Migration of Foregut Organs and Mesenteries

00:01 We’ll continue our investigation of how the foregut develops by looking at how the organs of the foregut rotate, migrate, and develop a set of mesenteries and specialized structures called omenta.

00:12 We’ve already seen how the stomach rotates and balloons out to create a greater curvature and a lesser curvature.

00:20 It’s hanging from its dorsal mesentery and as it moves, it’s gonna do some interesting things to this dorsal mesentery as other organs develop and force the stomach to rotate.

00:31 In particular, we’ve got the liver developing into the septum transversum and the ventral mesentery that are located anterior to the stomach.

00:41 The gallbladder and ventral pancreatic bud are also present and inferior to the liver.

00:47 We can’t see them in this illustration but do remember that they are there.

00:50 Whereas the dorsal pancreatic bud and the spleen will be developing within the dorsal mesentery.

00:56 As the liver enlarges, it has room to migrate to the right and that’s exactly what it does.

01:03 It’s tethered to the anterior body wall by the ventral mesentery which changes its name to the falciform ligament and connects the liver to the anterior body wall.

01:13 As the liver moves to the right side, it pulls on its connection to the stomach and forces the stomach to rotate so that the stomach’s anterior region is forced to rotate to the right.

01:25 As this occurs, that section of mesentery will now be called the lesser omentum.

01:32 So the lesser omentum connects the liver to the stomach and duodenum.

01:36 Posterior to that, we have the spleen developing within the dorsal mesentery and because of that, the dorsal mesentery between the stomach and the spleen will be known as the gastrosplenic ligament.

01:50 This is not a ligament as we’d expect in the musculoskeletal system that allows muscles to pull on bones but we simply name it because it is a stretch of connective tissue connecting two different structures.

02:01 The liver continues to enlarge, the stomach continues to rotate, and as the greater curvature balloons, the pyloric region and proximal duodenum are forced to the right.

02:12 This process creates a space posterior to the liver and posterior to the stomach that is called the lesser sac or the omental bursa.

02:22 It is part of the peritoneal cavity but the peritoneal cavity as a whole is only connected to this lesser sac by a small gap running right underneath the lesser omentum.

02:34 This is known as the omental foramen or more commonly to surgeons, the foramen of Winslow.

02:40 So the lesser omentum is going to be containing the entryway to the lesser sac or omental bursa from the rest of the peritoneal cavity which is often referred to as the greater sac.

02:53 Now, the dorsal mesentery that’s going to be running between the gastrosplenic ligament is going to expand tremendously along the stomach’s greater curvature.

03:02 As it expands, it kind of folds forward and loops back creating a great sheet of tissue that overlies the intestines.

03:13 This sheet of tissue is going to become the greater omentum and initially, it’s continuous with the space posterior to the stomach, the lesser sac or the omental bursa.

03:24 In this image, we can see in the sagittal cut how the lesser sac is present posterior to the stomach but anterior to the pancreas and that space between folds of the greater omentum is continuous with it.

03:36 As the greater omentum develops, those two folds will fuse together decreasing the space that’s present in the omental bursa.

03:45 At the same time, the greater omentum fuses with the mesentery of the transverse colon.

03:51 Because of that, when you encounter the greater omentum in the abdominal cavity and lift it up, you may move the stomach, but you’ll also move the transverse colon which is present just posterior to the greater omentum.

04:04 Now, I remember the first time I investigated the abdomen of a cadaver, I had no idea what the greater omentum was.

04:10 It was not an organ I’d ever heard of.

04:12 So upon opening the abdominal cavity, I saw nothing but a sheet of tissue and I had a minor freak out until my adviser came by, lifted up that apron of the omentum, and showed me that the intestines were indeed there.

04:25 Now, as the greater omentum is forming, we have a fusion of other regions of the mesentery.

04:33 The pancreas and spleen develop partially within the dorsal mesentery of the body but as the space in the abdomen becomes tighter and tighter, the pancreas and spleen get pushed towards the posterior body wall.

04:47 As that occurs, the mesentery of the pancreas is going to fuse with the mesentery lining the posterior body wall.

04:53 When that happens, the pancreas will become secondarily retroperitoneal.

04:58 It’s no longer freely hanging out inside the abdominal cavity, it’s stuck to the posterior body wall and the connection of mesentery between it and the spleen will then be called the splenorenal ligament.

05:12 Splenorenal because the left kidney is very close by and to follow that ligament posteriorly will take you into the vicinity of the left kidney and its connection to the spleen.

05:24 Thank you very much for your attention and I’ll see you on our next talk.