Okay, let's finish up talking about placentation going through weeks 4 through 12 of development. So, around week 4, the chorionic villi are going to develop and extend, forming blood vessels that will connect to the umbilical arteries and veins. And then, from weeks 5 through 12, the endometrium is undergoing massive changes at a cellular level to accommodate the pregnancy and placentation, although we're not going to cover all of that in detail. There are two main things that you should probably know.
First, the endometrium that is beneath the embryo or away from the lumen is going to contribute to the placenta. This will essentially become the maternal portion of the placenta. The endometrium that is toward the lumen or the cavity of the uterus will go on to surround the embryo and the amniotic sac. If you look down here at our images, we are showing weeks 4 to 12 of development. Focusing on week 4 first, just to orient you, we are still fully implanted within the uterine wall there. The lumen or the cavity of the uterus would be in this direction and the uterine wall, toward the myometrium, is over in this direction. You can see we have our little embryo inside of the amnion and a little early umbilical cord forming there with these umbilical blood vessels, which are the two umbilical arteries and the umbilical vein.
You can see how we have these chorionic villi, which now have these intricate networks of blood vessels within them, nestled within these intervillous spaces filled with maternal blood all throughout there. We also have our maternal blood supply depicted here. To give you a closer view, we have this image here. What we're seeing is the umbilical cord right there, the umbilical blood vessels, and how those are connecting to the blood vessels within that chorionic villi. The chorionic villi are, of course, embedded in the intervillous space full of maternal blood, with the maternal blood vessels supplying blood to that space. Essentially, all of this is the portion of the endometrium that will be contributing to our placenta, forming the maternal portion of the placenta. Meanwhile, the chorionic villi form the fetal portion of the placenta, jutting into that maternal portion.
This will be the main site of all the exchanges between the mother and the conceptus, such as nutrient delivery, waste removal, and gas exchange, which will occur within those chorionic villi. Another important note is that you can probably see quite easily that the chorionic villi on this side, toward the lumen, are much smaller than those on the side toward the uterine wall. This is because these villi are getting somewhat flattened. As the embryo grows and the embryonic sac pushes out, they're getting compressed against the sac and this endometrial tissue.
Over time, these will essentially flatten out, which we can see clearly here in our week 12 image, showing a cross-section of a fully-formed placenta. By week 12, our placenta should be fully formed and should be acting as the conceptus' main source of all of its necessities like nutrients, gas, and oxygen exchange, and waste removal. You can see how this side of the placenta has basically flattened out completely. We are looking at that here; this orangey peach-colored layer and then we have the endometrial tissue here in pink that is completely surrounding the embryo and the amniotic sac, extending all the way out and around the conceptus.
Alright, that is placentation. I will see you guys in our next video. Bye-bye.