What then is the placenta? The wide variety of morphological forms seen among mammals makes the organ hard to define, but the comparative placentologist Harland Mossman captured the essence by stating ‘The normal mammalian placenta is an apposition or fusion of the fetal membranes to the uterine mucosa for physiological exchange’. Hence, the placenta is often accorded ritual burial, for in some beliefs the soul must be reunited with its placenta before being able to pass through to the afterlife. This dependency is reflected in the way that various societal groups consider the placenta as a twin or guardian angel, and venerate it as a sacred object. Such complications may represent a trade-off against the provision for a large fetal brain.įor the nine months of its intrauterine existence, the human fetus is totally reliant on the placenta, a transient extracorporeal organ that interfaces with the mother, to sustain and protect it. These challenges, along with the immune interactions involved in maternal arterial remodelling, may explain complications of pregnancy that are almost unique to the human, including pre-eclampsia. Bipedalism has posed unique haemodynamic challenges to the placental circulation, as pressure applied to the vena cava by the pregnant uterus may compromise venous return to the heart. Furthermore, placental hormones have profound effects on maternal metabolism, initially building up her energy reserves and then releasing these to support fetal growth in later pregnancy and lactation post-natally. In addition, it adopts other strategies that are key to facilitating transfer, including remodelling of the maternal uterine arteries that supply the placenta to ensure optimal perfusion. The placenta is structurally adapted to achieve this, possessing a large surface area for exchange and a thin interhaemal membrane separating the maternal and fetal circulations. Its principal function is to supply the fetus, and in particular, the fetal brain, with oxygen and nutrients. During its transient existence, it performs actions that are later taken on by diverse separate organs, including the lungs, liver, gut, kidneys and endocrine glands. The placenta is arguably the most important organ of the body, but paradoxically the most poorly understood.
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