Secondarily retroperitoneal organs: the ascending colon
Secondarily retroperitoneal organs, such as the ascending colon, began on a mesentery (a fold of peritoneum, the membrane lining the abdomen) that later fused with the posterior abdominal wall.
Remember
Retro means behind, and secondarily means the organ had a mesentery first and then lost it. The ascending and descending colon are the classic examples, because their mesentery fused with the back wall.
The peritoneum is the thin membrane lining the abdominal cavity. Organs relate to it in three ways. Intraperitoneal organs such as the jejunum hang inside the cavity on a mesentery, a double fold of peritoneum that carries their vessels and nerves. Primarily retroperitoneal organs, such as the kidneys, developed behind the peritoneum from the start.
Secondarily retroperitoneal organs begin as intraperitoneal organs on a mesentery. As the embryo grows, the organ is pushed against the posterior abdominal wall, and its mesentery fuses with the parietal peritoneum, which is the layer lining the wall. The organ is then fixed in place behind the peritoneum, and only its front surface is covered.
The layer formed where the two peritoneal surfaces joined is the fusion fascia, a thin plane that a surgeon can open. The ascending colon and descending colon are the classic examples. Most of the pancreas is a further example. Because its mesentery is lost, the ascending colon is less mobile than the transverse colon and sigmoid colon.
Organs that keep a mesentery stay intraperitoneal. The jejunum keeps its mesentery and stays mobile. The transverse colon and sigmoid colon hang from a mesocolon (the mesentery of the colon). The stomach is suspended by omenta (folds of peritoneum running from the stomach to other organs), and the spleen is wrapped in visceral peritoneum, the layer that covers the organs.
The kidneys never had a mesentery, so they are primarily retroperitoneal, whereas the ascending colon had one and lost it.
Key facts
- Secondarily retroperitoneal
- the organ began on a mesentery, which later fused with the posterior abdominal wall.
- Classic examples
- the ascending colon and descending colon.
- Primarily retroperitoneal
- the organ never had a mesentery, as in the kidneys.
- Fusion fascia
- the layer formed where the visceral and parietal peritoneum joined.
- Peritoneal covering
- after fusion, only the front surface of the organ is covered by peritoneum.
- Organs that keep a mesentery
- the jejunum, transverse colon and sigmoid colon each keep a mesentery of their own, so they are not secondarily retroperitoneal.
Exam traps
- The jejunum also starts life suspended by a mesentery, but it keeps that mesentery rather than losing it by fusion, so it is not secondarily retroperitoneal.
- The transverse colon keeps its own mesentery, the transverse mesocolon, so it is not one of the standard examples, even though it lies between the ascending and descending colon.
- The omenta are the stomach's own mesenteries, so it stays intraperitoneal and is not fixed to the posterior abdominal wall.
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