The omentum is a large, apron-like fold of fatty tissue that hangs from the stomach and drapes over the intestines inside your abdominal cavity. It comes in two parts: the greater omentum, which is the big sheet most people mean when they say “the omentum,” and the smaller lesser omentum, which connects the stomach and the first part of the small intestine to the liver. Far from being inert padding, the omentum acts as a frontline immune organ, a wound-healing accelerator, and a metabolic player whose behavior has real consequences for conditions ranging from peritonitis to ovarian cancer.
Where Exactly It Sits
The greater omentum originates along the greater curvature of the stomach, the long outer curve on its left side, and cascades downward like a curtain in front of the small intestines. It can extend all the way to the pelvis in some people and folds back on itself before attaching to the transverse colon, the portion of the large intestine that runs horizontally across your upper abdomen. Its size varies considerably from person to person, influenced by body composition and how much fat is stored there. In thin individuals it can be nearly translucent; in others it forms a thick, fatty apron.
The lesser omentum is much smaller and sits higher up. It stretches between the lesser curvature of the stomach (the shorter, right-side curve) and the liver. Within this fold run the hepatogastric ligament, connecting the liver to the stomach, and the hepatoduodenal ligament, which connects the liver to the duodenum and carries the bile duct, the portal vein, and the hepatic artery.1PubMed Central. The vascular anatomy of the ligaments of the liver: gross anatomy, imaging and clinical applications These two omenta together form a significant portion of the peritoneal folds that organize and support the abdominal organs.
The “Abdominal Policeman”
Surgeons have called the greater omentum the “abdominal policeman” for well over a century, and the nickname is earned. When inflammation, infection, or injury occurs somewhere in the abdominal cavity, the omentum physically migrates toward the trouble spot and wraps around it. If your appendix perforates, for example, the omentum often walls off the leak, limiting the spread of contaminated material and buying time before surgery. This ability to seal off problems and control peritoneal contamination is one of the omentum’s most dramatic functions.2PubMed Central. Is omentectomy necessary in the treatment of benign or malignant abdominal pathologies? A systematic review
How it actually moves to these sites is still being studied, but the tissue’s loose, flexible structure and its rich blood supply allow it to shift around the peritoneal cavity with surprising agility. Its surface acts as a natural barrier, and its cellular machinery kicks in with lymphatic drainage and the engulfing of pathogens once it arrives at the scene.3PubMed Central. The Greater Omentum-A Vibrant and Enigmatic Immunologic Organ Involved in Injury and Infection Resolution
Milky Spots and Immune Surveillance
Scattered throughout the greater omentum are tiny clusters of immune cells called milky spots, named for their pale, whitish appearance under a microscope. These are not just random clumps of cells. They are organized immune outposts made up of macrophages, lymphocytes, and other white blood cells, and they serve as gateways for immune cells to enter and exit the peritoneal cavity.3PubMed Central. The Greater Omentum-A Vibrant and Enigmatic Immunologic Organ Involved in Injury and Infection Resolution A specialized set of immune cells patrols the peritoneal fluid and cycles through these milky spots, where they encounter bacteria or other threats and mount a response.4PubMed Central. Specialized immune responses in the peritoneal cavity and omentum
Milky spots also appear to support the production of certain immune cells beyond what you might expect from a piece of fatty tissue. Studies in mice have found that the milky spots contain not just lymphocytes and macrophages but also megakaryocytes and actively dividing neutrophil precursors, pointing to a form of blood-cell production happening right there in the omentum.5PubMed. Light and electron microscopic study of omental milky spots in New Zealand black mice, with special reference to the extramedullary hematopoiesis Related clusters called fat-associated lymphoid clusters, found in visceral fat including the omentum, help retain certain B cells in the peritoneal cavity and support their activation during immune challenges.6PubMed Central. Inflammation-induced formation of fat-associated lymphoid clusters
The recognition of omental milky spots as functional immune tissue came into focus in the 1980s, but the exact scope of what the omentum does immunologically is still being worked out.7PubMed. The omentum and omentectomy in epithelial ovarian cancer: a reappraisal. Part I–Omental function and history of omentectomy Researchers have increasingly described the omentum as a full-fledged immune organ rather than a passive layer of fat, and that reframing matters for how surgeons think about removing it.
Wound Healing and New Blood Vessels
The omentum has an unusual talent for stimulating the growth of new blood vessels, a process called angiogenesis. Surgeons noticed decades ago that placing omental tissue on ischemic or poorly healing wounds seemed to accelerate repair, and lab research has identified a likely reason: the omentum produces the highest levels of vascular endothelial growth factor (VEGF) of any tissue tested in rat models. The fat cells themselves, rather than the supporting structural cells, are the primary VEGF producers. When omental tissue is deprived of oxygen, VEGF output goes up further, which may explain why it is so effective at rescuing tissues with poor blood flow.8PubMed. Vascular endothelial growth factor is the major angiogenic factor in omentum: mechanism of the omentum-mediated angiogenesis
This revascularizing ability has pushed the omentum into research on neurological recovery as well. Its capacity to improve tissue perfusion, absorb fluid buildup, and support nerve repair has made it a subject of interest for conditions including spinal cord injury and ischemic brain disease.9PubMed Central. Greater Omentum: Multifaceted Interactions in Neurological Recovery and Disease Progression These applications are still largely experimental, but the omentum’s biological toolkit makes it a tempting material to work with.
Omental Fat and Metabolic Health
Not all body fat behaves the same way, and omental fat is a prime example of why location matters. Omental adipose tissue is metabolically more active than the subcutaneous fat beneath your skin. Proteomic studies comparing the two depots have found differences in glucose metabolism, lipid transport, protein synthesis, and markers of cellular stress, all elevated in the omental fat.10PubMed. Differential proteomics of omental and subcutaneous adipose tissue reflects their unalike biochemical and metabolic properties Lipidomic work has also revealed that omental fat in obese individuals has substantially higher cholesterol concentrations and contains cholesterol-derived signaling molecules that may drive further fat-cell development.11PubMed. Human omental and subcutaneous adipose tissue exhibit specific lipidomic signatures
This metabolic hyperactivity has a downside. Obesity is linked to chronic low-grade inflammation, and the omental fat depot is thought to be a key driver of that process.12PubMed. Inflammation is associated with a decrease of lipogenic factors in omental fat in women In people who are morbidly obese, omental tissue from those who are insulin-resistant shows significantly more macrophage infiltration and higher expression of inflammatory signaling molecules compared to omental tissue from equally obese but insulin-sensitive individuals. That difference was not seen in subcutaneous fat, underscoring that the omental depot has a distinct and outsized role in insulin resistance.13PubMed Central. Body mass index-independent inflammation in omental adipose tissue associated with insulin resistance in morbid obesity
Even at the level of individual enzymes, the two fat depots differ. Lipoprotein lipase, which helps pull fat from the bloodstream into storage, is expressed at levels roughly 20% lower in omental fat than in subcutaneous fat.14PubMed. Lipoprotein lipase steady-state mRNA levels are lower in human omental versus subcutaneous abdominal adipose tissue These molecular distinctions help explain why carrying excess weight around the midsection, where omental fat accumulates, poses different health risks than carrying it on the hips or thighs.
The Omentum and Cancer
The same immune-rich, well-vascularized environment that makes the omentum so good at fighting infection also makes it a frequent landing site for cancer cells that have broken free from their organ of origin. Ovarian cancer is the clearest example. Cancer cells shed from the ovary travel through the peritoneal fluid and preferentially home to the omentum, where omental fat cells actively promote their migration and invasion. Signaling molecules produced by omental adipocytes, including interleukin-8, help draw cancer cells in.15PubMed Central. Greater Omental Pancake Tumour due to Metastasis of Ovarian Cancer – A Cadaveric Study
When ovarian cancer extensively colonizes the omentum, the tissue transforms into a dense, tumor-laden mass commonly called an “omental cake.” The presence of omental cake significantly reduces the chances of achieving a complete surgical removal of all visible disease. Patients with this finding who do undergo successful surgery tend to require more extensive operations, including a higher number of intestinal resections.16PubMed Central. The Influence of “Omental Cake” Presence on the Completeness of Cytoreduction in Advanced-stage Ovarian Cancer Routine removal of the omentum became part of ovarian cancer surgical guidelines in the 1960s after surgeons observed how frequently it harbored metastatic disease and that removing all affected tissue improved outcomes.7PubMed. The omentum and omentectomy in epithelial ovarian cancer: a reappraisal. Part I–Omental function and history of omentectomy
Should the Omentum Be Removed During Surgery?
Given how many useful jobs the omentum performs, surgeons have been re-examining whether it should be removed as a matter of routine in operations where that has historically been standard. In gastric cancer surgery, for instance, a meta-analysis comparing omentum removal against preservation in advanced cases found no significant differences in overall survival, relapse-free survival, or rates of peritoneal recurrence. The only measurable advantage of preservation was less intraoperative bleeding.17PubMed. Omentectomy vs omentum preservation for advanced gastric cancer: A systematic review and meta-analysis Individual studies have suggested the preserved omentum may help reduce complications after stomach surgery, possibly by maintaining its immune and barrier functions in the abdominal cavity, though these trends have not reached statistical significance in larger pooled analyses.18Scientific Reports. Long-term outcomes of omentum-preserving versus resecting gastrectomy for locally advanced gastric cancer with propensity score analysis
The picture is different in ovarian cancer, where omentectomy remains standard because the tissue so reliably harbors metastatic disease. But even here, the tradeoff between removing a cancer reservoir and losing an immune organ is acknowledged. The decision hinges on the specific cancer, its stage, and how the surgeon weighs the risks.
Surgical Reconstruction With Omental Flaps
The omentum’s generous blood supply and flexible structure have made it valuable as a reconstructive tool in its own right. Surgeons can detach a piece of omentum while keeping its blood supply intact and relocate it to cover wounds, fill defects, or provide a vascular bed for healing. This technique, called an omental flap, has been used across multiple surgical specialties. In breast reconstruction after mastectomy, the omental flap has gained research interest as an alternative tissue source, and it has also been explored for treating post-mastectomy lymphedema through vascularized lymph-node transfer.19PubMed Central. Current Research on the Use of the Omental Flap in Breast Reconstruction and Post-Mastectomy Lymphedema: A Focus on Omental-Vascularized Lymph Node Transfer
The flap can also be harvested laparoscopically and used as a free flap, meaning it is fully detached and then microsurgically reconnected at a distant site. This approach has been applied to head, neck, and extremity wounds where a well-vascularized tissue source is needed to promote healing in areas with poor local blood supply.20PubMed. Laparoscopic Free Omental Flap for Craniofacial Reconstruction: A Video Article Demonstrating Operative Technique and Surgical Applications The omentum’s inherent angiogenic properties give it an edge over many other donor tissues for these purposes.
When the Omentum Itself Gets Sick
The omentum can develop its own problems, and because symptoms are vague, diagnosis is often delayed. Omental infarction occurs when the blood supply to part of the tissue is cut off, causing localized tissue death. This can happen spontaneously or as a result of omental torsion, where the tissue twists on itself. In one case report, a 32-year-old man presented three times over four days with worsening abdominal pain that mimicked more common causes of acute abdomen, with the location of pain shifting between visits, before imaging finally pinpointed omental infarction.21International Journal of Surgery Case Reports. Diagnosis and management of idiopathic omental infarction: A case report
Most omental infarctions resolve on their own because the tissue’s dense network of blood vessels allows collateral circulation to reroute blood flow around the blocked area.22PubMed Central. Left-sided omental infarction: a rare cause of abdominal pain, discovered by CT scan Omental torsion, where the tissue spirals and strangulates itself, is rarer and more likely to require surgery because the compromised blood flow can cause widespread tissue death.23PubMed Central. Acute Abdomen due to Primary Omental Torsion and Infarction CT scanning has become the primary tool for identifying these conditions, since the clinical symptoms alone are nearly indistinguishable from appendicitis, diverticulitis, or cholecystitis.24PubMed. Greater and lesser omenta: normal anatomy and pathologic processes
The Omentum in Tissue Engineering
Researchers are exploring the omentum not just as a surgical tool to move around the body but as a source of raw materials for regenerative medicine. The tissue can be decellularized, meaning its living cells are stripped away while the underlying three-dimensional scaffold of connective tissue is preserved. These scaffolds retain the structural architecture and could potentially be repopulated with stem cells for use in reconstructive procedures.25European Journal of Histochemistry. Decellularized omentum as novel biologic scaffold for reconstructive surgery and regenerative medicine
In one line of research, stem cells derived from human omental tissue were coaxed into becoming insulin-producing cells that showed markers of pancreatic beta cells. When transplanted into diabetic animal models, these cells survived, engrafted, and produced insulin in response to glucose levels for over four weeks.26PubMed Central. Self-organized insulin-producing β-cells differentiated from human omentum-derived stem cells and their in vivo therapeutic potential Work like this is still at the animal-model stage, but it illustrates the breadth of what researchers think omental tissue might eventually contribute to medicine, well beyond its traditional role as abdominal padding.