Abdominal Organs and Peritoneum
The following is a summary of the peritoneal relationships ofl the abdominal organs and a brief explanation of their developmental origins.
The following descriptions apply to abdominal organs, but not pelvic organs. Abdominal organs in the adult are found either attached to the posterior wall of the abdomen, posterior to the peritoneum, or enclosed within folds of peritoneum inside the peritoneal cavity. Structures of the urinary system (kidneys and ureters), the suprarenal glands, the great vessels of the abdomen and their paired branches all developed on the posterior wall of the abdomen. They were never enclosed in peritoneum at any point in their development (although their anterior or superior surfaces may be covered with parietal peritoneum. These structures are primarily retroperitoneal (meaning behind the peritoneum). They developed posterior to the peritoneum originally and remain there in the adult body form. Their positions are fixed. The kidneys have their own fascial coverings independently from the peritoneum.
The remainder of abdominal organs are either parts of the gut tube or derivatives of the gut tube with one exception, the spleen. All of these organs (including the spleen) originally developed enclosed within folds of peritoneum (mesentery). In the adult form, they separate into two categories. Most parts of the gut tube, derivatives of the gut tube and the spleen are enclosed within folds of peritoneum in the adult form. They are intraperitoneal. They receive their blood and nerve supply and lymphatic drainage from the abdominal wall between the folds of peritoneum. They are potentially mobile within the folds of peritoneum.
During development, the gut undergoes several rotations and much growth. In the process, some of the parts of the gut tube (enclosed within their peritoneal folds) are folded into contact with the posterior abdominal wall. Where the surface of the organ that is covered with visceral peritoneum folds such that one surface is in contact with the posterior wall, covered with parietal peritoneum, the peritoneal membranes that are in contact will be absorbed and disappear. Now the organ is covered over with a single layer of peritoneum, attached to the posterior body wall. It is retroperitoneal, but only became that way through development; it is secondarily retroperitoneal. Organs that developed as intraperitoneal and became retroperitoneal secondarily include the pancreas, parts 2, 3 and 4 of the duodenum and the ascending and descending colons. Their positions are fixed on the abdominal wall. All other parts of the digestive tract (including the liver, gallbladder and associated ducts) and the spleen, remain intraperitoneal in the adult.