What Is the Greater Omentum and What Does It Do?

The greater omentum is a large, apron-like fold of fatty tissue that hangs from the stomach and drapes over the intestines inside your abdominal cavity. Far from being inert padding, it is one of the most immunologically active structures in the body, housing specialized clusters of immune cells, producing growth factors that drive blood vessel formation and wound repair, and physically migrating toward sites of infection or injury to wall them off. Surgeons have long called it “the policeman of the abdomen” for this reason, and modern research keeps revealing new layers to its biology.

Where It Sits and What It Looks Like

The greater omentum attaches along the bottom curve of the stomach and the first part of the small intestine, then cascades downward like a curtain before folding back up to connect with the transverse colon. In a lean adult it can be surprisingly thin and translucent in places; in someone carrying more weight, it may be several centimeters thick with stored fat. Its total area is large enough to cover most of the intestines when spread flat.

Under a microscope, the omentum is made of two distinct tissue types. Some regions are rich in fat cells, well supplied with blood vessels, and covered by a continuous layer of mesothelial cells. Other regions are translucent and membrane-like, poorly supplied with blood vessels and perforated by small openings of varying size.¹ The translucent zones are thought to help with fluid and particle transport across the peritoneal cavity, while the fatty zones do much of the heavier immunologic and metabolic work.²

The Milky Spots That Power Its Immune System

Scattered throughout the omentum’s fatty tissue are clusters of immune cells called milky spots, named for their pale, whitish appearance. These are not standard lymph nodes. They sit right at the surface of the omentum, exposed to the fluid that bathes the abdominal cavity, and they act as collection points for antigens, bacteria, and debris floating in that fluid.³ Once milky spots capture foreign material, they can launch a range of immune responses, from inflammation to tolerance, depending on what they encounter.

Research has shown that milky spots function as secondary lymphoid organs in their own right, independently supporting both B cell and T cell responses to things encountered in the peritoneal space.⁴ They regulate immune cell trafficking, promote the self-renewal and differentiation of immune cells, and support local antibody production.⁵ In short, the omentum runs its own mini immune surveillance network, separate from but complementary to the lymph nodes and spleen.

How the Omentum Moves Toward Trouble

One of the omentum’s most remarkable behaviors is its ability to physically relocate within the abdominal cavity. When an organ becomes inflamed, when a surgical wound begins to heal, or when a perforation leaks gut contents, the omentum creeps toward the problem site. It adheres to the inflamed surface, seals off the area, and begins deploying immune cells and growth factors. Surgeons performing repeat operations routinely find the omentum stuck to the site of a previous incision or a healing ulcer, even when no one placed it there.

Exactly how it “knows” where to go is still debated. A recent anatomical study proposed that the omentum’s blood vessels act as a kind of hydraulic system: when inflammatory chemical signals reach the omentum, vessels on the side closest to the trouble engorge with blood, physically extending the tissue in that direction.⁶ Whatever the mechanism, the result is well documented. The metabolic activity of milky spots engaged in an inflammatory response increases ten- to twenty-fold, and both the number and size of the spots grow substantially.⁶ Animal studies confirm that the omentum acquires special properties under pathological conditions, including the ability to fuse with traumatic surfaces, stop bleeding, absorb fluid and microparticles, and launch a targeted immune response.⁷

A Powerhouse for Growing New Blood Vessels

The omentum is one of the body’s richest sources of vascular endothelial growth factor, the key protein that drives the formation of new blood vessels. Among all the tissues tested in one study, omental fat cells produced the highest concentration of this growth factor and secreted it at the highest rate.⁸ When tissue is starved of oxygen, the omentum ramps up production even further, which helps explain why surgeons have long placed omental grafts on ischemic wounds and seen new blood vessels sprout.

When the omentum is activated by injury or a foreign body, the transformation is dramatic. The non-fat portion of the tissue, concentrated in milky spots, can expand fifteen- to twenty-fold. Growth factor levels rise two to four times, blood vessel density triples, and the blood content of the tissue doubles.⁹ The activated omentum essentially converts itself into a regenerative toolkit loaded with growth factors and progenitor cells ready to repair damaged tissue.

Its Role in Metabolism and Obesity

The omentum is also a metabolically active fat depot, and its behavior changes with body weight. Omental fat is classified as visceral fat, the kind that accumulates around internal organs rather than under the skin. Visceral fat is more metabolically dangerous than subcutaneous fat, and central obesity driven by omental fat accumulation plays a direct role in the development of metabolic syndrome.

Omental fat cells secrete a variety of signaling molecules called adipokines. In people with obesity, the production of several inflammatory adipokines ramps up significantly. One study identified six adipokines from omental fat tissue whose secretion was elevated in obesity, including chemokines and inflammatory molecules linked to features of metabolic syndrome.¹⁰ Separately, omental tissue produces a unique adipokine called omentin that is found almost exclusively in visceral fat, with over 150-fold less expression in subcutaneous fat.¹¹ Omentin appears to help regulate insulin sensitivity, which makes the omentum a direct participant in how your body handles blood sugar, not just a passive storage site for calories.

Why Cancers Spread to the Omentum

The omentum’s rich blood supply, immune cell populations, and growth factor production make it a hospitable environment for healing. Unfortunately, those same properties can also make it a welcoming destination for cancer cells. Ovarian, gastric, and endometrial cancers in particular tend to seed the omentum when they spread through the abdominal cavity. On imaging, diffuse tumor infiltration of the omentum shows up as what radiologists call an “omental cake,” where the normal fatty tissue is replaced by a thickened mass of tumor.¹²

In one large series of over 800 patients who had their omentum removed during endometrial cancer surgery, about 6% had omental metastases. Among those patients, roughly two-thirds also had cancer in the lymph nodes and adnexal structures, suggesting that omental spread tends to accompany widespread disease.¹³ This is one reason the omentum is frequently removed during cancer surgeries: it is a common early stop for metastasizing tumor cells, and the stage of the disease may change based on what is found there.¹⁴

The Debate Over Removing the Omentum During Surgery

Because of its tendency to harbor metastatic disease, total omentectomy has been a standard part of many abdominal cancer operations, especially for ovarian and gastric cancers. But given everything the omentum does for immunity, wound healing, and fluid management, surgeons have increasingly questioned whether routine removal is always necessary.

For gastric cancer, the evidence is tilting toward preservation when the disease is not obviously invading the omentum. A meta-analysis comparing gastrectomy with and without omentectomy found no significant difference in five-year survival, relapse-free survival, recurrence rates, or complications. Meanwhile, preserving the omentum reduced operating time and blood loss.¹⁵ In one large series, only about 2% of patients undergoing radical gastrectomy had metastatic lymph nodes in the omentum, and their presence correlated with advanced disease markers.¹⁶ In other words, patients with early-stage disease rarely benefit from losing the omentum, and they do lose its protective functions. The conversation in surgical oncology is shifting toward more selective removal based on disease stage rather than reflexive omentectomy.

The Omentum as a Surgical Repair Tool

Surgeons have borrowed omental tissue for reconstructive procedures for decades, precisely because of its blood-vessel-growing and wound-healing properties. A piece of omentum, still connected to its blood supply or transplanted as a free flap, can be placed on a wound or defect where it promotes healing, fights infection, and helps new tissue grow.

The applications are surprisingly diverse. Omental flaps have been used to cover wounds and promote blood flow in the head and neck, extremities, chest wall, and abdominal cavity.¹⁷ In skull base surgery, laparoscopically harvested omental flaps have been used to reconstruct defects after tumor removal, with a reported flap survival rate of about 92% and no abdominal complications in one series.¹⁸ In breast reconstruction after mastectomy, the omentum offers an option for patients who do not qualify for more traditional tissue-donor flap techniques, with evidence suggesting it produces natural-appearing results.¹⁹

The omentum’s dense network of lymph nodes also makes it valuable for treating lymphedema, the chronic swelling that can develop after lymph node removal during cancer treatment. Transferring a vascularized piece of omentum carrying its lymph nodes to the affected limb can restore some lymphatic drainage. In one case of upper limb lymphedema, omental lymph node transfer reduced arm circumference by over 40% above the elbow and over 35% below the elbow within eight months.²⁰ Robotic-assisted harvesting of omental flaps is making this less invasive over time.²¹

When the Omentum Itself Gets Sick

The omentum can develop its own problems, though they are uncommon. Omental infarction occurs when blood flow to part of the tissue is blocked, usually because the omentum twists on itself. Torsion can happen on its own without any obvious trigger, or it can be caused by adhesions, hernias, or tumors acting as a pivot point.²² In rarer cases, infarction can occur without any torsion at all, possibly related to obesity, abdominal trauma, or blood vessel disease.

The symptoms are maddeningly nonspecific: localized abdominal pain, most often on the right side, developing over a few days.²³ Because it mimics appendicitis, diverticulitis, and other acute abdominal conditions, a CT scan is usually needed to pin down the diagnosis. The reassuring news is that the omentum’s abundant blood supply typically allows new connections to form around the blockage, so most cases resolve without surgery.²⁴

Emerging Research in Transplant Medicine

Some of the most forward-looking research on the omentum involves using it as a platform for cell transplantation. The idea is straightforward: the omentum’s rich blood supply, growth factor production, and immunologic environment make it an attractive site to implant living cells and have them survive. For people with type 1 diabetes, the omentum is being explored as an alternative location for transplanting insulin-producing pancreatic islet cells, which are traditionally infused into the liver.²⁵

In animal studies using both rats and primates, islets implanted onto the omentum within a biodegradable scaffold successfully engrafted, developed new blood vessel networks, and restored improved metabolic function.²⁶ The omentum’s ability to rapidly vascularize transplanted tissue is the key advantage here. Islets need a blood supply to sense glucose and release insulin, and the omentum provides that vascular scaffolding more readily than most other implantation sites. Clinical trials in humans are ongoing, and if the approach works, it could change the landscape of diabetes treatment by giving transplanted cells a better home than the liver’s congested blood vessels.

How the Omentum Changes With Age

The omentum is not a static organ. Its structure shifts meaningfully over a lifetime. In infants and young children, the omentum has a much denser network of blood vessels than it does in adults. One histological study found that omental blood vessel density was dramatically higher in children under two years old compared to older age groups, declining steeply by school age and remaining relatively low through adulthood.²⁷ This high early vascularity may explain why the omentum is particularly active in young children, who rely more heavily on innate peritoneal immune defenses before their adaptive immune systems are fully developed.

In adults, the omentum’s composition shifts toward more fat storage, especially with weight gain. The balance between its immune and metabolic functions tilts as a person ages and gains visceral fat, which may partly explain why omental fat becomes more inflammatory in older, heavier adults. Research into whether age-related changes in the omentum affect its surgical utility is still limited, but surgeons generally find that the tissue remains viable for flap reconstruction and other procedures well into older age.

The Omentum in Other Species

If you have a dog or cat, they have a greater omentum too, and veterinary surgeons use it in much the same way as their human counterparts. In dogs, the omentum is expansive and covers the intestinal loops thoroughly. In cats, it tends to be smaller. Structurally, the same two-zone architecture seen in humans, translucent membranes and fat-rich regions, appears across mammals.²⁸ Veterinary surgeons use omental flaps to wrap intestinal anastomoses, reinforce bladder repairs, and promote healing of abdominal wounds in companion animals. Much of the foundational research on omental biology, including early work on brain revascularization using omental grafts, was done in dog models.²⁹ The conservation of this organ’s structure and function across species underscores that it is not an evolutionary afterthought but a tissue with real survival value.