Ovine placental lactogen concentrations during premature delivery induced by infusion of dexamethasone or adrenocorticotrophin to the fetus

The effects of cortisol on the binucleate cell population in the ovine placenta during late gestation

Large granulated binucleate cells (BNCs) producing placental lactogen (PL) and pregnancy-associated glycoproteins (PAG) are present in all ruminant placentae throughout pregnancy. These BNC account for 15-20% of the cells in ovine trophectoderm for most of gestation but decrease in number close to term at the same time that fetal cortisol levels rise. The present study investigated the effects of cortisol on the BNC population using immunohistochemistry to count BNCs in ovine placentomes during late gestation and after experimental manipulation of the fetal cortisol level by fetal adrenalectomy and exogenous cortisol infusion. Abolition of the prepartum rise in fetal cortisol prevented the normal decline in BNC numbers towards term. Conversely, raising cortisol levels in immature fetuses to prepartum values prematurely reduced placental BNC numbers. However, a small population of BNC remained, even at the highest cortisol concentrations. When all the data were combined irrespective of treatment or gestational age, there was a significant inverse correlation between fetal plasma cortisol and the number of BNCs in the ovine placenta. These findings show that cortisol regulates the BNC population in ovine placenta during late gestation. They also have important implications for the production of PL and PAG during ovine pregnancy.

Comparison of changes in ovine plasma chorionic somatomammotropin concentrations in the fetus and mother before spontaneous vaginal delivery at term and adrenocorticotropin-induced premature delivery

Fetal and maternal plasma ovine chorionic somatomammotropin concentrations have been measured in eight pregnant ewes immediately prior to spontaneous vaginal delivery at 146 +/- 2 gestational days. In seven additional ewes, fetal and maternal plasma ovine chorionic somatomammotropin concentrations were measured immediately before and during induction of premature delivery by infusion of 1 microgram X hr-1 synthetic adrenocorticotropin (ACTH1-24) to the fetus commencing at either 120 to 122 gestational days (n = 3) or 130 to 132 gestational days (n = 4). Both fetal and maternal plasma ovine chorionic somatomammotropin concentrations fell significantly (p less than 0.05) in the 5 days before spontaneous term delivery; no change in the fetal to maternal ratio for plasma ovine chorionic somatomammotropin concentrations was observed. In contrast, the maternal plasma ovine chorionic somatomammotropin concentration rose (p less than 0.05) during infusion of ACTH1-24 to the fetus while the fetal concentration declined. During infusion of ACTH1-24 to the fetus the fetal to maternal ratio for plasma ovine chorionic somatomammotropin concentrations also fell. The finding that the parallel fall in ovine plasma ovine chorionic somatomammotropin concentrations in the fetus and mother in spontaneous term deliveries is not seen following ACTH-induced premature delivery suggests that mechanisms other than activation of the fetal adrenal axis induce the prepartum fall in ovine chorionic somatomammotropin secretion. These findings militate against a role for the fall in ovine chorionic somatomammotropin in the initiation of parturition.