaued, whereas no significant changes occurred in the concentration of actin mRNA over the gestational ages examined. The concentration of DNA in cotyledonary tissue (micrograms per mg wet tissue) increased (P 5 0.05) from days 60 through 120 and remained constant through day 135, such that when oPL mRNA was expressed on a picogram per rg DNA basis, no stage of gestation effect (P 2 0.10) was observed. The maternal serum oPL concentration increased (P s 0.05) from day 60 (7.1 rig/ml) to day 105 (417.7 rig/ml), followed by a large but nonsignificant (P 2 0.10) increase in maternal serum oPL occurring on day 135 (902.0 rig/ml). Fetal serum oPL concentrations increased (P 5 0.05) from day 60 (11.0 rig/ml) to day 90 (29.0 rig/ml) and then remained relatively constant. Maternal serum oPL (r = 0.68; P 5 0.01) and cotyledonary oPL mRNA levels (r = 0.61; P 5 0.05) were correlated with total fetal weight when adjusted for fetal number and gestational age, and together accounted for 80.6% (r* value) of the variation found in total fetal weight. The correlation between fetal serum oPL concentrations and total fetal weight was nonsignificant (P 5 0.10). Examination of placentome cross-sections by immunocytochemistry and in situ hybridization at the five gestational ages indicated that the chorionic binucleate cell was the sole source of oPL. These data provide evidence that, like maternal serum concentrations of oPL, oPL mRNA expression by chorionic binucleate cells increases until late gestation, whereas fetal serum concentrations of oPL plateau during midgestation. The secretion of oPL into maternal and fetal circulations appears to be regulated separately, and secretion into the fetal circulation may result from a population of binucleate cells that do not migrate into the maternal-fetal interface of the placentome. However, transcriptional control of the oPL gene within binucleate cells has yet to be defined. (Endocrinology 131: 2829-2838, 1992)