The size of the omentum varies from 300 to 2000 g with a surface area of 300–1500 cm². The greater omentum is a large apron-like fold of visceral peritoneum that hangs down from the stomach. It extends from the greater curvature of the stomach, passing in front of the small intestines, and reflects on itself to ascend to the transverse colon before reaching to the posterior abdominal wall. Since the greater omentum appears to float on the surface of the intestines, it is the first structure observed when the abdominal cavity is opened. The omentum has a rich vascular supply with numerous characteristic capillary convolutions which are termed omental glomeruli due to their similarity to renal glomeruli. These capillary beds lie directly under the mesothelium (Fig. 42.1).

The greater omentum is a primitive part of the gastrointestinal system containing a vast network of blood vessels and lymphatics. The omentum performs a number of functions during episodes of peritonitis. The first of these is the rapid absorption and clearance of bacteria and foreign material from the peritoneal cavity. The omentum is the only site, other than the diaphragmatic stomata, that has a documented ability to absorb particles from the peritoneal cavity.

The second function of the omentum is to supply leukocytes to the peritoneal cavity. In experimental animals with peritonitis, the omentum appears to be the principal site by which firstly macrophages and then neutrophils migrate into the peritoneal cavity.

The third function of the omentum is to adhere and seal off areas of contamination. The omentum can rapidly produce a layer of fibrin by which it adheres to the contaminated area at the point of contact.

The human omental microvascular endothelial cells (HOME cells) express the angiogenic peptide basic fibroblast growth factor. This process of neovascularization allows the omentum to provide vascular support to adjacent tissues such as the gut and promote function and healing in ischemic or inflamed tissue. Even if deprived of its own blood supply, it might survive by attacking arteries in the tissue in its vicinity, and Vineberg and Lwin [6] demonstrated in dogs that the free omental strips, within 3–8 days, had obtained a new blood supply from its vicinity. Free omental grafts, however, have a variable fate, they may gain a viable blood supply from the vicinity as stated earlier, or they may undergo complete infraction and necrosis. Infection greatly reduces the chance of the successful take of the free omental graft.

In the immediate operative period, a free omental graft behaves to some extent as an inert membrane, in that the biological defenses contained in the peritoneal adhesions may be denied adequate early access to the immediate area of operation, e.g., a suture line.

The result of the experimental work by Carter et al. [7] has shown that though a free omental graft may be used successfully, the pedicle omentum gives superior results, and its biological role is more reliable.

In a similar study, experimental and clinical results reported by Agarwal and Agarwal [8] were encouraging. In this clinical study, in 50 cases of critical ischemic limbs due to Buerger’s disease , allografts were taken from patients undergoing laparotomy for any other diseases. Simultaneously experimental study was done in 20 dogs in two groups: Group I was without ligation of femoral artery in ten dogs, and Group II was studied with ligation of femoral artery in another ten dogs.

Results of clinical and experimental studies showed that even mismatched graft is taken up and revascularizes the ischemic limbs .

Recent studies have revealed that the omentum, apart from being a great source of various growth factors, neurotransmitters, neurotrophic factors, and inflammatory mediators, also contains omnipotent stem cells that can differentiate into a variety of cell types, this omnipotent stem cell having the power to migrate (Solvason and Kearney [9]). In a study, it was found that the human fetal omentum, liver, and spleen are sites of B cell generation, but it was also demonstrated that the pro/pre-B cell compartment (CD24+, sIgM−) is detected in the omentum and liver but not in the spleen as early as 8 weeks of gestation. From 8 to 12 weeks of gestation, the proportions of IgM+ cells that were pre-B cells (cIgM+/sIgM−) in the omentum and liver were 53 ± 15 % and 45 ± 13 %, respectively, and IgM+ cells were not detectable in the spleen. After 12 weeks, the percentage of pre-B cells was unchanged in the fetal liver (41 ± 10 %) but decreased significantly in the omentum (25 ± 14 %); Pre-B cells were now detected in the spleen but at much lower percentages (2 ± 3 %) than either the omentum or liver. The nuclear enzyme, Tdt, was detected in approximately 25 % of the CD24+ cells in the omentum and liver during the 8–12-week time period; however, Tdt+ cells were not detected in the spleen. Approximately 40 % of the mature B cells found in the omentum and spleen were CD5+ compared with only 20 % in the liver. These results demonstrate that the fetal omentum, like the fetal liver and bone marrow, is a primary site of B cell development.

Solvason and Kearney [10] further studied the relationship between the omentum and liver during the development of Ly-1 B cells. The most obvious relationship between these two sites is that cells simply migrate from one location to the other, that is, precursor cells may migrate from the fetal liver into the fetal omentum and in this milieu give rise to exclusively Ly-1+ B cells or the sister population

García-Gómez et al. [11] studied angiogenic capacity of human omental stem cells. Human omental CD34+ cells were obtained from samples of human omentum by density gradient centrifugation in Ficoll. Proliferative pattern, marker expression (by flow cytometry), and angiogenic growth factor synthesis by omental cell cultures were determined.

In vivo angiogenic capacity of the cells was evaluated in rats. Omental stem cells showed a high rate of proliferation (Ki67 staining), expressed CD34 marker, and synthesized bFGF and VEGF. When implanted in rats, omental cells promoted neovascularization. Human omental cells were localized in rat tissue, mainly forming the endothelium of neo-vessels. Implantation of omental cells also facilitated angiogenesis of rat origin.

It was concluded that CD34+ cell population of human omentum could be responsible for the clinical benefit of omental transplantation by promoting angiogenesis and synthesizing angiogenic growth factors to facilitate revascularization of injured tissue .

These revascularizing properties of the omentum learned over the years were based on clinical and experimental studies. The exact mechanism of action has eluded the surgeons for many years. It appears to be answered by these recent studies, which showed that the omentum is full of omnipotent stem cells having power to migrate.

With the development and refinement of microvascular surgery, most of the ischemic tissue and organs are salvaged by direct reconstructive surgery. But many times, we are faced with situation where, either due to technical difficulty or complications like graft failure, these ischemic tissues and organs is lost.

In such situations, we have used the properties of the omentum by developing a simple technique to lengthen it and mobilizing it extra-abdominally. This technique can be used in all parts of the body from the scalp to the toe, to revascularize in end-stage ischemic limbs in Burger’s disease, where direct revascularization surgery is not feasible and amputation has to be done eventually [12]. We have been able to salvage limbs in 85 % cases in 15 years of follow-up [13].

Hoshino et al. [14] described the mobilization of the omentum from greater curvature of the stomach, and later Talwar et al. [15] described the technique of its elongation. Several techniques are available to mobilize and lengthen the omentum to reach inguinal region or mid-thigh. We have simplified the technique (Agarwal technique) to lengthen the omentum up to 1 m long with upper end attached to stomach.

The blood supply to the omentum comes mainly from the right and left gastroepiploic arteries which join along the greater curvature of the stomach to form the gastroepiploic arterial arch. The three major omental arterial trunks—the right, middle, and left—arising from this gastroepiploic arch form vascular arcade in the free border of omentum Barlow’s arcade which is not constant, and there are several variations. In some cases, it is not found on the free border of omentum but much higher which makes mobilization of omentum difficult. All the arteries are accompanied by veins (Fig. 42.2).

The abdomen is opened by left paramedian or median incision. The omentum is made free from its attachment to the colon and the stomach preserving the gastroepiploic arch. Attachment to the colon is avascular and easy to separate. To separate from the stomach, the ascending branches from the gastroepiploic arch to the stomach need to be divided. Depending upon the caliber, right or left gastroepiploic artery is divided. The omentum is then completely freed except for one pedicle attached to the stomach (Fig. 42.3). The omental graft is further designed using a cotton tape to get enough length to reach the ankle or wrist (Fig. 42.4).

In end-stage ischemic extremities, the lengthened omentum is brought out of the abdomen by an incision, lateral to deep inguinal region for lower extremity (Fig. 42.5) and near xiphisternum for the upper extremity, eye, and scalp (Fig. 42.6), and the abdomen is closed.