D. Holt , K.A. Agnello , in Feline Soft Tissue and General Surgery, 2014
Surgical anatomy
The peritoneum is a closed cavity that contains all of the abdominal organs except for the kidneys and the adrenal glands. The parietal peritoneum covers the abdominal wall and diaphragm. The visceral peritoneum covers the abdominal organs (Fig. 26-1). The peritoneum consists of a single layer of mesothelial cells covering a basement membrane.1 The peritoneal mesothelial cells have the same embryological origin as vascular endothelial cells2 and produce surfactant that acts as a lubricant.3 The basement membrane is a fibroelastic tissue containing glycosylated proteins, mast cells, macrophages, and lymphocytes and it covers a well-defined network of elastic fibers. The basement membrane is absent in parts of the diaphragm, omentum, and mesentery.1 The absence of the basement membrane in the diaphragm allows large gaps between the mesothelial cells ('stomata') to communicate directly with underlying lymphatic channels, facilitating absorption of fluid and particulate matter.
Peritoneum, Retroperitoneum, Mesentery, and Abdominal Cavity
Bevin Zimmerman , in Boorman's Pathology of the Rat (Second Edition), 2018
Abstract
The peritoneum is comprised of a single layer of mesothelial cells that resides on a basement membrane adjacent to a thin layer of delicate fibrous connective tissue. The peritoneum lines the abdominal cavity, reflects to cover the abdominal organs and mesentery, and separates the abdominal organs from the contents of the retroperitoneal space. Although seldom the primary site of toxicity, alterations to the peritoneum, retroperitoneum, or mesentery may be indicative of, or secondary to, changes that occur within the organs they line. The most commonly induced lesion of the peritoneum is the mesothelioma of tunica vaginalis.
Andrew J. Dart , Hannah-Sophie Chapman , in Robinson's Current Therapy in Equine Medicine (Seventh Edition), 2015
Anatomy and Physiology
The peritoneum is a single layer of squamous mesothelial cells resting on a loose connective tissue containing blood vessels, lymphatics, and nerves. Anatomically, the peritoneum is divided into a parietal and visceral peritoneum. The parietal peritoneum lines the diaphragm, abdominal walls, and pelvic cavity. The parietal peritoneum is continuous with the visceral peritoneum, which encloses the intraperitoneal organs and forms the omentum and mesenteries of the abdominal cavities. A small volume of peritoneal fluid lubricates the surface of the visceral and parietal peritoneum. Together the peritoneum and fluid are responsible for preventing adhesion formation. Normal peritoneal fluid is a transparent straw-colored ultrafiltrate of plasma with a total protein (TP) concentration of less than 1.5 g/dL (15 g/L) and total nucleated cell count (TNCC) of less than 2000 cells/µL (2 × 109 cells/L). The distribution and consistent turnover of peritoneal fluid ensures a highly effective clearance mechanism for bacteria, cells, and foreign material entering the peritoneal cavity. Neutrophils represent 24% to 60% of the cells found in peritoneal fluid. Protein concentrations greater than 2.0 to 2.5 mg/dL (20 to 25 g/L) and TNCCs greater than 5000 to10,000 cells/µL (5 to 10 × 109 cells/L) are considered abnormal.
Norm G. Ducharme , ... Ava M. Trent , in Farm Animal Surgery (Second Edition), 2017
Histology
The peritoneum is a serosal membrane that consists of a single layer of mesothelial cells and is supported by a basement membrane. The mesothelial cells are normally squamous in shape and have cilia that trap cellular products to help maintain the necessary gliding surfaces. The layer is attached to the body wall and viscera by a glycosaminoglycan matrix that contains collagen fibers, vessels, nerves, macrophages, and fat cells. The parietal submesothelial layer varies in thickness and cell concentrations among species and has a moderate thickness in the cow. The visceral submesothelial layer is thin, and the visceral peritoneum (serosa) closely adheres to the underlying viscera. The parietal peritoneum can be grossly separated from the underlying muscle and fascia. Except for the peritoneum that covers omentum, the visceral peritoneum cannot be manually separated intact from the underlying viscera.
Histologic studies in laboratory animals show that junctures between peritoneal cells are tight and passive diffusion limits bidirectional passage to relatively small molecules, including water, glucose, and electrolytes. Distinct openings (stomata) up to 12 µm in diameter are present in the parietal peritoneum that covers the diaphragm. These stomata are adequately sized to allow passage of large molecules and cells, primarily to remove cells, bacteria, particles, and molecules less than 10 µm in diameter. Similar openings occur in the peritoneum that covers both sides of the omentum over focal aggregates of lymphoid tissue called milk spots. Lymphoid and myeloid originating cells move into the peritoneal cavity through these openings, and cells and particulate matter move out of the peritoneal cavity through these openings.
Linda A. Ross , Mary Anna Labato , in Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice (Fourth Edition), 2012
Biology of the peritoneal membrane
The peritoneum is the serosal membrane that lines the abdominal cavity. The portion that covers the viscera and other intraabdominal structures is known as the visceral peritoneum, and that which lines the abdominal cavity is known as the parietal peritoneum. In humans, the surface area of the peritoneum is approximately the same as the body surface area (1 to 2 m2), and the visceral peritoneum accounts for approximately 80% of the total.10 Peritoneal surface area is proportionately larger in comparison to body surface area in infants and children,11 suggesting that this difference would also be true for dogs and cats.
Anatomically, the peritoneum consists of the mesothelium and underlying interstitial tissue (Figure 28-1). The mesothelium consists of a simple squamous epithelial-like monolayer supported by a basement membrane. The mesothelial cells have many apical microvilli that increase the functional surface area of the membrane. In humans, the basement membrane contains type IV collagen, proteoglycans, and glycoproteins. The interstitium is a layer of connective tissue below the basement membrane. Found within the connective tissue are extracellular matrix molecules, including collagen, fibronectin, and elastin. This layer has a gel-like character because of the presence of various proteoglycans. The peritoneal microvasculature is composed of true capillaries and postcapillary venules, which are supported by a negatively charged glycocalyx.61 These vessels are located at various distances from the mesothelial surface and can be found throughout the connective tissue layer. Lymphatics also are found in this layer, most commonly in the subdiaphragmatic peritoneum. These lymphatics drain primarily via stomata in the diaphragmatic peritoneum.10,48 The role of lymphatics in fluid and solute exchange from the peritoneum is poorly understood because of the difficulty in directly measuring lymph flow. Lymph flow is affected more by gravity than is blood flow through vessels, and therefore the upright posture of humans versus the quadruped stance of animals may mean that the role of peritoneal lymphatics differs between species.
The most important function of the peritoneal membrane is to provide a protective, lubricating surface for the abdominal organs. Mesothelial cells secrete glycosaminoglycans including hyaluronic acid, proteoglycans such as decorin and biglycan, and phosphatidylcholine-containing lamellar bodies that allow free movement of the visceral organs during respiration and gastrointestinal peristalsis.48,61 Mesothelial cells play a role in a number of other processes, including antigen presentation, control of inflammation, tissue repair, coagulation, and fibrinolysis.13,35 It is generally believed, however, that the mesothelium does not represent a significant barrier to water transport.61 The anatomic structures that appear to play the most important role in fluid and solute transport are the walls of the capillaries and the extracellular matrix located in the submesothelial cell connective tissue.27,28,63 Peritoneal capillaries are composed primarily of nonfenestrated endothelial cells supported by a basement membrane. Endothelial cells contain aquaporins, which are 20-kDa cellular membrane proteins that are responsible for water transport. Intercellular clefts between endothelial cells also play a role in solute transport.36
Although the anatomic surface area of the peritoneum is large, the effective surface area—that area involved in fluid and solute movement—is considerably smaller. This discordance is because transport of water and solutes is primarily dependent on the surface area of peritoneal capillaries, rather than the mesothelium.3,10,36
Brina Lopez , Kira Epstein , in Comparative Veterinary Anatomy, 2022
Peritoneum
The peritoneum (Fig. 12.7-5) is a serous membrane, which lines the intraperitoneal/peritoneal cavity. Thus, the peritoneal cavity is the space within the abdominal cavity between the parietal and visceral peritoneum. There is a small volume of fluid in the peritoneal cavity normally but nothing else. Based on location, there are 2 major divisions of the peritoneum: (1) visceral peritoneum—covering the digestive and urogenital systems (tube-shaped, serous membrane covering (complete and incomplete), and reflected at various places over abdominal and pelvic viscera); and (2) parietal peritoneum—lining the wall of the abdominal cavity and includes all peritoneum not directly attached to the abdominal viscera. The parietal peritoneum is connected to the visceral peritoneum through several components, all consisting of 2 distinct layers united. These peritoneal connections include:
Figure 12.7-5. Transverse section schematic defining the peritoneum and peritoneal cavity.
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Greater omentum—connects the greater curvature of the stomach to the dorsal body wall
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Lesser omentum—connects the lesser curvature of the stomach and duodenum to the liver
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Mesoduoenum—supports the duodenum
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Mesentery of the small intestine—supports the jejunum and ileum
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Mesocolon and mesorectum—support the colon and rectum
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Medial and lateral ligaments of the bladder—support the urinary bladder; an important distinction is that the medial ligament is the caudal vestige of the ventral mesentery between the umbilicus and the urinary bladder vs. the lateral ligaments, which are folds of the peritoneum and not the round ligaments of the bladder, and are the vestiges of the umbilical arteries
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Broad ligament of the uterus, suspensory ligaments of the ovaries, mesorchium, and mesoductus deferens—support and suspend their respective reproductive structures
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Hepatorenal ligament—supports the liver and kidneys
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Triangular (paired right and left) ligaments and coronary ligament of the liver—these ligaments are peritoneum from the diaphragm to the liver; specifically, the triangular ligaments attach to the right and left lobes of the liver and the coronary ligament connects the iver to the caudal vena cava
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Falciform ligament—a vestige of the cranioventral mesentery between the umbilicus and liver; this should not be confused with the round ligament of the liver, which is enveloped by the falciform ligament and represents the rudimentary umbilical vein
The peritoneal cavity space also has several parts and pouches (diverticula), including:
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Omental bursa—a segment between the greater omentum, which communicates with the peritoneal cavity through the epiploic foramen
(the area surrounded by the caudate lobe of the liver, gastropancreatic fold, and portal vein, and a site for small intestine herniation and incarceration)
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Ovarian bursa—opens to the peritoneal cavity via the ovarian fossa (see Fig. 13.4-6A and B in Case 13.4)
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Testicular bursa—the analogous or homologous to the ovarian bursa
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Rectouterine, vesicouterine, and pubovesical pouches/spaces—diverticula (L. divertere to turn) between the rectum and uterus, uterus and bladder, and bladder and bottom of the pelvic cavity, respectively
The epiploic foramen (L. foramen epiploicum) is also called the "foramen of Winslow" (after the anatomist, Jacob B. Winslow) and is uncommonly referred to as the "aditus" (L. entrance or approach), and finally as the "foramen omentale." It is the passageway into the omental bursa from the peritoneal cavity (see Case 12.3). Entrapment and strangulation of various parts of the intestinal tract have been reported and include the ileum, jejunum, cecum, and duodenum. The entrapment is generally in a left-to-right direction (>
95%) with the rare case of right-to-left entrapment. The ileum alone, or in combination with the jejunum, is the most common piece of intestine involved. Surgery is needed to correct the herniation with resection and anastomosis of any nonviable intestine.
Takanori Sugiyama , Helen M.S. Davies , in Comparative Veterinary Anatomy, 2022
Peritoneum
The abdominal cavity is lined by a thin mesothelial layer called the peritoneum, which gives rise to the term "peritoneal cavity." However, the peritoneal space is mostly a potential space, as the parietal and visceral layers of the peritoneum are normally separated by only a thin film of fluid, produced by this serosal membrane for lubrication. In normal individuals, the abdominal contents fill the peritoneal cavity, bringing the visceral and parietal surfaces of the peritoneum in direct contact throughout most of the abdomen. In the standing dog and cat, the peritoneal space typically is widest ventrally, as peritoneal fluid naturally gravitates to the lowest part of the peritoneal cavity. In laterally or dorsally recumbent patients, the peritoneal fluid gravitates to the dependent part of the peritoneal cavity.
The parietal peritoneum is closely adhered to the innermost layer of the abdominal wall, which is the deep fascia of transversus abdominis (Figs. 5.8-4 and 5.8-5).
Figure 5.8-4. Ventral abdominal wall of a cat. The skin, subcutaneous tissue, and linea alba have been incised to reveal the thin, translucent parietal peritoneum, which lines the abdominal wall (yellow arrows). The peritoneum was inadvertently punctured in two places in this prosection, which illustrates how readily the peritoneal cavity is entered on incising the linea alba (blue arrows). Note, too, that the belly of the rectus abdominis muscle is exposed (pink arrow) in the caudal portion of the incision, which deviated slightly from midline.
Figure 5.8-5. The direction of the muscle fibers of the left lateral abdominal wall from superficial to deep. The rectus abdominis muscle is confined to the ventral region and its fiber direction is cranial to caudal. Key: m., muscle.
The parietal peritoneum (generally called just the "peritoneum") is incised during celiotomy, and a small amount of peritoneal fluid is normally found within the peritoneal cavity. Care must be taken when incising the abdominal wall because the abdominal contents lie in direct contact with the peritoneum, except when the peritoneal cavity is distended with free fluid or gas. In closing an incision or defect in the linea alba or transversus/rectus abdominis muscle, the incised or torn edge of the peritoneum may be included with the suture needle, thereby closing the peritoneum with the internal layer of the body wall, or the peritoneum may be left to heal separately.
Most of the abdominal contents are encased by the visceral layer of the peritoneum, which is a deep invagination of the parietal peritoneum, separated from the parietal surface by folds of peritoneum which suspend these organs within the abdominal cavity. The folds are variously called the mesentery (intestine), mesocolon (colon), and mesovarium (ovary). The greater and lesser omentum are also peritoneal folds. However, several abdominal organs and vessels are located dorsal or dorsolateral to the parietal peritoneum in the retroperitoneal space, which is separated from the peritoneal cavity by the parietal peritoneum. The kidneys and most of the ureters, the adrenal glands, the abdominal aorta, the caudal vena cava, and the sublumbar lymph nodes are all considered retroperitoneal structures.
Abdominal Hernias
A true hernia comprises an opening in the abdominal wall (typically one of the 5 or 6 normal openings) bounded by a distinct fibrous rim (the hernial ring), through which an outpouching of the parietal peritoneum (the hernial sac) and some type of peritoneal content (fluid, omentum, intestine, etc.) protrudes to lie external to the abdominal cavity. Such hernias usually are congenital, and are named for their location—e.g., inguinal, umbilical. Depending on the type and amount of herniated tissue and the size of the hernial ring—i.e., on the risk and consequences of incarceration and strangulation of the tissue—the hernia may be symptomatic or asymptomatic, and thus may or may not require surgical repair.
Acquired hernias, such as traumatic and incisional hernias, are not "true" hernias because they may occur anywhere in the abdominal wall and do not contain a hernial sac. The herniated abdominal contents lie within the thoracic cavity (diaphragmatic hernia) or subcutis and may even be found external to the body if the skin is also disrupted. Acquired hernias usually are symptomatic and typically require surgical repair, because the inflammatory response in the injured peritoneum and body wall may narrow the defect and result in adhesions to the herniated tissue. This increases the risk of incarceration and strangulation in a defect that normally might be large enough to avoid these complications.
Herniation of abdominal contents through the weakened "pelvic diaphragm" (perineal hernia) is a specific type of acquired hernia seen almost exclusively in adult male dogs. It is discussed in Case 5.7.
Laparoscopic Surgery in the Elephant and Rhinoceros
Mark Stetter , Dean A. Hendrickson , in Fowler's Zoo and Wild Animal Medicine, 2012
Closure
The peritoneum is closed with 0 polyglyconate suture in a simple continuous pattern. The external abdominal oblique fascia and muscle are also closed in this pattern. The skin is closed in multiple horizontal mattress sutures using no. 5 stainless steel and plastic stents in the large incision, and a single horizontal mattress suture using no. 5 stainless steel in each of the accessory portals. The two ends of the wire suture are tightly twisted together (i.e., like cerclage wire) rather than tied with a knot. Although the elephant is often laid back down on the side as soon as possible, it should be noted that it is often faster to suture the skin while the elephant is suspended from the crane truck. Once closure is complete, the patient's perisurgical area is scrubbed and the animal is lowered and placed in lateral recumbency for anesthesia reversal.
Michele I. Slogoff , B. Mark Evers , in Encyclopedia of Gastroenterology, 2004
Embryology and Development
The peritoneum and other body cavities begin to develop from the intraembryonic coelom near the end of the third week of gestation. The coelom has a parietal wall and a visceral wall, both lined by mesothelium; the parietal mesothelium is derived from somatic mesoderm whereas the visceral mesothelium is derived from splanchnic mesoderm. By the fourth week of gestation, the coelom appears as a horseshoe-shaped cavity in the cardiogenic and lateral mesoderm. The curve of the "horseshoe" represents the future pericardial cavity, and the lateral and caudal extensions represent the eventual pleural and peritoneal cavities (Fig. 1). These lateral areas communicate with the extraembryonic coelom. The development of the midgut involves a herniation through this communication into the umbilical cord, where the midgut develops into the small intestine and part of the large intestine. At this point, the intraembryonic coelom is divided into right and left halves that are divided by the ventral and dorsal mesenteries.
FIGURE 1. (A) Drawing of a dorsal view of a 22-day-old embryo, showing outline of the horseshoe-shaped intraembryonic coelom. (B) Transverse section through embryo at the level shown in A. Reproduced with permission from Moore and Persaud (1998).
Toward the end of the fourth week of gestation, the lateral parts of the intraembryonic coelom move onto the ventral aspect of the embryo and the ventral mesentery degenerates, creating a large peritoneal cavity with a dorsal mesentery (Fig. 2). Until the seventh week of gestation, this peritoneal cavity communicates with the pericardial and pleural cavities through pericardioperitoneal canals; during the fifth and sixth weeks of gestation, folds form near the cranial and caudal ends of these canals. Fusion of these membranous folds with mesoderm ventral to the esophagus separates the pericardial and pleural cavities while fusion of the caudal pleuroperitoneal membranes forms the diaphragm and separates the pleural and peritoneal cavities. The peritoneal cavity then loses its connection with the extraembryonic coelom during the tenth week of gestation, when the intestines return to the abdomen from the umbilical cord. At this point, the peritoneum lines the abdominal wall and invests the abdominal viscera.
FIGURE 2. (A) Transverse section illustrating embryonic folding and its effects on the intraembryonic coelom and other structures. (B) Transverse section illustrating formation of the ventral body wall and disappearance of the ventral mesentery. Arrows indicate the junction of the somatic and splanchnic mesoderm. Reproduced with permission from Moore and Persaud (1998).
Susanne M. Stieger-Vanegas , Paul M. Frank , in Textbook of Veterinary Diagnostic Radiology (Seventh Edition), 2018
Radiography of the Peritoneum
The peritoneum is typically not seen on survey radiographs, but its evaluation is an essential aspect in interpreting abdominal radiographs; although this analysis is performed indirectly by analyzing anatomic organ relationships and radiographic opacity variations in the abdomen.3 Identifying the origin of peritoneal disease is based on evaluating the shape, size, location, radiopacity, and margination of organs and the radiopacity throughout the abdominal cavity. In the diseased state, connecting organs can be displaced and altered in shape and radiopacity, which can help in identifying the disease origin and the ranking of differential diagnoses.3
Fat is typically present throughout the abdomen, primarily in the falciform ligament, the greater omentum, the mesentery, and the retroperitoneal space. The presence of abdominal fat is important for visceral organ visualization in radiographs, because fat provides contrast between viscera because of its lesser radiographic opacity (Fig. 39.2). The greater omentum, which is located mainly ventral in the abdomen and very movable, contains variable amounts of fat and therefore contributes to the areas of decreased radiopacity noted ventrally on abdominal radiographs.3
The amount and character of intraabdominal fat depends on the age of the animal and the body habitus. In emaciated patients, the abdomen is often tucked up, which can be visualized on radiographs (see Fig. 39.2C); however, the possibility of coexisting peritoneal fluid or peritonitis cannot be excluded. Normal dogs and cats younger than a few months of age lack sufficient fat to provide intraabdominal contrast; thus, the abdomen appears to be of relatively uniform soft tissue opacity (see Fig. 39.2D). Another factor is that young patients have a relatively higher proportion of brown (multilocular) fat than adults. Brown fat has opacity closer to that of soft tissue because of its higher water content. As young animals mature, the water content of the brown fat decreases.6 As brown fat is replaced by white fat, the contrast between intraabdominal soft tissues increases.
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