What is the Difference Between Omentum and Peritoneum

The main difference between omentum and peritoneum is that omentum is an abdominal structure formed from the visceral peritoneum with a structure similar to the mesentery whereas peritoneum is the thin, serosal membrane, which lines the abdominal and pelvic cavities, covering most of the viscera. Omentum consists of four or two layers of the visceral peritoneum, while peritoneum is a double-layer structure composed of a layer of mesothelium and a thin layer of connective tissue. 

Basically, omentum and peritoneum are two types of membranous structures that occur in the abdominal cavity, mainly providing support to the internal organs. Greater omentum and lesser omentum are the two types of omenta while parietal peritoneum and visceral peritoneum are the two layers of peritonea. 

Key Areas Covered 

1. What is Omentum
     – Definition, Anatomy, Function
2. What is Peritoneum
     – Definition, Anatomy, Function
3. What are the Similarities Between Omentum and Peritoneum
     – Outline of Common Features
4. What is the Difference Between Omentum and Peritoneum
     – Comparison of Key Differences

Key Terms 

Greater Omentum, Lesser Omentum, Omentum, Parietal Peritoneum, Peritoneum, Visceral Peritoneum

What is Omentum 

Omentum is an abdominal structure derived from the visceral peritoneum, extending from the stomach and the proximal part of the duodenum to the other organs of the abdomen. Generally, the two main types of omenta are the greater omentum and the lesser omentum.  

Greater Omentum 

The greater omentum consists of four layers of the visceral peritoneum. It originates from the greater curvature of the stomach and the proximal part of the duodenum. It is attached to the anterior surface of the transverse colon by folding back up. Moreover, the ‘abdominal policemen’ refers to the greater omentum due to its role in immunity, preventing visceral infections. The three components of the greater omentum include the gastrocolic ligament, which is the largest component, the gastrosplenic ligament that extends up to the hilus of the spleen, and the gastrophrenic ligament. 

Figure 1: The Greater Omentum

Lesser Omentum 

The lesser omentum, which is smaller than the greater omentum, consists of two layers of visceral peritoneum. It originates from the lesser curvature of the stomach and to the proximal part of the duodenum. And, at the end, it is attached to the liver. Furthermore, the two components of the lesser omentum include the gastrohepatic ligament, which is flat and broad, and the hepatoduodenal ligament, which is the free edge. Moreover, the gastrohepatic ligament is attached to the left lobe of the liver, while the hepatoduodenal ligament contains the portal triad; the portal vein, hepatic artery, and the common bile duct. 

Figure 2: The Lesser Omentum

What is Peritoneum 

The peritoneum is the serous membrane which covers the peritoneal cavity. Therefore, it serves as a continuous membrane, covering the abdominal viscera, providing support and pathways for blood and lymphatic vessels to pass in and out from the viscera. However, the peritoneum is a double-layered structure consisting of a mesothelial layer made up of simple squamous epithelium and a thin connective tissue layer. Usually, the two types of peritonea are the parietal peritoneum and the visceral peritoneum. 

Parietal Peritoneum 

The parietal peritoneum is the part of the peritoneum lining the internal surface of the abdominopelvic wall. Since it develops from the somatic mesoderm, the parietal peritoneum receives the same somatic nerve supply. The parietal peritoneum is sensitive to pressure, laceration, pain, and temperature. Meanwhile, its pain is a well-localized signal.

Figure 3: Parietal and Visceral Peritonea

Visceral Peritoneum 

Visceral peritoneum is the second part of the peritoneum, covering the outer layer of most of the abdominal viscera. However, it develops from the splanchnic mesoderm and has the same autonomic nerve supply. It is sensitive to stretch and chemical irritation while producing poorly localized pain.  

Peritoneal Cavity 

The peritoneal cavity is the space between the parietal and visceral peritonea. It is filled with a small amount of lubricating fluid. Furthermore, depending on the location, the abdominal viscera has two divisions; they are the intraperitoneal and retroperitoneal organs. Typically, the intraperitoneal organs occur inside the peritoneal cavity such as the stomach, liver, spleen, etc. while the retroperitoneal organs occur outside the peritoneal cavity. Moreover, the primary retroperitoneal organs occur completely outside of the peritoneal cavity; for example, esophagus, kidney, rectum, etc. Secondary retroperitoneal organs, on the other hand, become retroperitoneal with the fusion of the mesentery to the posterior abdominal wall. For example, the ascending and descending colon. 

Similarities Between Omentum and Peritoneum 

• Omentum and peritoneum are two types of membranous structures found in the abdominal cavity. 
• Also, they are made up of two layers: one layer of mesothelium and a thin layer of connective tissue. 
• Moreover, their main function is to support internal organs while keeping them in place. 
• They also store fat, support blood vessels, lymph vessels, and nerves in the abdomen. 

Difference Between Omentum and Peritoneum 

Definition 

Omentum refers to a fold of visceral peritoneum connecting the stomach with other abdominal organs. Meanwhile, the peritoneum refers to the serous membrane, which lines the cavity of the abdomen, covering the abdominal organs. 

Correspondence 

The omentum is a structure derived from the visceral peritoneum, whereas the peritoneum is the serous membrane, which lines the abdominal and pelvic cavities. 

Structure  

Moreover, omentum may contain four or two layers of visceral peritoneum with prominent patched of fat, and thus, giving a lace-like appearance. Meantime, the two layers of the peritoneum are a mesothelium and a connective tissue layer.  

Types 

The two types of omentum are the greater omentum and the lesser omentum, while parietal peritoneum and visceral peritoneum are the two types of peritonea. 

Origin 

Greater omentum origins from the greater curvature of the stomach while lesser omentum origins from the lesser curvature of the stomach. In comparison, parietal peritoneum lines the abdominal and pelvic cavities while visceral peritoneum covers the external surfaces of most of the internal organs.  

Function 

While omentum supports the intestines and the liver, peritoneum supports intraperitoneal organs. 

Conclusion 

Basically, the omentum is an abdominal structure derived from the visceral peritoneum. The greater omentum and the lesser omentum are the two types of omenta; both of them mainly originate from the stomach. In addition, they form a lace-like structure, supporting the intestine and the liver respectively. The peritoneum, on the other hand, is the serous membrane that supports the intraperitoneal organs located in the peritoneal cavity. The parietal and visceral peritoneum are the two types of peritonea. Basically, parietal peritoneum lines the wall of the abdomen and pelvis while visceral peritoneum covers the outer surfaces of most of the internal organs. Therefore, the main difference between omentum and peritoneum is their correspondence, structure, and function. 

References:

1. O’Neill, Katie. “The Peritoneum.” TeachMeAnatomy, 1 Oct. 2018, Available Here.

Image Courtesy:

1. “Sobo 1909 564” By Dr. Johannes Sobotta – Atlas and Text-book of Human Anatomy Volume III Vascular System, Lymphatic system, Nervous system and Sense Organs (Public Domain) via Commons Wikimedia   
2. “Lesser omentum EN” By Olek Remesz – Own work (CC BY-SA 3.0) via Commons Wikimedia  
3. “General Distribution of the Peritoneum” By Dennis M DePace – Own work (CC BY-SA 4.0) via Commons Wikimedia