Corpus: Peritoneum
from ancient Greek: περί- ("perí-") - around, around; τόνος ("tónos") - taut or περιτόνιον ("peritóneion") - peritoneum
1. Definition
The peritoneum is a serous membrane lining the peritoneal cavity. It is composed of two layers that allow smooth movement of the abdominal organs.
2. Anatomy
2.1. Classification
The peritoneum has two layers:
- Parietal peritoneum: lines the abdominal wall
- Visceral peritoneum: covers parts of the abdominal organs
These two layers are connected by mesenteries. In adults, the total surface area of the peritoneum is between 1.6 and 2.0 m². In the lesser pelvis, it is referred to as the urogenital peritoneum.
2.2. Innervation
Just below the diaphragm the parietal peritoneum is innervated by the phrenic nerve and other segmental spinal nerves, providing somatosensory innervation. The visceral peritoneum receives sensory input from visceral afferent fibers that accompany sympathetic nerves, with no somatosensory innervation, except in the liver and gallbladder.
2.3. Special structures
The peritoneum forms five longitudinal folds on the anterior abdominal wall:
- Left and right lateral umbilical folds
- Left and right medial umbilical folds
- Median umbilical fold along the center
Additionally, there are several peritoneal pockets between the abdominal wall, organs, and mesenteries.
2.4. Topography
Several anatomical spaces are defined relative to the peritoneum. The peritoneal cavity itself, which is enclosed by the parietal sheet, is also known as the intraperitoneal space. The extraperitoneal space, which is located outside the peritoneum. Accordingly, we speak of intraperitoneal or extraperitoneal abdominal organs. The extraperitoneal space is further divided into three parts:
- Retroperitoneal space: located dorsal of the parietal peritoneum, containing structures such as the abdominal aorta and kidneys
- Preperitoneal space: located between the parietal peritoneum and the anterior abdominal wall
- Subperitoneal space: located beneath the peritoneum, housing structures like the urinary bladder, rectum, prostate, cervix, and vagina
3. Histology
The peritoneum, like other serous membranes, consists of a secretory and absorptive mesothelium (surface layer) known as the lamina epithelialis serosae. The mesothelial cells are connected by adhesion and tight junctions and have an apical surface densely covered with microvilli, increasing surface area. Beneath lies the lamina propria serosae, a thin layer of connective tissue. Under the microscope, only the nuclei of the mesothelial cells are usually visible in standard staining.
A third tissue layer, called the subserosa, contains fat cells, blood and lymphatic vessels, and pain receptors (nociceptors).
Unique to the peritoneum are milky spots, aggregates of immune cells, including macrophages, B and T lymphocytes, and isolated mast and plasma cells. These immune clusters are located in the lamina propria and are especially common in the greater omentum.
4. Physiology
The peritoneum secretes and reabsorbs peritoneal fluid, a viscous liquid that reduces friction, allowing smooth movement of intraperitoneal organs. Under physiological conditions, the fluid volume is about 50–70 ml.
The peritoneum also has an immunological role. The mesothelium in the milky spots is permeable, allowing direct contact between immune cells and peritoneal fluid in the lamina propria. The immune cells can recognize and ingest antigens. These antigens are subsequently removed through fine lymphatic vessels. Macrophages can also migrate from the milky spots into the peritoneal cavity.
5. Clinic
Leakage from the gastrointestinal tract or other sources can cause painful inflammation of the peritoneum, known as peritonitis.
Excessive production of peritoneal fluid results in ascites. The peritoneum’s large surface area and absorptive capability are used in peritoneal dialysis.
Malignant tumors may spread within the peritoneal cavity, leading to peritoneal carcinomatosis. Rarely, a primary cancer can arise from the peritoneum itself, known as primary peritoneal carcinoma (PPC).