Abdominal Wall, Mesentery, and Peritoneal Cavity: Imaging Approach and Differential Diagnosis

Embryology and Relevant Anatomy

The fetal gut is suspended between the anterior and posterior abdominal walls by the ventral and dorsal mesenteries, which separate to enclose the developing alimentary tube. Important viscera develop within the mesentery of the caudal part of the foregut, such as the liver, pancreas, spleen, and biliary tree. The various mesenteries either regress or elongate. The dorsal mesentery lengthens with the progressive elongation of the small intestine. The ventral mesentery resorbs, which allows communication between the right and left sides of the peritoneal cavity in adults.

Variations in the complex rotation, fusion, growth, and resorption of mesenteries and the viscera that develop within them result in common variations in peritoneal and retroperitoneal spaces in adults with clinical manifestations, such as internal hernias. All peritoneal recesses potentially communicate, although adhesions and other pathologic processes may seal off loculated collections of fluid, such as infected or malignant ascites.

Peritoneal Cavity

The abdominal cavity contains all of the abdominal viscera, both intra- and retroperitoneal, and is not synonymous with the peritoneal cavity. The abdominal cavity is limited by abdominal wall muscles, diaphragm, and pelvic brim.

The peritoneal cavity is a potential space within the abdomen that lies between the visceral and parietal peritoneum. It usually contains only a small amount of fluid (for lubrication).

The peritoneal cavity is composed mostly of the greater sac (or general peritoneal cavity). The lesser sac (omental bursa) communicates with the greater sac through the epiploic foramen (of Winslow) and is bounded in front by the caudate lobe, stomach, and greater omentum and in back by the pancreas and left kidney. To the left, the lesser sac is bound by the splenorenal and gastrosplenic ligaments and on the right by the lesser omentum and epiploic foramen.

While the lesser sac is in communication with the rest of the peritoneal cavity, ascites usually does not enter into it readily. Lesser sac fluid collections usually result from a local source (e.g., pancreatitis or perforated gastric ulcer) or from generalized infection or tumor (e.g., infectious or malignant ascites).