Derivatives of Lateral Plate Mesoderm: Body Wall and Cavities

00:01 Hello, we're now gonna begin investigating how the body cavities develop which seems like a strange topic.

00:06 How do we discuss the development of a space? What you may recall is that the body cavities develop as a consequence of the trilaminar embryo folding.

00:17 And one thing about the trilaminar embryo is it likes to fold.

00:20 You may notice as you look at your friends and family, they're not flat disks of cells walking around, they're actually relatively cylindrical.

00:28 We have body cavities, a body wall, and we had neurulation occur which brought our central nervous system into the body.

00:36 We have our gut tube form from the endoderm bringing it inside the body.

00:40 But we need one final set of tubes to form to actually seal off our entire body and make an anterior body wall and enclose the various body cavities.

00:50 So starting with the trilaminar embryo.

00:52 We're gonna revisit the lateral plate mesoderm.

00:55 That's the mesoderm that's on the farthest lateral edge of the developing embryo and it's going to develop a split in it.

01:02 Now, that split is known as the extraembryonic coelom because it's a space located outside of the developing embryo.

01:10 As that split gets wider and wider, we have a somatic layer of lateral plate mesoderm that's in contact with the overlying ectoderm and amniotic cavity.

01:21 Below that, we have a visceral or splanchnic layer that's in contact with the endoderm.

01:26 Now, as development proceeds, the endoderm and visceral layer of the lateral plate mesoderm are gonna pinch together to form the gut tube and have the yolk sac extend off of them.

01:39 At the same time, the lateral plate mesoderm's somatic layer and the overlying ectoderm are going to stretch way, way far forward.

01:48 Almost like the wings of a manta ray.

01:50 And eventually, they're going to surround the developing gut tube.

01:53 So here, this image is showing how that combined layer of a lateral plate mesoderm plus ectoderm now called the somatopleure is wrapping around the gut tube and in the process, it is enclosing the extraembryonic coelom and turning it into the intraembryonic coelom.

02:14 That shift in nomenclature simply denotes that that space was once outside the embryo and now it is inside the embryo.

02:21 As the lateral plate mesoderm, somatopleure, grow closer and closer together, they're actually gonna seal together anteriorly and create a completely intact body wall and that's what we see here.

02:33 The gut tube is hanging out inside the intraembryonic coelom.

02:38 It's connected posteriorly by a dorsal mesentery and anteriorly by a ventral mesentery and those are gonna support and maintain the gut tube.

02:48 Now, with time, that ventral mesentery will simply go away because there's not too much in there.

02:55 The only place it sticks around is in the liver as a structure called the falciform ligament.

02:59 But we need our dorsal mesentery because that's how blood innervation and other structures are getting to and from the gut tube from its blood supply, the dorsal aorta, and its venous drainage which is going to be the hepatic portal veins.

03:14 Now, what can go wrong in this process? A variety of body wall defects occur when the somatopleure doesn't quite make it all the way to the front.

03:25 On the milder side, you can have sternal defects like a little gap or foramen in the sternum but you can also occasionally have the sternum completely separate on the left and the right, but covered by some skin.

03:38 So not an obvious defect but an underlying bony defect.

03:43 Far more serious though are body wall defects where the body wall doesn't grow together and the underlying organs are exposed.

03:50 If this occurs in the thorax, we wind up with something called ectopia cordis or an ectopic, outside heart.

03:57 As the somatopleure is travelling anteriorly, if the left side and right side can't meet each other, the heart is developing completely separately and the heart may develop relatively normally.

04:10 But since there's no body wall for it to be constrained by, it will literally hang outside the body.

04:17 So an ectopic heart is the heart developing but completely exposed to the external environment.

04:23 And this is incredibly serious and will require surgical correction to take into account and hopefully, give some quality of life to the child who's born with it.

04:33 If a body wall defect occurs in the abdomen, a variety of problems can occur.

04:38 Gastroschisis or splitting of the belly is quite literally, a defect in the anterior abdominal wall that allows the intestines to exit.

04:47 Now, during development, the intestines have to herniate into the developing umbilical cord for a period of time.

04:54 They run out of space inside the abdomen and have to move into the cord.

04:58 But as the body gets larger, the intestines gradually return.

05:02 In the case of gastroschisis, the pressure pushes them out the defect and there's never any underlying push to get them back in.

05:11 So children born with gastroschisis will have to have supportive care given keeping the intestines nice and moist while they're very gradually reintroduced back into the body wall so that it can expand to accommodate them, and then, be sutured together.

05:26 What initially appears to be very similar to gastroschisis is actually different and it's a condition called omphalocele.

05:33 In an omphalocele, you have herniation of gut, both the liver, the stomach, the intestines, any of those abdominal organs not out of a defect in the body wall but into the umbilical cord, and that's what's imaged here.

05:47 Unlike gastroschisis, an omphalocele will have a covering of peritoneum pushing out into the umbilical cord and this is usually tied to other severe defects of development and is usually far more serious not just because of the body wall defect, but because of other linked symptoms.

06:05 Now, let's return to the intraembryonic coelom.

06:08 Initially, it has kind of a horseshoe shape coming from the eventual abdominal region, up around the head, and surrounding the heart.

06:16 As the heart folds forward into the thoracic cavity, it's going to pull that cavity with it.

06:23 And the cavity that surrounds the heart is called the pericardial cavity.

06:27 But it's important to note that it's simply a subdivision of the intraembryonic coelom and at this point, it is not separate from the rest of that body cavity.

06:36 So here in grey, within the embryo, we have demonstrated the entire intraembryonic coelom.

06:45 In the abdominal area, we have the peritoneal cavity and in the thoracic region, we have the pericardial cavity.

06:53 Now, this single horseshoe shaped cavity will be subdivided to form not just the peritoneal cavity and the pericardial cavity but also, the pleural cavity and we'll discuss that as we discuss how the septum transversum becomes the diaphragm and how we completely separate the thoracic and abdominal organ systems.

07:16 Thank you very much for your attention.