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Summary: The major source of PGE2 in ovine pregnancy is the placenta, with secretion occurring bidirectionally into fetal and maternal circulations. The placental output of PGE2 appears to increase when demand on placental function is increased, suggesting that the normally observed increase in its concentration towards term is driven by the growing demands of the fetus. The fetal pituitary is also involved in the control of PGE2 synthesis. PGE2 has potent stimulatory actions on the fetal pituitary to increase both the absolute concentration and the bioactive fraction of ACTH-containing peptides in the fetal circulation. It also directly stimulates glucocorticoid secretion from the fetal adrenal gland. We propose that PGE2 provides a tonic stimulation of the fetal HPA axis in late gestation, contributing to phenomena such as the apparent insensitivity of the pituitary to Cortisol feedback and the increasing sensitivity of the fetal adrenal. Because of its apparent responsiveness to placental workload, it may transduce stimuli from the placenta and transmit them to the fetal HPA axis, giving a possible biochemical basis to the empirically observed correlation between fetal metabolic demand and gestation length.

To determine whether the adrenal gland of the bandicoot is actively producing steroid hormones at birth, the cellular structure was examined by electron microscopy, and the concentration of cortisol in the gland was determined by radioimmunoassay. Two distinct cell populations were seen in the adrenal gland. One group of cells possessed dark staining granules, normally observed in catecholamine-secreting cells, and the second group contained large amounts of smooth endoplasmic reticulum and mitochondria with tubulo-vesicular cristae. Both features are characteristic of steroid-secreting cells. A concentration of 0.094 ng of cortisol per adrenal was determined by pooling glands from nine individual newborn bandicoots. This evidence of cortisol secretion from the bandicoot adrenal at birth indicates that, as in many eutherians, the marsupial adrenal may play an important role in determining the length of gestation.