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Greater omentum

Overview of the greater omentum and neighbouring abdominal viscera.

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Transcript

We all understand the duty of police officers. Yes, I’m talking about those real-life superheroes who protect us, our property, and investigate crimes. I’m not talking about those friendly folk you might bump into at a cops-and-robbers theme night at one of your local nightclubs. But did you know that you actually have a police officer in your abdomen? Well, today, we’re going to be lauding about that officer, which is the greater omentum.

Before we start talking about the greater omentum, it’s important that we orientate ourselves, and in this illustration, which will feature heavily in this tutorial, we can see an anterior view of the abdominal cavity. The layers of the anterior abdominal wall including the skin, the fat, and muscles have been dissected away, and this has been done to reveal the membranes and organs within the abdomen.

Here we can see the greater omentum and some associated structures such as the small intestine, the large intestine, the stomach, and the liver. If I told you that the greater omentum is the largest fold of visceral peritoneum in the abdominal cavity, it might not mean that much to you. So, I think it’s important that I give you some background information and define a few key terms.

Let’s start with the peritoneum. If we look at our image, we can see the peritoneum highlighted in green in the midsagittal plane and this is the plane that effectively cuts the body into two equal halves. So, the peritoneum continuous serous membrane that lined the abdominal cavity and wraps around the abdominal organs. It can be divided into two parts – the parietal peritoneum and the visceral peritoneum.

The parietal peritoneum is the portion of the peritoneum that lines the abdominal cavity as you can see in our image here, whereas the visceral peritoneum wraps around the abdominal organs or the viscera. And between these two layers, there’s a potential space known as the peritoneal cavity which contains a small amount of lubricating fluid.

The peritoneum consists of many folds and ligaments, and the largest fold of visceral peritoneum in the abdominal cavity is the greater omentum. So, now you’ll definitely know what I’m talking about.

Now that we’ve oriented ourselves and are provided some background information, let me give you a quick overview of what we’re going to be talking about in today’s tutorial. We’ll start with the structure and function of the greater omentum followed by the blood supply, and then we’ll go on to look at some associated structures. And, finally, we’ll conclude our tutorial with some clinical notes relating to the greater omentum.

So let’s begin by talking about the structure of the greater omentum.

The greater omentum is also known as the epiploon which is derived from the Greek word “epipleo”, meaning floating on the surface of something, which in this case, is the surface of the intestines. The greater omentum is usually thin and has a perforated appearance. It contains variable amounts of adipose tissue or fat and consists of an anterior sheet and a posterior sheet, each made up of two layers. Therefore, the greater omentum consists of four layers in total.

So, if we go back to our image showing the abdominal cavity in the midsagittal plane, we can see the organization of the greater omentum a lot more clearly. The anterior sheet descends from the greater curvature of the stomach then folds under itself to become the posterior sheet. The posterior sheet then passes superiorly to attach to this structure here which is the transverse colon.

Okay, now that we’re familiar with the structure of the greater omentum, let’s move on to talk about its function.

So, this is a section of the tutorial where we’ll find out why it is called the policeman of the abdomen. So one of the functions of the greater omentum is fat deposition as it contains variable amounts of adipose tissue. Macrophages contained within milky spots of the greater omentum help fight infections of the peritoneum and it also may physically limits the spread of intraperitoneal infections. The greater omentum often adheres to inflamed visceral organs within the abdomen which is thought to limit the spread of infection. And the last two functions are why it’s commonly called the police officer of the abdomen.

Now for the blood supply. The blood supply of the greater omentum is derived from these arteries just here – the right and the left gastroepiploic arteries. And these arteries run along the greater curvature of the stomach to anastomose with one another within the anterior sheet of the greater omentum. They give off many omental branches, which we can see the start of just here that travel the length of the greater omentum supplying it with oxygenated blood. The venous drainage of the greater omentum mirrors that of the arterial supply draining into the gastroepiploic veins.

Let’s move on now to look at some relations or associated structures of the greater omentum, because knowing the relations of the greater omentum is really important in clinical practice when palpating the abdomen and even more so in abdominal surgery.

So let’s go back to our original image and start with the liver – specifically, the right lobe of the liver. So superior to the greater omentum, we can see the right lobe of the liver, and the right lobe of the liver is located in the right upper quadrant of the abdomen, which is also known as the right upper hypochondrium. If we look at the liver on its own, we can clearly see that the right lobe is the larger of the two lobes.

To the left-hand side of the right lobe is, of course, the left lobe of the liver. The left lobe is located in the epigastrium and towards the upper left hypochondrium, and as we just mentioned, the left lobe is the smaller of the two lobes. Note that there are two further accessory lobes of the liver – the caudate lobe and the quadrate lobe. And these lobes arise from the right lobe and are located on the visceral surface of the liver, therefore, we can’t see them in these images.

Let’s have a look at some peritoneal ligaments associated with the liver starting with the falciform ligament of the liver. And as you can see, these ligaments separates the right and the left lobes of the liver and connects the liver to the anterior abdominal wall. To the right of the falciform ligament, we can see another peritoneal ligament, which is the round ligament of the liver, and this is also commonly known as the ligamentum teres.

The round ligament of the liver is a cord-like ligament that is actually the remnant of the fetal left umbilical vein which normally deteriorates soon after birth. So if we take a look at the liver on its own, we can see the falciform ligament here and at its space or free edge, we can see the round ligament.

Tucked under the right lobe of the liver, we can see this little structure here, which is the gallbladder. If we reflect the liver upwards, we can see this pouch-like organ a little bit better, and the gallbladder stores bile, which is produced by the liver.

The next structure we’re going to talk about is the stomach, and the stomach is part of the GI tract and is involved in digestion. And as we mentioned earlier, it is also the site of attachment for the anterior sheet of the greater omentum.

Let’s move on to look at the large intestine starting with the ascending colon. The ascending colon is the second part of the large intestine and it comes after the cecum, which we can see if we look at the large intestine in isolation. As we can see from our first image, it is located to the right of the great omentum and descends from the lower right quadrant of the abdomen to the upper right quadrant.

If we dissect away the greater omentum and some of the abdominal organs, we can also see the right colic flexure. And the right colic flexure is the transition point between the ascending colon and the transverse colon. So, let’s look at the transverse colon.

The transverse colon is the third part of the large intestine and we can see it here highlighted in green. As we mentioned earlier, it is the site of attachment for the posterior sheet of the greater omentum. While we’re here, let’s talk about the blood supply of the transverse colon, and just so you know what we’re looking at.

In our image, the greater omentum has been reflected upwards so that we can see the structures we’re about to talk about more clearly. And running within the transverse mesocolon – a fold of peritoneum that connects the transverse colon to the posterior wall of the abdomen – we can see the middle colic artery. And the middle colic artery is a branch of the superior mesenteric artery and it supplies the transverse colon. Immediately inferior to the middle colic artery is the middle colic vein – a tributary of the superior mesenteric vein – and the middle colic vein drains the transverse colon and follows the path of the middle colic artery.

Now back to the large intestine. If we look to the left of the greater omentum, we can just about see a structure highlighted in green which is in continuation with the transverse colon. This structure is named the left colic flexure. So, in the following illustration, the greater omentum and several organs have been dissected away to give us a better view of the left colic flexure and this is also known as the splenic flexure because as you can see, this flexure is in close proximity to the spleen. The left colic flexure is a point at which the transverse colon turns downwards to become the descending colon.

So, let’s have a look at the descending colon. Again, we’ve reflected the omentum upwards so that we can see the descending colon, which is the fourth part of the large intestine and it descends from the left upper quadrant of the abdomen to the left lower quadrant. And note that the large intestine consists of five parts, and the fifth part is known as the sigmoid colon, which we can see here.

Now, let’s move on and talk about the small intestine, starting with the jejunum.

So as we mentioned earlier, the greater omentum floats on the surface of the intestines, so we have to reflect it upwards to see this structure. The jejunum is continuous with the duodenum which is the first part of the small intestine, therefore, it is the second part of the small intestine, and as we can see, it occupies the central part of the abdominal cavity. Next, we can see the third and final part of the small intestine which is the ileum. The ileum connects to the large intestine via the ileocecal junction.

Inferior to the ileum and moving downwards towards the lower abdomen, we can see the urinary bladder. And the urinary bladder stores urine transported by the kidneys before it leaves the body via the urethra.

So before we go on to our clinical notes, I have one more structure that I must mention. If we have a greater omentum, then we must, of course, have a lesser omentum. So, the lesser omentum is a double layer of visceral peritoneum and it attaches from the lesser curvature of the stomach and the proximal part of the duodenum to the liver.

Okay, time for some clinical notes on the greater omentum.

An omentectomy refers to the complete or partial surgical removal of the omentum, and this surgery is usually recommended to patients if cancer cells have been invaded or metastasized to the greater omentum. Most commonly, these cancers have originated in the stomach, the ovaries or in the large intestine, and as the metastases develop, the greater omentum becomes thickened, and in radiology, this is referred to as an omental cake. And that’s definitely not something you’d want to serve out what it is.

Okay, thanks for sticking with me. We’ve now reached the end of our tutorial. Before we bring in to a close, however, let’s, of course, quickly summarize what we’ve learned today.

So, today, we looked at the greater omentum and we talked about its structure and how it attaches to the greater curvature of the stomach and to the transverse colon. We then talked about its function as the police officer of the abdomen. We talked about its blood supply which is derived from the right and left gastroepiploic arteries. Then we talked about some associated structures including the liver, the stomach, and the large and small intestine, and finally, we concluded our tutorial with some clinical notes on the greater omentum.

So that brings us to the end of our tutorial on the greater omentum. I hope you enjoyed it. Thanks for watching. Happy studying!

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