The last post got a little lengthy, so here is the second part of that week 3 of embryological development. Specifically, neurulation – the formation of the neural tube! It kind of works to split it up like this anyhow, since this process really won’t complete itself until the end of week 4. That makes it sort of an inbetweener. We’ll also discuss the embryonic mesoderm as well.

Okay, so the notochord is in place. Remember that? It’s a long tube that has mesoderm on either side of it. Above it is the ectoderm and below is the endoderm. It’s not connected to any of them. It will, however, get the ectoderm above it to thicken, which is going to form a bunch of epithelial cells into a long structure called the neural plate.

On day 18, that neural plate is going to fold along its central axis to form a groove. The parts that stick up on either side, then, is called the neural folds. By the end of week three, they’re going to fuse together to turn the neural plate into the neural tube. Just like how the notochord split itself off, the neural tube is going to do the same thing.

In addition, the cells of the ectoderm that were next to on either side of the neural folds are going to become the neural crest. As those folds come up and fuse, they’re also going to be released from the ectoderm and become neural crest cells. So, they’re lying between the neural tube and the ectoderm. They are the start of our formation of spinal ganglia, pigment cells, adrenal cells and connective tissue component of the head.

So, now about that mesoderm. The cells in it are organized into three parts – the paraxial mesoderm, the intermediate mesoderm and the lateral mesoderm.

The paraxial mesoderm is along the neural tube. Have you seen pictures of early embryos and they have the square ridges on their backs? Those are somites and they’re formed along the neural tube by the paraxial mesoderm. Eventually, they’ll become our axial skeleton, along with the musculature and dermis that goes with it. We have somewhere between 40-44 of them. It’s good to have an understanding of them because they are often used for determining an embryo’s age.

The lateral mesoderm will have cavities in both it and the cardiogenic mesoderm. The cavities will eventually hook up and form the intraembryonic coelom which is the beginning of the primordial body cavity. That coelom is splitting the lateral mesoderm into the somatic/parietal lateral mesoderm and the splanchnic/visceral lateral mesoderm. The somatic mesoderm hooks up with the ectoderm on top to form the somatopleure – the embryonic body wall. The splanchnic mesoderm hooks up with the endoderm underneath to form the splanchnopleure – the embryonic gut. Kind of cool that things are actually taking shape now, huh?

Speaking of forming things, that intraembryonic cavity we just made? In the 2nd month, the coelom is going to divide into the peritoneal cavity, the pericardial cavity and the pleural cavities. Boom!

Let’s add a couple more things here that also happen in week 3. This is also an important week since we are forming the beginnings of our vascular system. For this, we need to go back to the extraembryonic mesoderm of the umbilical vesicle where it will start. Don’t get lost here, the extraembryonic mesoderm is mesoderm NOT in the embryo. So, we’re looking at parts of the uterus. This process is going to work its way up the connecting stalk, into the chorion and then into the embryo. The two important parts here are vasculogenesis (blood vessel formation) and angiogenesis (branching of the blood vessels).

Starting with vasculogenesis, we’re going to use more mesenchymal cells, since after all, they are pluripotent. They’re going to start to make angioblasts which will get together and form blood islands in the umbilical vesicle and endothelial cords in the embryo. All of those are going to start to form cavities inside of them. The angioblasts are then going to develop endothelial cells around the cavities to form endothelium. That sound familiar? What lines all of your blood vessels? All of these are going to fuse to form networks of endothelial channels and the mesenchymal cells surrounding them will differentiate into…what else do you have in blood vessel walls? Yup, muscular and connective tissue. Starting to all make sense, right?

Next, angiogenesis. Those vessels are going to spread into other areas by budding and fusing with the other vessels they meet furthering the network that we started in vasculogenesis. Blood cells are also going to be derived from the endothelial vessels in the umbilical vesicle. And yes, we’re still in week 3. Kind of crazy, all that stuff that happens. Huh?

In all of this, they’re are also two long tubes that are near each other in the cardiogenic area. They’re fusing into the primordial heart tube which is connecting to all of the blood vessels. By day 21, maybe day 22, it will begin beating.