Greater and lesser omentum

Peritoneum

The abdominal wall and the abdominal organs are lined by a thin serous membrane called the peritoneum. Peritoneum consists of two layers; parietal peritoneum that covers the abdominal wall, and visceral peritoneum which covers the viscera. There is a potential space between these two layers called the peritoneal cavity.

Depending on how they are lined with visceral peritoneum, abdominal organs are divided into intraperitoneal or retroperitoneal. Intraperitoneal organs are completely wrapped in peritoneum. Their peritoneal covering is continuous with peritoneal folds called mesenteries. A mesentery suspends an intraperitoneal organ from the abdominal wall and carries its neurovascular bundle. Mesenteries are named according to the organ they suspend. For example, the mesentery of the transverse colon is called the transverse mesocolon. Retroperitoneal organs are located posteriorly to the peritoneum, and only their anterior surfaces are covered with peritoneum. They do not have a mesentery.

You can learn more about the peritoneum and peritoneal relations using this collection of helpful anatomical resources.

Peritoneal cavity

As stated above, the peritoneal cavity is the space between the parietal and visceral peritoneal layers. It is divided into two spaces that communicate with each other; the greater sac and the lesser sac (omental bursa). The greater sac is the larger of the two spaces. It extends from the diaphragm to the pelvic cavity. It is divided into two compartments by the transverse mesocolon. 

• The supracolic compartment, which is above the transverse mesocolon, contains the stomach, liver and spleen. 
• The infracolic compartment lies below the transverse mesocolon and mostly contains the small and large intestines. 

The omental bursa, also called the lesser sac, is located posteriorly to the stomach and the lesser omentum (which will be discussed later), and anteriorly to the pancreas and duodenum. The function of the omental bursa is to allow unrestricted movement of the stomach. The greater sac and omental bursa communicate with each other through an opening called the omental foramen (also called the foramen omentale or epiploic foramen).

If you would like to learn about the more intrinsic details of the peritoneum and peritoneal cavity with its recesses, you can use this series of anatomical resources.

Omenta

Omenta are the fused peritoneal folds that connect the stomach and duodenum with other abdominal organs.  There are two omenta, the greater omentum and the lesser omentum.

The greater omentum attaches the stomach to the transverse colon. The lesser omentum attaches the stomach and the duodenum to the liver. The omenta are derived from the embryological ventral and dorsal mesenteries. The greater omentum is derived from the dorsal mesentery, while the lesser omentum originates from the ventral mesentery.

Lesser omentum

The lesser omentum extends from the lesser curvature of the stomach and duodenal bulb (first part of duodenum) to the liver. One of its roles is to separate the greater sac from the omental bursa. The lesser omentum consists of two ligaments: medially located hepatogastric ligament, and laterally located hepatoduodenal ligament. The hepatogastric ligament connects the lesser curvature of the stomach to the visceral surface of the liver. The hepatoduodenal ligament passes from the duodenal bulb towards the visceral surface of the liver. It ends as a free border, forming the anterior margin of the omental foramen, The hepatoduodenal ligament carries the portal triad (hepatic portal vein, proper hepatic artery and bile duct). 

The lesser omentum transports the arteries for the lesser curvature of the stomach; the right and left gastric arteries. The left gastric artery (one of the three branches of the celiac trunk) passes into the lesser omentum and travels downwards along the lesser curvature of the stomach in a left to right direction. The right gastric artery branches from the proper hepatic artery (also a branch of the celiac trunk). It ascends along the lesser curvature of the stomach in a right to left direction to form an anastomosis with the left gastric artery. 

The right and left gastric veins follow the same direction as the arteries through the lesser omentum. They drain directly into the hepatic portal vein. The visceral branches of the vagus nerve and the hepatic and gastric lymph nodes also pass through the lesser omentum.

Embryology

The development of the greater omentum is quite a complex process and can be difficult to understand. During development, the stomach is suspended from the dorsal wall by the dorsal mesentery and from the ventral wall by the ventral mesentery.

As the organs grow, their position changes in the abdomen. The stomach rotates 90 degrees in its longitudinal axis so that it’s left side faces anteriorly and its right side faces posteriorly. As a result, the dorsal mesentery is displaced to the left and becomes folded, and the ventral mesentery is moved to the right. This rotation forms a separate space behind the stomach, the omental bursa. The stomach then rotates a second time, and here the cranial end moves inferiorly and the caudal end moves superiorly, giving the stomach its final position in the abdomen. This second rotation causes the double folded dorsal mesentery to move downwards, becoming the greater omentum. Later, it fuses anteriorly with a layer of peritoneum hanging from the lesser curvature of the stomach, and posteriorly with the mesentery of the transverse colon, to become the familiar apron like structure that is characteristic of the greater omentum.

The development of the lesser omentum is a little more straight-forward. It is formed from the ventral mesentery (which is derived from the septum transversarum). During rotations of the stomach, the ventral mesentery is moved to the right hand side and then superiorly. As the liver grows into the ventral mesentery, it divides it into the falciform ligament and the lesser omentum.

Learn more about this topic with our article on the development of the digestive system.