Omentum Explained

Overview

The omentum is often referred to as the “policeman of the abdomen,” forms a protective covering around the peritoneal cavity ensuring that the areas are protected. Previously, it was believed to be redundant fat that was placed over the intestines, however, in recent times surgeons’ attitude towards omentum has changed. It is now considered as an individual organ with a wide spectrum of functions, including its ability to inhibit the spread of sepsis such as in peritonitis, and acts as a source of angiogenic factors actively involved in tissue repair. It has also been identified as a potential source of stem cells which may play an important role in tissue engineering and in vascular grafts synthesis.

Structure

The omentum was first recognised during Egyptian times and was called as great epiploon by Aristotle. The embryonic development takes place from the dorsal mesogastrium. It is divided anatomically into two main components, namely greater and lesser omentum. The greater omentum is formed by a double-layered peritoneum that follows the greater curvature of the stomach, encompasses the small intestine and then overlaps on itself to join the peritoneum in the anterior surface of the transverse colon. The lesser omentum forms the boundary of the lesser sac at the anterior portion, extending between the lesser curvature of the stomach and liver.

Ultrastructure

The omentum consists of two mesothelial sheets comprising adipocytes along with loose connective tissue. Additionally, there is an accumulation of mononuclear phagocytic cells. Microscopic examination reveals that it is composed of two unique tissue types, an adipose rich area and a thin fenestrated translucent area. The main role of the translucent areas has not been completely established, although it is believed to be actively involved in the transport of fluid and solute. On the other hand, the adipose area of the omentum contains milky spots or “taches laiteuses”. These milky spots were described by Ranvier and play a crucial role in the clearance of bacteria and promote the maturation and proliferation of macrophages and B cells.

Illustration of the greater and lesser omentum. Image by ASCCC OERI

Function

Omentum has the ability to adhere to certain intra-abdominal foreign bodies including drains and catheters. Therefore, patients who require placement of catheters in the abdominal cavity for a long duration such as those requiring peritoneal dialysis can have drainage problems as the omentum may occlude the tubes. It has been observed that removal of omentum significantly minimized the catheter blockage and improved drainage.

The omentum plays several important roles during episodes of peritonitis. It maintains the integrity of the peritoneal cavity by promoting the clearance of bacteria and foreign agents. The macrophages especially B lymphocytes located within the milky spots act as the principal site for phagocytosis. This in turn removes the bacteria and other harmful agents from the peritoneal cavity.

The omentum promotes angiogenic activity in the adjoining structures wherever it is applied. Studies have shown that lipid component derived from the omentum activates angiogenesis and is available abundantly. It has been reported that human omental microvascular endothelial cells (HOME cells) induce the release of the ‘basic fibroblast growth factor’, an important angiogenic peptide.  Hence, the omentum is actively engaged in the process of neovascularization and serves as a source of vascular supply in areas that are ischaemic or inflamed tissue.

Neurovascular Supply

The omentum consists of sympathetic nerve fibres. The nerve fibres are present around small blood vessels, and less frequently individual fibres can be detected between lymphoid cells. The omental milky spots (OMSs) located within the greater omentum play a crucial role in regulating the peritoneal immune homeostasis. The sympathetic nervous system and neurotransmitters (noradrenaline) regulate the key neural processes.

The right, left and middle omental arteries form the main blood supply of the omentum. These arteries originate from the right and left gastroepiploic arteries. The anterior surface is supplied by the larger right omental artery, whereas the posterior surface is supplied by the smaller left omental artery.

Real picture of the vasculature to the omentum of a mammal. Image by Léodras

Clinical Relevance and Associated Diseases

Omentum may be the site causing tumours. Occasionally, primary tumours of the omentum may occur. It is one of the main sites leading to the metastasis of carcinomas of the stomach, ovaries, and colon. In the case of ovarian epithelial carcinomas, omentectomy is often performed in order to prevent local recurrence.  Omentectomy is also suggested for carcinomas that undergo metastasis through the peritoneal cavity. Omental adipose tissue may increase the level of circulating glucocorticoids which may contribute to obesity and insulin resistance. This is referred to as the “Cushing’s disease of the omentum.”

Recent research has highlighted that omentum is a source for several important components such as adipokines (leptin, RANTES), and resistin. This may help to establish a relationship between metabolic dysfunction and intra-abdominal obesity.

References

Logmans A, Schoenmakers CH, Haensel SM, et al. High tissue factor concentration in the omentum, a possible cause of its hemostatic properties. Eur J Clin Invest 1996;26:82–83. doi:10.1046/j.1365-2362.1996.107247.x.

Bikfalvi A, Alterio J, Inyang AL, Dupuy E, Laurent M, Hartmann MP, et al. Basic fibroblast growth factor expression in human omental microvascular endothelial cells and the effect of phorbol ester. J Cell Physiol 1990;144:151–158. doi:10.1002/ jcp.1041440120.

Singh AK, Patel J, Litbarg NO, Gudehithlu KP, Sethupathi P, Arruda JA, Dunea G, et al. Stromal cells cultured from omentum express pluripotent markers, produce high amounts of VEGF, and engraft to injured sites. Cell Tissue Res 2008;332:81–88. doi:10.1007/s00441-007-0560-x.

Shimotsuma M, Simpson-Morgan MW, Takahashi T, Hagiwara A. Activation of omental milky spots and milky spot macrophages by intraperitoneal administration of a streptococcal preparation, OK432. Cancer Res 1992;52:5400–5402.

Florey H, Walker JL, Carleton HM. The nature of the movement of the omentum. J Pathol Bacteriol 1926;29:97–106. doi:10.1002/path.1700290111.

Oloumi MM, Derakhshanfar A, Molaei M, Tayyebi M. The angiogenic potential of autogenous free omental graft in experimental tibial defects in rabbit: Short-term preliminary histopathological study. J Exp Anim Sci 2006;43(3):179–187. doi:10.1016/j.jeas.2006.02.002.

Matoba Y, Katayama H, Ohami H. Evaluation of omental implantation for perforated gastric ulcer therapy: findings in a rat model. J Gastroenterol 1996;31:777–784. doi:10.1007/ BF02358602.

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