Embryologic Considerations

Knowledge of the development of the subperitoneal space is a prerequisite to recognizing pathologic conditions and understanding the pathogenesis of direct spread.8-11 The conceptualization of the abdomen and pelvis as one space, the subperitoneal space, and its continuity with the thorax requires the reexamination of standard embryology from a holistic perspective.1'4'5'7'12 The primitive coelom is formed at the end of the 3rd week as the intraembryonic mesoderm and either side of the midline differentiates into a paraxial and intermediate portion and a lateral plate. The appearance at this time of intracellular clefts laterally divides the lateral plate into the somatic mesodermal layer and splanchnic mesodermal layer (Fig. 13-1). A serous membrane also forms and lines these layers and the coelomic cavity. Subsequent development divides the single embryonic coelom into the thorax and abdomen. The thorax contains the pleural and pericardial spaces and the abdomen the peritoneal space. It is important to remember that throughout development continuity is preserved between the pleural and peritoneal spaces; the pericardial space becomes isolated by the 5th to 6th week (Fig. 13-2a'b).

Fig. 13—1. Schematic drawing of a transverse section through an embryo at the end of the third week of gestation.

The somatic mesoderm and the splanchnic mesoderm result from the division of the lateral plate. The serous membrane is formed from the tissue lining the intraembryonic cavity. (Reproduced from Oliphant et al. )

The somatic mesoderm forms the parietal layer of the serous membrane and eventually lines the pleural and peritoneal cavities. The splanchnic mesoderm forms the visceral layer of the serous membrane and covers the abdominal and thoracic viscera. The space subjacent to the serous membrane defines the single subserous space, which in the adult is the subperitoneal space of the abdomen/pelvis and the subpleural space of the thorax.

The respiratory diaphragm forms by the 8th week and divides the embryonic coelomic cavity into the tho rax and abdomen (Fig. 13-3). During this division, the integrity of the subserous space is maintained by that portion of the subserous space traversing the diaphragmatic hiatuses and apertures—the thoracoabdominal continuum.12 The esophageal hiatus lies ventral and cranial to the aortic hiatus with the esophagus, vagus nerves, and esophageal vessels coursing within the sub-serous space. The aortic hiatus is an osseoaponeurotic opening between the diaphragm and vertebral column containing the aorta, hemizygous and azygous veins,

Subserous Membrane

Fig. 13—2. (a) Schematic drawing of a transverse section through an embryo at 4—5 weeks' gestation.

The lung buds are growing into the pericardioperitoneal canals, forming the pleuropericardial folds. The serous membrane lines the coelomic cavity. The subserous space is the stippled area subjacent to the serous membrane. (b) Schematic drawing of a transverse section through an embryo at 5—6 weeks' gestation.

Complexity of the serous membrane results as it fuses ventrally forming the pericardium (PerC = pericardial cavity) and as the pleuropericardial folds fuse bilaterally at the root of the lungs. The serous membrane lines the pleural cavities as the visceral pleura and parietal pleura. The subpleural space is the stippled area subjacent to the pleura. The common cardinal veins form the superior vena cava (SVC). A = aorta; E = esophagus; Pl cavity = pleural cavity. (Reproduced from Oliphant et al.12)

Fig. 13—2. (a) Schematic drawing of a transverse section through an embryo at 4—5 weeks' gestation.

The lung buds are growing into the pericardioperitoneal canals, forming the pleuropericardial folds. The serous membrane lines the coelomic cavity. The subserous space is the stippled area subjacent to the serous membrane. (b) Schematic drawing of a transverse section through an embryo at 5—6 weeks' gestation.

Complexity of the serous membrane results as it fuses ventrally forming the pericardium (PerC = pericardial cavity) and as the pleuropericardial folds fuse bilaterally at the root of the lungs. The serous membrane lines the pleural cavities as the visceral pleura and parietal pleura. The subpleural space is the stippled area subjacent to the pleura. The common cardinal veins form the superior vena cava (SVC). A = aorta; E = esophagus; Pl cavity = pleural cavity. (Reproduced from Oliphant et al.12)

and the thoracic duct within the subserous space. The vena caval foramen is the most ventral of the three hiatuses and transmits only the inferior vena cava. The wall of this vessel is adherent to the margins of the foramen and interrupts continuity of the subserous space.13 Ven-trally, there are small apertures between the sternum and costal cartilage, allowing continuity of the subserosal space as the superior epigastric branch of the internal mammary artery and lymphatics course from the thorax to the abdomen.

The abdominal cavity, formed by the 8th week, provides the space within which the viscera grow, shift position, and move without hindrance. To achieve this goal, the developing abdominal organs are suspended by two opposing splanchnic mesodermal layers that form a double-layered mesentery at 3/ weeks—the primitive mesentery.

The gut arises by the infolding of ectoderm at 3 weeks to form a tube. The splanchnic mesoderm contains the gut and extends as a double layer from the dorsal to the ventral walls of the coelomic cavity. Thus, the primitive mesentery divides the coelomic cavity in halves and contains the gut within its component layers. The gut at this time is a straight tube and divides the primitive mesentery into the dorsal mesentery and the ventral mesentery (Fig. 13-4a.) At this time, the liver appears, partially enclosed within the ventral mesentery.

The primitive mesentery contains a layer of connective tissue beneath its epithelium. The development of the vascular system is heralded by the appearance of numerous islands that form plexiform networks throughout this mesenchyme. These plexuses fuse and give rise to the abdominal vessels. At the end of 4 weeks, the aorta has formed and has developed three prominent ventral branches: the celiac artery in the stomach-pancreas region, the superior mesenteric artery in the small intestine region, and the inferior mesenteric artery in the large intestine region. These three vessels course through the mesenteries to the gastrointestinal system.

Over the next several weeks, the small intestine mesentery grows along with the intestine as it loops outside the coelomic cavity. It reenters the cavity at 12 weeks. During this time, the ventral and dorsal mesenteries undergo specialization.

The ventral mesentery is intimately related to the liver, the lower esophagus, stomach, and upper duodenum. The rapid growth of the liver splits the ventral mesentery (Fig. 13-4b) into anterior and posterior portions, the falciform ligament and the lesser omentum, respectively. The free edge of the falciform ligament contains the left paraumbilical vein, which obliterates at birth, forming the round ligament. The lesser omentum is also termed the gastrohepatic ligament, and its free edge,

Diagrams Falciform Ligament

Fig. 13—3. Schematic drawing of transverse section illustrating the hiatuses of the respiratory diaphragm at the fourth month of life.

The pleuroperitoneal membrane (PPM) fuses with the septum transversum and the esophageal mesentery, forming the respiratory diaphragm. Esophageal mesentery encloses that portion of the subserous space that contains the esophagus (E) and the inferior vena cava (IVC). Stippled area = subserous space.

The serous membrane lines the diaphragm and invaginates dorsomedially, encasing the subserous space. A = aorta; asterisk = muscular rim from body wall, forming periphery of diaphragm.

(Reproduced from Oliphant et al.12)

Fig. 13—3. Schematic drawing of transverse section illustrating the hiatuses of the respiratory diaphragm at the fourth month of life.

The pleuroperitoneal membrane (PPM) fuses with the septum transversum and the esophageal mesentery, forming the respiratory diaphragm. Esophageal mesentery encloses that portion of the subserous space that contains the esophagus (E) and the inferior vena cava (IVC). Stippled area = subserous space.

The serous membrane lines the diaphragm and invaginates dorsomedially, encasing the subserous space. A = aorta; asterisk = muscular rim from body wall, forming periphery of diaphragm.

(Reproduced from Oliphant et al.12)

the hepatoduodenal ligament, which contains the common bile duct, the portal vein, and the hepatic artery. The lesser omentum rotates from the sagittal to the frontal position as a result of the growth of the liver and rotation of the stomach.

The liver is surrounded by the ventral mesentery except for its upper surface, where it is in contact with the diaphragm (bare area of the liver). Where the peritoneal covering of the liver is continuous with the coelomic peritoneum, the reflections are named the coronary ligaments.

The dorsal mesentery extends from the lower end of the esophagus to the rectum. Throughout its length it serves as a pathway for blood vessels, lymphatics, and nerves to the gut. It is a continuous mesentery suspending the gut, and its subsegments take their names from the regions served, i.e., region of the stomach, the dorsal mesogastrium; region of the duodenum, the dorsal me-soduodenum; region of the colon, the dorsal mesoco-

Pancreas Transverse Section

Fig. 13—4. (a) Diagrammatic transverse section through an embryo at the end of 4 weeks.

The splanchnic mesoderm, the heavy black line outlining the coelomic cavity (CC), has fused in the midline and formed a double-layered membrane, the primitive mesentery. The gut (G) is contained within and divides the primitive mesentery into the dorsal mesentery (DM) and ventral mesentery (VM). Ao = aorta; K = mesonephros. (b) Diagrammatic transverse section through the liver (L), stomach (St), spleen (S), and pancreas (P).

With subsequent development, the ventral mesentery (VM) will form the falciform ligament and gastrohepatic ligament, and the dorsal mesentery (DM) will form the gastrosplenic ligament and the splenorenal ligament as the pancreas becomes retroperitoneal. K = kidney; Ao = aorta. (Modified from Langman.8)

Fig. 13—4. (a) Diagrammatic transverse section through an embryo at the end of 4 weeks.

The splanchnic mesoderm, the heavy black line outlining the coelomic cavity (CC), has fused in the midline and formed a double-layered membrane, the primitive mesentery. The gut (G) is contained within and divides the primitive mesentery into the dorsal mesentery (DM) and ventral mesentery (VM). Ao = aorta; K = mesonephros. (b) Diagrammatic transverse section through the liver (L), stomach (St), spleen (S), and pancreas (P).

With subsequent development, the ventral mesentery (VM) will form the falciform ligament and gastrohepatic ligament, and the dorsal mesentery (DM) will form the gastrosplenic ligament and the splenorenal ligament as the pancreas becomes retroperitoneal. K = kidney; Ao = aorta. (Modified from Langman.8)

lon; and the region of the jejunum and ileum, the mesentery proper. The dorsal mesogastrium continues to grow after the stomach completes its rotation. This ongoing growth forms a duplication of the mesogastrium upon itself anterior to the transverse colon and small intestine. Later, the four leaves fuse and are suspended from the greater curvature of the stomach as the greater omentum.

The spleen appears between the folds of the meso-gastrium at the 5th week and, with growth, bulges into the left upper portion of the coelomic cavity. The dorsal mesogastrium connecting the spleen and stomach is the gastrosplenic ligament. The dorsal mesogastrium between the spleen and dorsal midline fuses with the posterior abdominal wall, whereas the remaining part connects the spleen and left kidney and is designated the splenorenal ligament.

The head and body of the pancreas grow within the dorsal mesoduodenum and extend into the mesogas-trium. As the pancreas grows, the stomach rotates and the duodenum moves from the midline to the right. The duodenum and pancreas lie against the dorsal abdominal wall, and the right dorsal mesoduodenum fuses at that site, except at the pylorus and duodenal bulb. The tail of the pancreas lies within the mesogastrium, which usually fuses with the adjacent dorsal body wall. It is important to recognize that the pancreas, while positioned beneath the coelomic peritoneum, is connected by the subperitoneal space within the mesenteries to the other abdominal organs.

The dorsal mesocolon undergoes extensive loss of its mesentery. After the ascending and descending portions of the colon come to lie in their lateral positions, their mesocolons fuse with the dorsal wall of the coelom. However, the appendix and caput of the cecum retain their mesenteries. The transverse mesocolon persists. It fuses and covers the duodenum at the colon's crossing, forming the duodenocolic ligament. At the splenic flexure, the left lateral extension ofthe transverse mesocolon to the diaphragm forms the phrenicocolic ligament. The formed mesenteric connection between the stomach and transverse colon is designated the gastrocolic ligament. The sigmoid mesocolon persists. The primitive mesorectum obliterates.

The mesentery of the small intestinal loop undergoes dramatic changes as the small intestine elongates faster than the coelomic cavity grows. The mesenteric attachment grows correspondingly as it is carried out into the umbilical cord with the bowel loops. The completed rotation and reentry of the small bowel and its mesentery occur by the 12th week. In this process, the cecum comes to lie on the right, the transverse colon crosses ventral to the duodenum, and the small intestine lies caudad and to the left of the ascending colon. The rotation of this mesentery occurs about the axis of the superior mesenteric artery. The focal point of the rotation is the root of the superior mesenteric artery as it originates from the aorta. From its narrow origin, the mesentery of the intestine spreads out like a fan. The intestine is freely movable on the mesentery until the 14th week, when the secondary fusions affix portions of gut, forming new lines of attachment. The root of the small bowel mesentery finally affixes itself posteriorly and extends dorsally from the left upper to the right lower abdomen. The root of the small intestine mesentery is in continuity with the attachment of the transverse mesocolon in the left upper abdomen and the peritoneum overlying the ascending colon on the right side. In this manner, the root of the small bowel mesentery interconnects the upper and lower portions of the abdomen.

The broad ligament is formed by the mesenchymal shelf that bridges the lateral walls of the pelvis. In the center is the uterus, and the lateral sheets extending to the lateral pelvic walls form the broad ligament. The broad ligament is covered by peritoneal folds continuous from the pelvic walls to cover the uterus. Within the broad ligament, the blood vessels and nerves course from the pelvic wall to the uterus. The mesovarium and me-sosalpinx accompanied by their blood, lymphatic, and nerve supply are intimately related with the broad ligament. The subperitoneal space within the broad liga ment interconnects the female pelvic organs with the remainder of the abdomen.

With the completion of development of the mesenteries and ligaments and the planes of attachments and fixations, the interconnections between contiguous and remote sites within the abdomen formed by the sub-peritoneal space are complete.

The subperitoneal space can be defined as the anatomic continuum deep to the peritoneum that both covers the coelomic cavity and the abdominal and pelvic viscera and extends within the folds that are the mesenteries and ligaments.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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