Functional significance and cortisol dependence of the gross morphology of ovine placentomes during late gestation.

The gross morphological appearance of ovine placentomes is known to alter in response to adverse intrauterine conditions that increase fetal cortisol exposure. The direct effects of fetal cortisol on the placentome morphology, however, remain unknown, nor is the functional significance of the different placentome types clear. The present study investigated the gross morphology of ovine placentomes in relation to placental nutrient delivery to sheep fetuses during late gestation and after experimental manipulation of the fetal cortisol concentration. As fetal cortisol levels rose naturally toward term, a significant decrease was observed in the proportion of the D-type placentomes that had the hemophagous zone everted over the bulk of the placentomal tissue. When the prepartum cortisol surge was prevented by fetal adrenalectomy, there were proportionately more everted C- and D-type placentomes and fewer A-type placentomes with the hemophagous zone inverted into the placentome compared with those of intact fetuses at term. Raising cortisol concentrations by infusion before term reduced the incidence of D-type placentomes and lowered the proportion of individually tagged placentomes that became more everted during the 10- to 15-day period between tagging and delivery. Cortisol, therefore, appears to prevent hemophagous zone eversion in ovine placentomes during late gestation. The distribution of placentome types appeared to have no effect on the net rates of placental delivery of glucose and oxygen to the fetus under normal conditions. When fetal cortisol levels were raised by exogenous infusion, however, placental delivery of glucose, but not oxygen, to the fetus, measured as umbilical uptake, was reduced to a greater extent in fetuses with a higher proportion of C- and D-type placentomes. The gross morphology of the ovine placentomes is, therefore, determined, at least in part, by the fetal cortisol concentration and may influence placental nutrient transfer when fetal cortisol concentrations are high during late gestation. These findings have important implications for the placental control of fetal growth and development, particularly during adverse intrauterine conditions.