Development of the placenta

Definitions & Key takeaways

The placenta is a temporary organ that develops in the uterus during pregnancy to support the developing embryo or fetus. It connects the developing fetus to the mother's blood supply. It acts as a bridge between the mother and the embryo or fetus, allowing nutrients and oxygen to flow from mom to fetus and waste products to flow from the fetus back to the mother. The placenta forms a few weeks after the embryo implants in the wall of the uterus.

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The placenta is an organ that’s co-created by the fetus and the mother during development. The way it works is that deoxygenated fetal blood gets to the placenta through two umbilical arteries, and the blood picks up oxygen and glucose while dropping off carbon dioxide.
Oxygenated fetal blood, then heads back towards the heart through a large umbilical vein. That umbilical vein and the two umbilical arteries collectively form the vessels of the umbilical cord.
Development of these structures goes back to the first week of development, when the fetus is just a little ball of cells called a blastocyst.
Going forward, we’ll refer to this blastocyst as the fetus just to avoid changing names. By day 7 or 8, the fetus implants on the surface of the endometrial wall or decidua, and the area that it implants into is called the decidua basalis.
To snuggle deeper into the decidua basilis, trophoblast cells from the outer layer of the fetus assemble into two layers of cells called the cytotrophoblast and the syncytiotrophoblast.
The cytotrophoblast is an inner layer of mononucleated cells, and the is an outer layer of multinucleated cells with no distinct cell boundaries.
Syncytiotrophoblast cells don’t undergo cell division which means that the population of cells is doomed to simply die out over time.
To replenish the numbers of syncytiotrophoblast cells, there is a steady flow of cytotrophoblast cells that fuse with the syncytiotrophoblast cells - literally merging right into them and forming an expanding syncytium.
It’s a bit like maintaining the population in a retirement community where individuals aren’t raising youngsters and occasionally pass away, by attracting a steady inflow of new retirees.
The multinucleated syncytiotrophoblast grows larger and moves like an octopus deeper into the decidua basalis. Around day 14 of development, cells of the syncytiotrophoblast start to protrude out to form little protrusions called primary villi - each one looks a bit like a tree.
These primary villi “trees” form all the way around the fetus, and cells start to clear out from between the primary villi, leaving behind empty spaces called lacunae.
While this is all happening, arteries and veins from mom start to grow into the decidua basalis. Normally we think red blood cells as staying confined to blood vessels, but as the placenta develops, an interesting thing happens - tiny arteries merge with the lacunae.
So these empty spaces become filled with oxygenated blood. Veins also merge with lacunae and bring that blood back to the heart.
Now over time, more and more of these little pools of blood develop and they start merging together to form a single large pool of blood with many arteries delivering blood into it and many veins taking blood away.
This large pool is called the junctional zone. So lots of fetal villi “trees” next to one another are basically submerged in the junctional zone.
The villi in the junctional zone start to grow more and more branches, whereas the villi that are outside of it regress.
Around day 17 of development, fetal mesoderm cells enter the primary villi and they start forming tiny blood vessels, establishing a basic system of fetal arteries, capillaries, and veins within each villi.
The villi capillaries connects with blood vessels in the umbilical cord, and this links up the placental and fetal circulation.
At this point, the placenta is really taking shape. The basal plate, sometimes called the decidual plate, is the thick layer of decidua basalis tissue that the maternal arteries - called spiral arteries - and veins have to pass through to get to the junctional zone.
This is the maternal contribution to the placenta. On the other side, we’ve got a forest of villi “trees” called the chorionic frondosum, and they emerge out of the chorionic plate.
The chorionic plate is parallel to the basal plate - and sandwiches the junctional zone. This is the fetal contribution to the placenta.
Now, while the placenta has been forming on one side, a space forms around the fetus. These spaces form as cells sort of move out of the way, and this new cavity is called the chorionic cavity.
Within the chorionic cavity is the amniotic cavity, yolk sac, and the fetus itself, and the wall of this cavity where the syncytiotrophoblast villi regressed is known as the chorionic leave.
Stuck to the outside of the chorionic leave is a thin layer of decidua, called the decidua capsularis. On an ultrasound, the chorionic cavity shows up as a relatively large, dark space, and it’s used to identify a pregnancy even before the fetus can be seen.
During the fourth and fifth months of development, decidual septa or walls form and they divide the placenta into 15 to 20 different regions, called cotyledons.
About 100 spiral arteries pump blood into each cotyledons, providing a steady supply of oxygenated blood for the villi to bathe in.
Oxygen and glucose, as well as other molecules like immunoglobulins, hormones, and even certain toxins are able to move across the syncytiotrophoblast which lines the villi wall, and then move across the endothelial cells lining the fetal capillaries.
Meanwhile, carbon dioxide makes the opposite journey, leaving the fetal capillaries and entering the blood in the junctional zone.
As the fetus grows, so does the uterus and the placenta - with the placenta covering about 15-30% of the uterine wall at any given time during development.
The placenta also thickens and by the time the fetus reaches full term, it’s about 20cm across, the size of a frisbee. During the third stage of labor, the placenta pulls away from the wall of the uterus, since it is no longer needed at that point, and it’s expelled from the body as afterbirth.
All right, as a quick recap… The placenta forms a few weeks after the fetus implants in the wall of the uterus. On the maternal side, spiral arteries send blood into the junctional zone, and maternal veins take that blood away.
On the fetal side, chorionic villi with tiny capillaries within them dip into the junctional zone. The thin endothelial cell wall and the syncytiotrophoblast lining of the villi are all that separate fetal red blood cells and maternal red blood cells - and throughout pregnancy gases and nutrients hop across that barrier to help the fetus grow.