The first body part of the biweekly series
Some cultures choose to bury it. Others eat it. Others turn it into skin creams. Often, it goes straight into a biohazard waste bag. Despite being our very first organ—one with a vast range of duties, one that brings two people’s bloodstreams so close but never in contact—the placenta is unfamiliar to many, and often incompletely understood.
The placenta forms during pregnancy, attached to the inside of the uterus. The fetus does not breathe through its lungs or eat through its mouth. A twisted white cord, containing three vessels, connects the fetus to placenta. One vein in the umbilical cord carries oxygenated blood and nutrients to the fetus, and two arteries send blood out to the placenta to pick up what’s needed and deliver it back. We only have a placenta for the first nine months of our lives, but during these months, it has numerous functions. It acts as lungs to bring the fetus oxygen and it acts as a digestive system to deliver nutrients. It takes on the role of the liver to detoxify any harmful elements that may be present in the mother’s blood, but allows important things, like antibodies, to pass. It acts as an endocrine organ to secrete many hormones; one reduces maternal uptake of glucose so more is reserved for the fetus, another maintains the required uterine environment for the fetus.
All of these functions occur without the maternal and fetal blood ever touching. The placenta is made up entirely of fetal cells that form branch-like structures. The outer layer of these branches is composed of a continuous layer of cells, called a syncytium. The branches, called chorionic villi, bring oxygen- and nutrient-deprived fetal blood into a bath of maternal blood, where an exchange occurs between mother and fetus. The fetal blood will pick up oxygen, glucose, proteins, and fatty acids from the maternal side, and this rich blood returns to the developing fetus through the umbilical cord. When the oxygen demands of the fetus increase, at the tips of the villi, the nuclei of the syncytium are pushed away so that a layer thinner than a cell separates maternal and fetal blood and provides optimal gas exchange.
The demands of the growing fetus call for about 600ml of blood to enter the placenta every minute. To allow for such high volumes of blood to enter the space between the villi (intervillous space), changes must occur in the maternal vasculature. In 1774, anatomist William Hunter produced the first image of spiral arteries, the vessels that supply blood to the uterus. During pregnancy, extravillous trophoblasts—fetal cells which have moved beyond the trophoblasts that make up the villous structures of the placenta—invade into the maternal spiral arteries. They replace the maternal cells lining the vessels, remodelling the spiral arteries into high capacitance and low resistance structures, to achieve the high volume of blood flow that the fetus demands. Only in the beginning of the 1900s were these extravillous trophoblasts that invade the maternal tissue confirmed to be of fetal origin.
When the remodelling of the spiral arteries does not occur properly, the diameter of the arteries is far smaller than what it should be. Consequently, the fetus does not receive enough oxygen and nutrients from the mother, resulting in neurological and developmental issues, and growth restriction. The placenta, starved for oxygen and nutrients, will begin releasing various factors into maternal blood, in an attempt to get more blood delivered to the intervillous space of the placenta. The released factors cause endothelial damage in the mother, most notably resulting in an increase in blood pressure, as well as kidney failure; when severe, this disorder of pregnancy is termed preeclampsia, and affects about five percent of pregnancies. The only cure presently known is delivery of the baby and placenta.
Though often called the afterbirth, it is before birth that the placenta feeds, protects, breathes for, and connects to the fetus. It is before birth that this tree-like structure, its name meaning simply “flat pancake,” continues to mystify researchers. Though you will never yourself form another placenta, perhaps recall those extravillous trophoblasts that ensured your healthy arrival into the world, and the tree that was once your neighbour.
Stay tuned for the next featured organ: (to be confirmed)
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