Embryology

To fully understand the peritoneum, a basic understanding of its embryologic development is necessary.^4–7 However, to explain the complex and detailed embryologic development of the peritoneum is not the purpose of this chapter. Briefly, the primitive gut is suspended within the peritoneal cavity by a dorsal and ventral primitive mesentery, which divides the peritoneum into a right and a left cavity. Unlike the ventral mesentery, the dorsal mesentery does not stop its attachment at the stomach, but continues to connect the primitive gut to the posterior abdominal wall inferiorly. The absence of the ventral mesentery in the lower abdomen allows for communication between the left and the right abdominal cavity.

The liver develops within the ventral mesentery. The pancreas and spleen develop within the dorsal mesentery. The ventral mesentery anterior to the liver and attaching it to the anterior abdominal wall later becomes the falciform ligament, containing the umbilical vessels. The ventral mesentery between the liver and the stomach will develop into the gastrohepatic and hepatoduodenal ligaments.

Further growth, organ development, elongation, cavitation, and rotation form the adult peritoneum (Figures 32-1 and 32-2). This includes the formation of the lesser sac, which is isolated from the remainder of the peritoneum or greater sac, except at a small opening called the foramen of Winslow.

Figure 32-1 Embryologic rotation of the peritoneal structures to the eventual adult position. The liver (L) will grow and fill the right side of the peritoneal cavity. The spleen (Sp) moves similarly to the left. GHL, gastrohepatic ligament; GSL, gastrosplenic ligament; K, kidney; S, stomach; SRL, splenorenal ligament.

(Illustration courtesy of David Bier.)

Figure 32-2 Peritoneal structures in the adult, position and relationships.

(Illustration courtesy of David Bier.)

A single layer of mesothelial cells forms the peritoneum. It is separated from the submesothelial layer of connective tissue by a basal lamina. The submesothelial layer of connective tissue consists of collagen, elastic fibers, fibroblast-like cells, arteries, veins, and lymphatics.⁶ The submesothelial layer or subperitoneal space is a virtual space that allows continuity between the mesenteries, the ligaments, the abdominal wall, and the retroperitoneum, as are described further.

Individual mesothelial cells are flat, contain abundant cytoplasm, and have long slender microvilli. This is especially true for cells that cover the viscera. The microvilli enables the cells to entrap hyaluronite, which aids to lubricate organ motion and movement. The peritoneum normally also contains a small amount of sterile fluid for lubrication, bacterial defense, and fluid transport.

Visceral and Parietal Peritoneum

The peritoneum is classified as either visceral or parietal.⁸ The abdominal and pelvic walls are lined by the parietal peritoneum. This includes the anterior surface of the retroperitoneum, the abdominal wall, and the undersurface of the hemidiaphragm. Conversely, the visceral peritoneum covers the intraperitoneal organs or viscera and forms the omenta and mesenteries.

In males, the greater peritoneal cavity is a closed continuous cavity. Conversely, in females, it is discontinuous at the ostia of the oviducts, providing a communication between the peritoneal cavity and the lower pelvis, which is extraperitoneal.⁶

Double folds of the peritoneum result in formation of ligaments and mesenteries. They suspend and form the supporting structure for the peritoneal organs. For example, the mesentery suspends the small bowel within the peritoneal cavity. The mesentery serves also to carry the arteries, lymphatics, and nerves. The omenta are formed from a double fold of the visceral peritoneum that extends from the stomach (Figures 32-3 and 32-4). The peritoneal cavity is divided into various interconnecting compartments by the ligaments and mesenteries.

Figure 32-3 The peritoneal structures in midsagittal view, diagrammatic representation.

(Illustration courtesy of David Bier.)

Figure 32-4 The peritoneal structures in oblique view. Diagrammatic representation and relationship of the stomach (S), pancreas (P), and transverse colon (TC) and their attachments.

(Illustration courtesy of David Bier.)

Supramesocolic and Inframesocolic Space

The peritoneal cavity is divided into a supramesocolic and an inframesocolic space by the transverse mesocolon⁹ (Figure 32-5). The supramesocolic space is further divided into the right and left supramesocolic space by the falciform ligament. The left supramesocolic space is then further subdivided into the anterior and posterior perihepatic spaces. The right supramesocolic space also is subdivided into the anterior perihepatic space and a posterior compartment known as the lesser sac. The right and left supramesocolic space communicates via the foramen of Winslow, allowing communication between the lesser sac and the remainder of the peritoneal cavity or greater sac. There is continuity between the right paracolic gutter and the right supramesocolic space. However, the phrenicocolic ligament acts as a barrier between the left paracolic gutter and the left supramesocolic space.

Figure 32-5 A and B, Coronal view of the peritoneal spaces and attachments, including the supramesocolic and inframesocolic spaces. Note that there is no communication between the left supramesocolic and the left paracolic spaces, owing to the phrenocolic ligament.

(Illustration courtesy of David Bier.)

Important supporting ligaments in the supramesocolic space include the gastrohepatic ligament, hepatoduodenal ligament, gastrocolic ligament, gastrosplenic ligament, and splenorenal ligament (Figures 32-6 to 32-8). The ligaments are anatomically connected and continuous and their location and relationship can be identified by certain landmarks, mostly vasculature (Table 32-1).

Figure 32-6 Computed tomography (CT) with intravenous contrast. The transverse mesocolon is delineated by the middle colic artery and its branches (arrow). Note the solid enhancing mass in the right kidney from renal cell carcinoma (line).

Figure 32-7 CT with intravenous contrast. The gastrosplenic ligament is delineated by the short gastric artery (arrow) and left gastroepiploic vessels (not shown). The gastrosplenic ligament extends from the greature curvature of the fundus and upper body of the stomach to the splenic hilum.

Figure 32-8 CT with intravenous contrast. The hepatoduodenal ligament contains the portal vein (arrow), hepatic artery, extrahepatic bile duct, and nodal stations. It connects the lesser curvature of the stomach to the hepatic hilum.

Table 32-1 Supramesocolic Ligaments: Their Organ Relationship and Landmarks

The inframesocolic compartment is divided into a right and a left inframesocolic space by the obliquely oriented small bowel mesentery. The ascending colon provides the lateral border of the right inframesocolic space. The inframesocolic compartment consists of the root of the mesentery, jejunal mesentery, ileal mesentery, ascending mesocolon, descending mesocolon, sigmoid mesentery, and the pelvic floor and peritoneal folds (Table 32-2).

Table 32-2 Inframesocolic Compartment, Organ Relationship, and Landmarks

SMA, superior mesenteric artery; SMV, superior mesenteric vein.

Paravesicular Spaces

Peritoneal folds or reflections in the pelvis also result in potential spaces and compartments^10–12 (Figures 32-9 and 32-10). The urinary bladder, obliterated umbilical arteries, and inferior epigastric vessels indent upon the parietal peritoneum to form the anterior and posterior paravesicular spaces.

Figure 32-9 The paravesicular spaces in cross-sectional view. Ascites distend the pelvic cavity and help delineate the folds and spaces. The anterior paravesicular spaces are divided into the supravesicular space, medial inguinal fossae, and lateral inguinal fossae. The supravesicular space covers the dome of the urinary bladder and in between the medial umbilical folds, which is formed by indentation of the obliterated umbilical vessels. The medial and lateral inguinal folds are separated by the lateral umbilical fold, which is formed by the inferior epigastric vessels.

(Illustration courtesy of David Bier.)

Figure 32-10 Posterior paravesicular spaces. Sagittal views of the male (A) and female (B) pelvis demonstrate the peritoneal covering of the superior bladder, uterus, and upper one third of the rectum.

(Illustration courtesy of David Bier.)