Peritoneum

• The embryology of the alimentary canal means that it can be divided into 3 segments: the foregut (containing the oesophagus and pharynx as well), the midgut, and the hind gut. The division of these segments are determined by their arterial blood supply:

  • The foregut receives blood from the coeliac trunk.
  • The midgut receives blood from the superior mesenteric artery.
  • The hindgut receives blood from the inferior mesenteric artery.
• In the embryo, the gut tube is suspended in the middle of the abdominal cavity by 2 double-folds of membrane called mesenteries.

  A mesentery is a double-fold of membrane supporting the gut. It usually contains fat, nerves, and blood vessels.

  In the embryo, there is a dorsal mesentery that attaches the gut to the posterior wall, and a ventral mesentery that attaches the gut to the anterior wall.
• The dorsal mesenteries initially line the entire alimentary tract, but as the tract elongates and twists, some sections of the dorsal mesenteries blend into the parietal peritoneum of the posterior wall and become lost.
• Organs that keep their dorsal mesenteries are known as peritoneal organs. Organs that don't keep their dorsal mesenteries are known as retro-peritoneal organs.
• There are 2 types of retro-peritoneal organs: primary retro-peritoneal organs, and secondary retro-peritoneal organs.
• Primary retro-peritoneal organs are abdominal organs that developed behind the peritoneal cavity and never had a mesentery. Examples of primary retro-peritoneal organs are: the kidneys, suprarenal glands, and the major blood vessels (aorta and inferior vena cava).
• Secondary retro-peritoneal organs are abdominal organs that begun as intraperitoneal organs, but had their mesenteries absorbed after being pushed back to the posterior abdominal wall. Examples of secondary retro-peritoneal organs are the pancreas, duodenum, the ascending colon, and the descending colon.

  Growth of the liver caused the pancreas and duodenum to be pushed back to the posterior wall. Gut rotation caused the ascending and descending colons to be pushed back to the posterior wall.
• Retro-peritoneal organs can be partially covered by visceral peritoneum.
• (In the developing embryo), the mesenteries of the stomach in the upper abdominal cavity are called mesogastria. The mesogastria are known as the ventral mesogastrium and the dorsal mesogastrium. In the ventral mesogastrium is the developing liver. In the dorsal mesogastrium, there is the developing pancreas and spleen. On the posterior wall itself, there are the two kidneys.
• As development progresses, the liver grows at a very fast rate, and grows more to the right side than the left. The liver is also still connected to the anterior abdominal wall by the falciform ligament. The expansion of the liver forces the other organs (the stomach and spleen) to the left; these organs are still connected to each other by their respective ligaments.
• As the liver adopts its position of the right side of the abdominal cavity, the right hand side of the peritoneal cavity is pushed backwards behind the stomach and lesser omentum; this becomes cavity behind the stomach becomes known as the lesser sac.

  The left side of the peritoneal cavity expands and becomes the greater sac.
• The greater and lesser sacs are spaces between the parietal and visceral peritoneal membranes. The lesser sac is located behind the stomach behind the lesser omentum, and the greater sac and is the rest of the space. There is a connection between the greater and lesser sacs, which is through the epipolic foramen. The epipolic foramen is a hole located just under the right free edge of the lesser omentum.
• During development, the rotation of the midgut and foregut is driven by the expansion of the liver. The stomach for instance, is pushed to the left and rotated.

  Before these changes, the stomach has its dorsal and ventral mesogastria. On the left and right sides of the stomach are the left and right vagus nerves respectively.
• During the changes that occur during development, the stomach is then pushed to the left, and rotates on a longitudinal plane and antero-posterior plane. The stomach rotates longitudinally 90 degrees clockwise. This causes the right side of the stomach to become posterior, and the left side of the stomach to become anterior (hence the left and right vagus nerves supply the anterior and posterior abdominal walls respectively.

  During this rotation, one side of the stomach grows at a faster rate than the other side. This results in the greater and lesser curvatures of the stomach.
• The greater and lesser omenta are attached to the greater and lesser curvatures of the stomach respectively.
• The greater omentum hangs down in front of the intestines. It's attached to the greater curvature of the stomach and on route, also fuses with the transverse colon to form the gastrocolic ligament. The greater omentum (and lesser omentum) are full of fat and contain many blood vessels.
• The greater omentum localises infections, trapping the pathogens and preventing them from spreading through the abdominal cavity. It is able to do this by sticking to the infected region(s). The greater omentum is also higher vascular, and it's through this vasculature that the greater omentum can bring lymphocytes to the infected region.
• The greater omentum is originally formed from 4 layers/2 mesenteries. One is the mesentery associated with the stomach, and the other is a mesentery associated with the pancreas. The inner membrane of these 2 mesenteries form the peritoneal lining of the lesser sac.
• To form the greater omentum, the inner layers of the mesenteries fuse together inferiorly at the omental bursa, and also expand. As they expand, they fuse with the membranes of the transverse colon, forming the transverse mesocolon.

  Once the inner mesentery membranes fuse together, the space and the omental bursa disappear, and the gastrocolic ligament is formed between the greater omentum and the transverse colon. (Surgeons can gain access to the lesser sac by lifting up the greater omentum and cutting through the gastrocolic ligament).
• The parietal peritoneum lines the abdominal cavity.

  The visceral peritoneum lines the organs and is continuous with the parietal peritoneum at the front and back. The visceral peritoneum also connects abdominal organs to each other. These connections are called ligaments.
• The falciform ligament connects the parietal peritoneum to the front of the liver.
• The gastrohepatic ligament is a ligament that connects the liver to the lesser curvature of the stomach. The gastrohepatic ligament is also known as the lesser omentum.
• The gastrosplenic ligament connects the left side of the stomach to the spleen.

  The lieno-renal ligament connects the spleen to the posterior abdominal wall (where the kidneys are).
• In the embryo, the dorsal mesentery runs all the way down the posterior wall of the abdomen, attaching to the entire length of the gut. As the gut tube elongates, twists, and folds, some parts of the gut tube are pushed back to the posterior abdominal wall. In these cases, there is no longer a requirement for a dorsal mesentery, and it is subsequently lost.
• The anterior mesentery forming the lesser omentum remains.

  The greater omentum is an expansion of the dorsal mesentery, and since it moves to a more anterior position, it remains.

  The mesentery of the jejunum and the ileum remain, and this mesentery is called The Mesentery.
• • Mesenteries of the ascending colon (part of the midgut) and the descending colon (part of the hindgut) are lost.
  • The caecum usually loses its mesentery, but is highly variable.
  • The transverse colon remains suspended by the transverse mesocolon, and becomes fused with the greater omentum via the gastrocolic ligament.
  • The sigmoid colon retains its mesentery: the sigmoid mesocolon.
• Fluid movements within the greater sac of the peritoneal cavity are limited by the attachments of the mesenteries, and by the spread of infections within the sac.
• The greater sac is divided into 2 compartments by the transverse mesocolon. These compartments are called: the supra-colic compartment, and the infra-colic compartment.
• The infra-colic compartment is continuous inferiorly with the pelvic cavity, so infra-colic infections can travel to and from the pelvis.
• The pelvic organs are partially covered by peritoneum from above. There are 3 pelvic organs in both sexes.
• In females, the 3 pelvic organs are the bladder, uterus, and rectum. The peritoneum drapes over all three of these organs, forming pouches between them.

  Between the bladder and uterus is the vesicouterine pouch, and between the uterus and rectum is the rectouterine pouch. The rectouterine pouch is the lowest point of the peritoneal cavity in females, and this is often where pelvic infections track down to.
• In males, the three pelvic organs are the bladder, rectum, and prostate gland. The prostate gland is below the bladder, so it isn't covered by peritoneum like the bladder and rectum. Hence, there is only the rectovesical pouch between the bladder and rectum in males.