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Mesentery

Overview of the mesentery on the anterior view of abdomen with the greater omentum reflected and small intestine removed.

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Did you know that your small intestine is approximately five meters in length? That’s nearly as tall as an adult male giraffe or two and a half times the size of your average basketball player. So, how is it that the not-so-small small intestine fits into our abdominal cavity? Well, that’s what we’re going to find out today in our tutorial about the mesentery.

Before we begin, I’d like to give you a quick overview of what we’re going to talk about in this tutorial. In our image here, which is going to feature throughout this tutorial, we can see inside the abdominal cavity. The greater omentum has been reflected upwards and the small intestine has been removed, so that we can see the star of the show – the mesentery.

We’ll start off our tutorial by discussing what the mesentery actually is then we’ll move on to talk about some relations of the mesentery and wrap up our tutorial with some clinical notes.

So what is this structure that we’re learning about and what is it for? Before we start talking about the mesentery, I’m first going to introduce you to the peritoneum. Here, we can see the abdominal cavity in the sagittal plane. I’ve chosen this image because it nicely shows how the peritoneum lines the walls of the abdominal cavity and most of the organs or the viscera contained within it. The peritoneum is a double layer of serous membrane or mesothelium and can be divided into two parts: the parietal peritoneum and the visceral peritoneum.

The parietal peritoneum which we can now see highlighted in green lines the walls of the abdominal cavity, whereas the visceral peritoneum covers most of the abdominal viscera. The peritoneum can form folds such as mesenteries, omenta and ligaments.

Okay, so now that we’re familiar with the peritoneum, let’s move on to talk about some of its folds. The mesenteries are peritoneal folds that contain viscera and anchor them to the posterior abdominal wall. The largest mesentery is the mesentery proper or the mesentery of the small intestine. It is a fan-shaped double layer of peritoneum that connects the jejunum and the ileum to the posterior abdominal wall. Superiorly, the mesentery of the small intestine is attached at the junction between the duodenum and the jejunum. It then travels obliquely downwards to terminate at the ileocecal junction. The two peritoneal layers of mesentery proper contain the blood vessels, nerves, and lymphatics that supply the jejunum and the ileum.

The mesentery proper isn’t the only mesentery of the human body. We actually have a few more.

Next stop, we have the transverse mesocolon, which we can now see highlighted in green. The transverse mesocolon anchors the transverse colon of the large intestines to the posterior abdominal wall. It leaves the posterior abdominal wall from the anterior surface of the pancreas and travels outwards to enclose the transverse colon. The transverse mesocolon contains the blood vessels, nerves, and lymphatics that supply the transverse colon.

Next, we have the sigmoid mesocolon, which is an inverted v-shaped peritoneal fold that attaches the sigmoid colon of the large intestines to the posterior abdominal wall. The apex of the V is near the division of the left common iliac artery into its internal and external branches. The sigmoid mesocolon contains the sigmoid and superior rectal vessels along with the nerves and lymphatics associated with the sigmoid colon.

It’s worth noting that the vermiform appendix also has the mesentery associated with it which is called the mesoappendix. This is the fourth and final mesentery of the abdominal cavity. It is attached close to the inferior end of the mesentery proper of the small intestine close to the ileocecal junction. It usually extends right down to the tip of the vermiform appendix. The mesoappendix contains the blood vessels, nerves, and lymphatics that supply the vermiform appendix.

Now that we’ve covered the mesenteries, let’s look at some relations. We’ll start with some more folds of the peritoneum – the omenta and the ligaments. Then we’ll move on to talk about the viscera associated with the mesenteries. In our current image, we can see the greater omentum reflected upwards so that we can see the mesentery. Let’s flip it back down so that we can see it in its anatomical position.

The greater omentum is also a double fold of the peritoneum. It hangs down from the greater curvature of the stomach and reflects back on itself to attach to the transverse colon. We can actually see this best if we switched to a sagittal view of the abdominal cavity.

So here we can see the stomach and over here we can see the greater omentum doubling back on itself to attach to the transverse colon. Because the peritoneum is made up of two layers and the greater omentum reflects back on itself, the structure is made up of four layers in total. If you’d like to learn more about the greater omentum, please watch our video or read about it on our website.

If you have a greater omentum, you must also have a lesser omentum. The lesser omentum extends from the lesser curvature of the stomach and the proximal duodenum to the liver. If we go back to our original image, you’ll notice that we can’t see the lesser omentum as it’s hidden by the reflected greater omentum.

Okay, so let’s have a look at the peritoneal ligaments we can see in this image, starting with the median umbilical fold. This unpaired structure is a raised ridge of parietal peritoneum that overlies the median umbilical ligament which runs from the apex of the bladder to the umbilicus. The medial umbilical ligament is a remnant of the urachus – a canal that connects the bladder to the umbilicus during fetal development.

Located on either side of the median umbilical fold, we find the medial umbilical folds. The medial umbilical folds are a pair of raised ridges of parietal peritoneum that overlie the medial umbilical ligaments. The medial umbilical ligaments are the remnants of the fetal umbilical arteries.

Moving laterally, we find the lateral umbilical folds. The lateral umbilical folds are a pair of raised ridges of parietal peritoneum that overlie the inferior epigastric vessels. I’ll show you these vessels now before we move on to talk about the viscera related to the mesentery.

Here we can see the inferior epigastric artery which is a branch of the external iliac artery. This vessel supplies the anterior abdominal wall. Accompanying the inferior epigastric artery, there is an inferior epigastric vein. The inferior epigastric vein arises from the superior epigastric vein and drains into the external iliac vein. This vessel drains the anterior abdominal wall.

Now that we’re familiar with the omenta and the ligaments associated with the mesenteries, let’s move on to talk about the organs or viscera. We’ll start with the small intestine. The small intestine can be divided into three parts. The duodenum, the jejunum, and the ileum. In today’s tutorial, we’ll focus on the parts associated with the mesentery proper, the jejunum, and the ileum.

First, we’re going to look at the jejunum. The jejunum is the middle segment of the small intestine between the duodenum and the ileum. Here we can only see the start of the jejunum. Let’s change our image so that we can see the jejunum suspended from its mesentery in the abdominal cavity. The jejunum plays an important role in digestion and its functions include absorbing water and nutrients. Its blood supply is carried by the jejunal arteries and its venous blood drains via the jejunal veins.

The sympathetic innervation of the jejunum is carried by the nerves of the celiac plexus and superior mesenteric plexus. The parasympathetic innervation is supplied by the vagus nerve. Lymph from the jejunum drains to the superior mesenteric nodes. Remember that these vessels and nerves are transmitted to and from the jejunum via the mesentery proper.

Next, we’ll talk about the ileum which is the terminal portion of the small intestine. The ileum is a continuation of the jejunum and ends at the ileocecal valve which separates it from the cecum. Here we can only see the end of the ileum. Let’s change our image again so that we can see the ileum suspended from its mesentery.

The ileum functions to absorb bile salts, vitamin B12, and any other nutrients that haven’t been absorbed yet. Its blood supply is carried by the ileal arteries and its venous blood drains via the ileal veins. Like the jejunum, its sympathetic innervation is carried by the nerves of the celiac plexus and the superior mesenteric plexus whereas its parasympathetic innervation is supplied by the vagus nerve. Lymph from the ileum drains to the superior mesenteric nodes. Similar to the jejunum, these vessels and nerves are transmitted to and from the ileum via the mesenteric proper.

Now that we’ve looked at the small intestine from this perspective, let’s move on to the large intestine. The large intestine can be divided into eight parts – the cecum, the ascending colon, the right colic flexure, the transverse colon, the left colic flexure, the descending colon, the sigmoid colon, and the rectum. I’ll introduce you to all of these parts but I’ll provide more detail on the sections associated with the mesenteries, the transverse colon, and the sigmoid colon.

As I’ve mentioned previously, the large intestine begins with the ileocecal valve. The first part of the large intestine is the cecum, which is a patch just below the ileocecal valve. The appendix is a fingerlike blind-ended tube connected to the cecum. The next part is the ascending colon, which starts immediately after the cecum and travels up to the right colic flexure. The right colic flexure is also known as the hepatic flexure because of its proximity to the liver. The right colic flexure lies between the ascending colon and the transverse colon.

The next part of the large intestine is the transverse colon which is suspended from the posterior abdominal wall by the transverse mesocolon. The transverse colon receives digestive food from the small intestine from which it absorbs water and electrolytes to form feces. Its blood supply is carried by the right, middle, and left colic arteries and its venous blood drains via the middle colic vein. The transverse colon is innervated by the nerves of the superior mesenteric plexus and the inferior mesenteric plexus. Lymph from the transverse colon drains to the superior mesenteric nodes. These vessels and nerves are transmitted to and from the transverse colon via the transverse mesocolon.

The transverse colon travels across the abdomen to the other colic flexure – the left colic flexure. It’s also known as the splenic flexure for, as you might have guessed, its proximity to the spleen. The left colic flexure lies between the transverse colon and the descending colon. The next part of the large intestine is the descending colon. It’s located on the left side of the abdomen and it travels straight down to the sigmoid colon.

The sigmoid colon which we can now see highlighted in green is suspended from the posterior abdominal wall by the sigmoid mesocolon. The sigmoid colon transports feces from the descending colon into the rectum. It is also responsible for the absorption of water and nutrients. Its blood supply is carried by the sigmoid arteries and its venous blood drains via the sigmoid veins. The sigmoid colon is innervated by the nerves of the inferior mesenteric plexus. Lymph from the sigmoid colon drains to the inferior mesenteric nodes. These vessels and nerves are transmitted to and from the sigmoid colon via the sigmoid mesocolon. The final part of the large intestine is the rectum. When the rectum pierces the pelvic floor, it becomes the anal canal.

Before we move on to our clinical notes, I just wanted to quickly point out some general features of the large intestine. Here we have the tinea coli highlighted in green, which are three-thickened bands of longitudinal muscle. In this image, we can also see the haustra, which are the sacculations of the colon as well as the omental appendages. These three features can be used to help distinguish between the small intestine and the large intestine.

Now that we’re familiar with the mesentery and its associated structures, let’s get clinical.

In today’s clinical notes, we’re going to talk about mesenteric ischemia, which occurs when the blood flow to the small intestines is restricted. It occurs as a result of narrowed or blocked arteries and can lead to necrosis of the small intestine. There are two types of mesenteric ischemia – acute mesenteric ischemia and chronic ischemia. Acute mesenteric ischemia is most commonly caused by an embolus and requires emergency surgery. Surgery involves excision of any necrotic bowel and revascularization of the bowel to removal of the embolism.

When the blood supply to the small intestine deteriorates over time, it is called chronic mesenteric ischemia. Chronic mesenteric ischemia is usually a result of atherosclerosis and management of this condition reflects this. Patients will be given an antiplatelet and a statin along with the advice and support relating to lifestyle modification. These measures are taken to help stabilize any plaques. In severe cases, surgery such as stenting may be required.

So before we bring our tutorial to a close, let’s quickly summarize what we’ve learned today. First, we established what the mesentery actually is by looking at the peritoneum, which can be subdivided into the parietal peritoneum and the visceral peritoneum. The parietal peritoneum lines the walls of the abdominal cavity and the visceral peritoneum covers most of the abdominal viscera. Once we established that the mesenteries are derived from the peritoneum, we went on to talk about each one, including the mesentery proper which is associated with the jejunum and the ileum, the transverse mesocolon which is associated with the transverse colon, the sigmoid mesocolon which is associated with the sigmoid colon, and the mesoappendix which is associated with the vermiform appendix.

We then went on to talk about some relations of the mesentery starting with the greater omentum and the lesser omentum. We then looked at some peritoneal ligaments including the median umbilical fold which overlies the median umbilical ligament, the medial umbilical fold which overlies the medial umbilical ligament, and the lateral umbilical fold which overlies the inferior epigastric artery and the inferior epigastric vein.

Next, we discussed the organs or the viscera related to the mesenteries. We started with the small intestine focusing specifically on the jejunum and the ileum, then we looked at the large intestine which can be divided into the cecum, the ascending colon, the right colic flexure, the transverse colon, the left colic flexure, the descending colon, the sigmoid colon, and the rectum. When the rectum pierces the pelvic floor, it becomes the anal canal.

We also looked at some general features of the large intestine such as the tinea coli. Finally, we concluded our tutorial with some clinical notes about mesenteric ischemia.

So that brings us to the end of our tutorial on the mesentery. I hope you enjoyed it. Thanks for watching and see you next time!

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