By Margaret Dedloff
My favorite organ is one that’s often forgotten: the placenta. The placenta is a specialized organ that forms in the uterus during pregnancy. The placenta is a fetal organ, composed of fetal cells containing fetal DNA. In humans, the placenta begins to form around seven to ten days after conception, or when the egg becomes fertilized with sperm. The placenta attaches to the wall of the uterus and continues to form slowly throughout the first three months of pregnancy, or the first trimester. After the first trimester, the placenta can begin doing some of its important functions necessary for the growing fetus.
So what does the placenta do? And why is it so cool? The placenta has many important roles during pregnancy. One role is the transfer of nutrients from the mother to the fetus. While the maternal and fetal blood never mix, the mom’s blood flows over special cells, called syncytiotrophoblasts, that line the placenta (Figure 1). This allows for nutrients and oxygen from mom’s blood to diffuse into the syncytiotrophoblasts and then into the fetal bloodstream. The same process allows for fetal waste and carbon dioxide to get removed!
Figure 1. Diagram showing the placenta. Syncytiotrophoblasts form the syncytiotrophoblast layer. Maternal blood flows over the syncytiotrophoblast layer, which allows for nutrients to diffuse into the fetal cells and then into fetal blood. Image made by author in BioRender.
Another important role for the placenta is protecting the fetus from maternal infections. The placenta acts as a physical barrier to block pathogens from infecting the fetus. Additionally, the placenta has specialized immune mechanisms to protect the fetus. These include specialized cell types, like Hofbauer macrophages, which are a special type of cell that essentially “eat” viruses and bacteria. The placenta also produces special signaling molecules, called cytokines, that help to protect it and the fetus from viruses and bacteria.
The placenta also produces important hormones that help to maintain a healthy pregnancy. These hormones include estrogen and progesterone. The placenta also produces human chorionic gonadotropin (hCG), a hormone that is only produced during pregnancy. In fact, at home pregnancy tests measure the presence of hCG to determine pregnancy! These three hormones are all critical for a healthy pregnancy.
Placentas are especially cool because they are different in different animals. Placentas can be classified based on their shape and where the placenta and maternal tissue touch. There are four kinds of placenta shapes (Figure 2 top). The first is a diffuse placenta, which has villi (where the fetal blood supply is) all over the sack the fetus is in. Horses and whales have diffuse placentas. The second is cotyledonary placenta, which is essentially a bunch of little placentas instead of one big placenta. Sheep and cows have this type of placenta. Next, the discoid placenta, which is what humans have, is what I typically think of as a placenta. It is just one big, disc-like organ. The final shape of placenta is the zonary placenta, which is a band of placental tissue that encircles most or all of the fetus. Dogs and seals have this kind of placenta.
Placentas can also differ based on how many layers of tissue separate maternal and fetal blood (Figure 2 bottom). Epitheliochorial placentas have the fetal blood supply separated from the maternal blood supply by fetal connective tissue, fetal epithelial cells, maternal epithelial cells, and maternal connective tissue. Endotheliochorial placentas have the fetal blood supply separated from the maternal blood supply by fetal connective tissue and fetal epithelial cells. Hemochorial placentas have the fetal blood supply separated from the maternal blood supply by fetal epithelial cells. Humans have hemochorial placentas.
Figure 2. Placental classification. There are four different shapes of placenta (top). There are three different amounts of layers between the maternal and fetal blood supply (bottom). Image made by author in BioRender.
The placenta is the coolest organ, with so many important functions essential for growing life, but it’s often forgotten! There are so many different kinds of placentas, but despite their differences, they are all critical for healthy pregnancies. Finally, there is still so much unknown about the placenta, and so many areas of study left to explore.
