Has the mechanism by which the endocrine placenta scavenges the mother whilst sparing the foetus been unmasked?
Throughout pregnancy, the placenta controls the metabolism of the mother for the benefit of the foetus, sometimes to such an extent that in humans the mother's health is compromised. As there are no neural connections, the main means by which the placenta controls the mother's metabolism must be endocrine in nature. Unfortunately the foetal blood that perfuses the placental/uterine boundary cells to carry oxygen and nutrients back to the foetus will also collect the very hormones the placenta is producing in the boundary cells in amounts sufficient to diffuse into the maternal circulation and reach concentrations that can affect the mother's metabolism. Despite this the foetus seems to be able to develop on its own with its physiological systems maturing naturally, without any significant interference from these placental endocrine products. Whilst we understand much of the complexity of the peripheral endocrine organs and systems in the non-pregnant state, we know little of the endocrinology of the placenta; in fact, despite the sophisticated level of technology available today most of the workings of the placenta remain an enigma.
The advantage of a placenta is amply demonstrated in comparing the birth of a wildebeest with that of the kangaroo, in the former a fully mature calf is born whereas in the latter the primitive embryo makes a hazardous journey, crawling up through the mother's fur and eventually reaching the pouch, where it latches onto a teat.
The processing of biologically active peptide hormones from inactive precursors evolved in single-cell eukaryotes resulting in endocrine tissues in higher animals storing the final active products in granules ready for secretion. Recent work in my laboratory would indicate that the placenta has developed a different mechanism for processing peptide hormone precursors which protects the foetal endocrine systems whilst being able to control the mothers metabolism.