omentum

**Greater omentum** - Structure: 4-layered apron of peritoneum (two folded double layers) hanging from the greater curvature of the stomach and first part of the duodenum, over the transverse colon and small intestines. Fat-rich. - Subdivisions/ligaments: gastrocolic (stomach to transverse colon), gastrosplenic (stomach to spleen, contains short gastric and left gastroepiploic vessels), gastrophrenic (stomach to diaphragm). - Blood supply: right and left gastroepiploic (gastro-omental) arteries. The right arises from the gastroduodenal artery (from the celiac trunk). The left arises from the splenic artery. They anastomose along the greater curvature. - Function: immune and protective roles (milky spots with macrophages and lymphoid tissue), fat storage, and thermoregulation/cushioning. It migrates to sites of inflammation and walls off infection (e.g., appendicitis, diverticulitis), hence "abdominal policeman." It also has angiogenic properties. - Clinical uses/associations: omental flaps (e.g., for sternal wound, perineal, or perforated ulcer repair, "Graham patch"). Omental caking, seen with peritoneal carcinomatosis (ovarian, GI, e.g., ovarian cancer), appears as a thickened omentum on CT. Omental infarction is a mimic of appendicitis or cholecystitis. Omental metastases can occur in many cancers. It can be a site of herniation or a lead point for adhesions and small-bowel obstruction. Omental torsion is rare.

**Lesser omentum** - Structure: 2-layer peritoneal fold from the lesser curvature of the stomach and the proximal duodenum to the liver (fissure for ligamentum venosum). - Parts: hepatogastric ligament (contains the left and right gastric arteries) and hepatoduodenal ligament (free edge, contains the portal triad: hepatic artery proper (anterior-left), common bile duct (anterior-right), and portal vein (posterior)). - Pringle maneuver: compressing the hepatoduodenal ligament to control hepatic bleeding. If bleeding continues, suspect hepatic veins/IVC injury or an accessory/replaced left hepatic artery from the left gastric artery. - Epiploic foramen (of Winslow): the opening posterior to the free edge of the hepatoduodenal ligament, connecting the greater sac to the lesser sac (omental bursa). Boundaries: anterior = hepatoduodenal ligament, posterior = IVC, superior = caudate lobe of liver, inferior = first part of duodenum. - Lesser sac: posterior to the stomach and lesser omentum. Fluid collections from pancreatitis (pseudocysts) and perforated posterior gastric ulcers can localize there.

**Embryology** - Both omenta derive from the ventral and dorsal mesogastrium. The greater omentum derives from the dorsal mesogastrium, and the lesser omentum from the ventral mesogastrium. Rotation of the stomach during development creates the lesser sac.

**High-yield pearls** - Greater omentum: gastroepiploic arteries. Lesser omentum: gastric arteries (right and left), plus the portal triad. - Greater omentum is intraperitoneal "mobile" tissue that localizes infection. - Pringle maneuver and the epiploic foramen of Winslow are classic exam tie-ins. - Posterior gastric ulcer perforation can bleed into the lesser sac or erode the gastroduodenal or splenic artery.