This week, let’s keep our theme of embryology going. We’re going to skip ahead a bit to the formation of the second week of development and what’s known as the chorion cavity and the bilaminar disc. 2nd week, bilaminar disc. Should be easy to remember, right? We think the best way to conceptualize this portion is to imagine balloons being grown inside of balloons. This portion of development is about making the right structures that the embryo will need to support its development. That starts with building the structures inside of which it will do its business! But let’s start at the with implantation.

So, implantation occurs between day 6 and 10. At this point, think of the embryo as a little hollow ball of cells. Surrounding that, the outside layer is the syncytiotrophoblast. This is the layer that is going to actually invade the endometrium to implant. Immediately deep to the syncytiotrophoblast is the cytotrophoblast whose job it is to keep the syncytiotrophoblast supplied with fresh cells. Together they form the trophoblast layer.

Inside this capsule, there is a collection of cells that are gathered to one side. This is the embryoblast, which as the name sounds, will become the actual embryo. There is also a layer of cells that separate the embryoblast from the rest of the hollow cavity (the blastocystic cavity), and that layer is called the hypoblast.

Okay, so that’s pretty much where we are at by day 8. We’re mostly deeply embedded with more or less that structure going on. The syncytiotrophoblast is doing some cool stuff to start supplying blood from the mother to the embryo, but we’re going to focus more on the internal structures instead. That embryoblast that we talked about? At this point its going to differentiate into two layers – they hypoblast and the epiblast. Use your language skills. Which one is on top? Yes, the EPIblast is above the HYPOblast. This is our bilaminar disc. From this point, we’re not quite ready to start forming all the parts, yet. We have a few things to do before we are prepared for that amount of cell differentiation and such.

At the same time that these are forming, there is a small cavity that opens up within that epiblast. This is the start of the amniotic cavity. It’s going to be surrounded by cells that separate from the epiblast. The cells are called amnioblasts and we call that layer, which encloses the amniotic cavity, the amnion.

Also at the same time (a lot goes on at the same time, huh?), at the other end of the hollow ball, the outside part, cells are coming of the hypoblast. They’re working on forming a thin membrane called the exocoelomic membrane. This membrane along with the hypoblast forms the lining of the primary umbilical vesicle. The umbilical vesicle and the amniotic cavity we also formed make it possible for the cells of the embryonic disc to move. That’s important so we can get all the right cells in all the right places for proper development of a human.

Back to the surrounding layers, the cells from the umbilical vesicle are going to form a layer of connective tissue. This is going to surround the umbilical vesicle and the amniotic cavity, both. This is called the extraembryonic mesoderm. Eventually, that increases in size and little gaps start forming in it. These are extraembryonic coelomic spaces. At some point, those are going to all get together and form one big cavity (so surrounding the umbilical vesicle and the amnionic cavity). Welcome to the EXTRAembryonic coelom. This is how we add layers to layers, yeah? Now we call things by slightly different names. The lining of cytotrophoblast and the amnion is called extraembryonic somatic mesoderm. The part that surrounds the umbilical vesicle is the extraembryonic splanchnic mesoderm.

As this all is happening, we need to go back to the hypoblast real quick and work on forming the chorionic cavity. Those are also separating again inside the primary umbilical vesicle so that they can form a secondary or definitive umbilical vesicle. The primary umbilical vesicle sort of just drops down to the opposite side of the cavity, the side that is more towards the surface of the endometrium.

Next, we have that big thick layer of extraembryonic somatic mesoderm that is surrounding everything. This is going to get the cytotrophoblasts to start growing quickly and invade the syncytiotrophoblast lasyer. These little fingers that are going to start coming through are the primary chorionic villi.

So we have this hollow ball of cells that has developed three small hollow balls inside of it. This whole thing is embedded in the endometrium with the outermost layer being the syncytiotrophoblast which had done the original invading of the endometrial wall. As we discussed, it did some also very cool stuff to start getting blood and nutrients from the mother. That’s where these villi or fingers are going into. Now, the extraembryonic coelom is called the chorionic cavity. It’s surrounding is made of the extraembryonic somatic mesoderm, the cytotrophoblast and the syncytiotrophoblast. We also call this wall the chorionic sac.

Last for this blog post, day 14, we’re going back to the hypoblast part of our bilaminar disc again. The part of the hypoblast that will end up developing the head portion of the fetus is going to start to thicken. It’s also going to fuse with the epiblast. This forms the prechordal plate. Our first step of making an actual body part!

It’s a lot to keep track of, but as with most things, go slow. There are more details than here, but hopefully this gives you the frame work you need!