Development of the placenta
At the end of pregnancy, a number of changes in the placenta lead to exchange between the maternal and fetal circulations.
The placenta is an organ that’s co-created by the fetus and the mother during development.
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; the point of contact is called the decidua basalis.
To snuggle deeper into the decidua basalis, 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 syncytiotrophoblast is an outer layer of multinucleated cells with no distinct cell boundaries.
Syncytiotrophoblast cells don’t undergo cell division which means that their population of cells would be doomed to simply die out over time without the help of the cytotrophoblast.
To replenish the number of syncytiotrophoblast cells, there’s a steady flow of cytotrophoblast cells that fuse with the syncytiotrophoblast cells, literally merging with 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 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 the mother start to grow into the decidua basalis.
Normally we think of red blood cells as staying confined to blood vessels, but as the placenta develops, an interesting thing happens—tiny arteries start to merge with the lacunae, and these previously empty spaces become filled with oxygenated maternal blood.
Veins also merge with lacunae and bring that blood back to the maternal heart.
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—because this is where the maternal and fetal circulations come into close contact.
Lots of fetal villi trees next to one another are basically submerged in the pool of maternal blood 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 connect with blood vessels in the umbilical cord, and this links up the placental and fetal circulatory networks.