Moving on to more of week 4 and headed through week 8! We made our bilaminar disc and then our trilaminar disc, now we’re going to fold them all up like tacos! Well, sort of like tacos, but lots of folding, for sure!

Two things you need to keep in mind. First, that we’re going to fold on two planes – median and horizontal. So in a way, it’s really more of a pinching effect. The second is that folding is that even though the folding along the carniocaudal axis occurs faster, the folding occurs at the same time.

So, starting with cranial fold, the neural tube has been formed in where the head is more or less. It’s going to keep growing beyond that oropharyngeal membrane that we had talked about, the future site of your mouth. It sort of keeps going and going until it starts to wrap around overhangs the developing heart. This is going to sort of slide everything around so that the developing heart, the oropharyngeal membrane adn the septum transversum (a thick mass of cranial mesenchyme) are going to move to the ventral (forward facing) surface. Some of the endoderm of the umbilical vesicle is coming along for the ride, too. It’s starting the foregut. At this point, the septum transversum is going to separate the abdominal and thoracic cavities as well.

Next, our caudal fold. Again, it’s the growth of the neural tube that is causing this folding to take place. It’s going to end up curling around, much like on the cranial end as well. Here, we’re starting the hindgut. The end is also going to dilate into the cloaca, which is the beginning of the rectum and the urinary bladder. The connecting stalk, which is the early version of the umbilical cord, is also brought around on that fold, so now its on the bentral surface of the embryo as well. A portion of the allantois (a membrane below the chorion) is also incorporated. It’s going to help form part of the umbilical cord.

Okay, now our lateral fold. Keep in mind, these are all happening essentialy simultaneously. This one is being caused by the growth of the spinal cord and those somites we formed earlier. It’s going to come from the top, down and around and meet towards the middle. We’re rolling the endoderm up on itself as well, so that is going to create our midgut.

With our lateral fold done, the connection between our midgut and the umbilical vesicle is reduced, forming the omphaloenteric duct. And no, no idea how to pronounce that. The important thing is that the connection between the body cavity (intraembryonic) and the chorionic cavity (extraembryonic) to a much narrower pathway. Guess what we’re forming.

As this fold happens, its also pulling the amnion down and around it, like a baby blanket. The embryo is now lying comfortably in the amniotic cavity.

Sort of a short one, but that’s all for now! We’ll keep the embryology rolling, so check back again, soon!