The greater omentum is a significant and complex structure within the abdominal cavity, known for its large, apron-like appearance. It hangs from the greater curvature of the stomach and extends down to cover the small intestines before folding back upon itself to ascend to the transverse colon, connecting eventually to the posterior abdominal wall. This unique arrangement allows the greater omentum not only to protect the abdominal organs but also to store fat deposits, provide insulation, and contain immune cells that help fight infection.
Unlike the lesser omentum, which is smaller and extends from the liver to the lesser curvature of the stomach, the greater omentum is substantial in size and has more extensive protective and immunological functions. The term “epiploic” relates to the greater omentum and is derived from the Greek “epipleein,” meaning to float or sail over, which vividly describes how the greater omentum overlays the intestines.
Functionally, the greater omentum is involved in several critical bodily processes. It plays a role in immune responses by isolating infected areas and forming adhesions to wall off infections from the rest of the body, a process known as “peritoneal defense mechanism.” Additionally, it acts as a fat reservoir, contributing to energy storage and metabolic regulation.
The greater omentum’s ability to move and mold itself around the intra-abdominal organs also aids in the healing of injured areas within the abdomen by adhering to and protecting the sites of inflammation or injury. Moreover, due to its rich blood supply and lymphatic vessels, it assists in the transport of lymph and the clearance of cellular debris.
Location
The greater omentum is located in the abdominal cavity. It originates from the greater curvature of the stomach and the beginning part of the duodenum. From there, it hangs down like an apron in front of the intestines. After extending downwards, it folds back on itself and attaches to the transverse colon. This placement allows the greater omentum to cover the small intestines like a protective layer. The area between where the greater omentum attaches to the stomach and where it attaches to the transverse colon encompasses its specific location within the abdominal space. This unique positioning enables the greater omentum to move freely within the abdominal cavity, allowing it to contribute to various physiological functions, including immune responses and abdominal organ protection.
Structure
The structure of the greater omentum is intricate and multifunctional, consisting primarily of a sheet-like, apron-like fold of visceral peritoneum, which is a thin layer of tissue forming the lining of the abdominal cavity:
Layers
The greater omentum is made up of four layers of peritoneum. These layers result from the double sheet folding back on itself, creating a large, fatty apron. These four layers are not distinct and uniform; they can vary in thickness and fat content based on the individual’s age, nutritional status, and overall health.
Blood Supply and Lymphatics
The omentum is richly supplied with blood vessels and lymphatics. The blood vessels, including arteries and veins, are primarily branches from the gastroepiploic arteries, which run along the greater curvature of the stomach. This extensive vascular network supports its roles in immune response, fat storage, and injury repair. The lymphatic vessels and nodes within the omentum contribute to the body’s immune defense by filtering pathogens and facilitating the immune response.
Fat Deposition
The greater omentum is known for its fat deposits. These deposits can vary significantly between individuals and serve multiple purposes, including energy storage, insulation, and protection. The amount of fat can influence the omentum’s buoyancy and positioning within the abdominal cavity.
Connective Tissue
The layers contain connective tissue that helps in holding the cells together and provides structural support. This connective tissue also contains macrophages and other immune cells that play a crucial role in the body’s immune response, especially in the context of abdominal infections or inflammations.
Omental Milieu
Within the layers of the greater omentum, there exists a unique microenvironment or ‘milieu’ that is conducive to the proliferation of immune cells. This environment aids in the omentum’s ability to act as an immune organ, responding to infections and facilitating healing processes.
Flexibility and Movement
The greater omentum is notably flexible and mobile, which allows it to wrap around or adhere to organs in response to injury or infection. This movement is crucial for its role in isolating infected areas, preventing the spread of infections, and supporting the healing process.
Adipose Tissue
The adipose tissue within the omentum not only serves as an energy store but also secretes various cytokines and hormones involved in metabolic and immune responses.
Function
The greater omentum serves several crucial functions in the body, including:
- Protection: It acts as a physical barrier, protecting the abdominal organs underneath. By draping over the intestines and other abdominal structures, it helps shield them from injury and infection.
- Immune Response: The greater omentum contains a significant amount of lymphoid tissue, which produces white blood cells and antibodies. This makes it an essential part of the body’s immune system. It can identify and respond to infections, isolating affected areas to prevent the spread of infection within the abdominal cavity.
- Fat Storage: The omentum stores fat, which can serve as an energy reserve. The fat within the omentum can vary in quantity among different individuals and can be metabolized by the body as needed.
- Inflammatory Response: In case of injury or infection in the abdomen, the greater omentum can adhere to the affected area, walling it off and preventing the spread of infectious agents or inflammatory processes to other parts of the body. This is known as the omental response and is a crucial aspect of the body’s defense mechanism.
- Wound Healing and Tissue Repair: The greater omentum contributes to the healing of damaged tissues within the abdomen. It can adhere to areas of inflammation or injury, supplying white blood cells and facilitating the repair process through its blood vessels and fibrous tissue.
- Insulation: By storing fat, the omentum helps insulate and maintain abdominal temperature, which is vital for the proper functioning of the organs within the abdominal cavity.
- Metabolic Functions: Besides storing fat, the greater omentum is involved in various metabolic processes, including the regulation of insulin sensitivity and the metabolism of lipids and carbohydrates. It secretes hormones and cytokines that can influence the body’s metabolic rate and immune responses.
- Prevention of Peritoneal Adhesions: The omentum can help prevent the formation of adhesions (bands of fibrous tissue that can form between abdominal tissues and organs) following surgery or inflammation, thus maintaining the organs’ mobility within the abdominal cavity.
Greater omentum vs Lesser omentum
| Feature | Greater Omentum | Lesser Omentum |
|---|---|---|
| Location | Hangs from the greater curvature of the stomach and duodenum, covering the intestines. | Extends from the lesser curvature of the stomach and the first part of the duodenum to the liver. |
| Structure | Four layers of peritoneum, filled with adipose tissue, extensive blood vessels, and lymphatics. | Two layers of peritoneum, contains the hepatic artery, portal vein, and bile duct within the hepatoduodenal ligament. |
| Size and Appearance | Larger, apron-like, and fatty. | Smaller, thinner, and less fatty. |
| Functions | Protection of abdominal organs, fat storage, immune response, insulation, metabolic processes, healing promotion. | Provides a conduit for structures entering the liver, acts as a barrier between the stomach and liver. |
Clinical significance
The greater omentum has significant clinical implications due to its unique structure, functions, and responses to disease:
- Source of Infection and Inflammation: While the greater omentum aids in isolating and combating intra-abdominal infections, it can itself become a site of infection or inflammation, leading to conditions such as omentitis or primary omental torsion, where the omentum twists upon itself, causing acute abdominal pain.
- Cancer and Metastasis: The omentum can be a site for the spread (metastasis) of cancers from other parts of the abdomen, particularly ovarian, colon, and stomach cancers. Due to its extensive blood supply and lymphatic drainage, cancer cells can easily seed and grow in the omental tissue. The removal or biopsy of the omentum, known as omentectomy, is often performed during abdominal surgeries to assess or reduce the spread of cancer.
- Surgical and Therapeutic Uses: The greater omentum is utilized in various surgical procedures due to its ability to promote healing and provide vascularized tissue for repairing or covering internal organs. It can be used to cover wounds, support anastomoses (surgical connections between tissues), and treat infections or defects within the abdominal cavity, such as in the repair of abdominal wall hernias or in the management of chronic infections.
- Indicator of Metabolic Syndrome: Excessive fat deposition in the omentum is associated with metabolic syndrome, a cluster of conditions including high blood pressure, high blood sugar, excess body fat around the waist, and abnormal cholesterol levels. The size and fat content of the greater omentum can serve as an indicator of an individual’s risk for developing conditions such as type 2 diabetes, heart disease, and stroke.
- Role in Immunological Research: Due to its rich immune cell content, the greater omentum is of interest in immunological research, particularly in understanding how the body’s immune system responds to abdominal infections, injuries, and malignancies. Studies of the omentum may lead to new therapeutic strategies for treating abdominal diseases and enhancing immune responses.
- Adhesions and Complications: Postoperative adhesions involving the greater omentum can lead to complications such as chronic abdominal pain, bowel obstruction, and difficulty during subsequent surgeries. Understanding the omental response and how to manage it is crucial in abdominal surgical procedures.
- Omental Infarction: Although rare, omental infarction is a condition where part of the omentum becomes ischemic (loses blood supply) and necrotic (dies), leading to acute abdominal pain. It requires prompt medical attention to prevent further complications.
