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1 Body Cavities Formation of the Intraembryonic Cavity At the end of the third week the lateral plate mesoderm acquires a cleft that splits it into two layers. The somatic mesodermal layer takes a course towards the amniotic wall while the splanchnic mesoderm layer becomes continuous with the wall of the yolk sac. Because of its intimate relationship with the yolk sac through the yolk stalk, the gut can be divided into a foregut an opened (with yolk sac) bottom midgut, and a hindgut. The pharyngeal gut or pharynx extends from the buccopharyngeal membrane to where the tracheobronchial diverticulum will form. The foregut lies caudal to the pharyngeal tube and extends down to where the outgrowth of the liver will form. The midgut begins caudal to the liver and extends to the mid-portion of the transverse colon. The hindgut extends from the transverse colon to the cloacal membrane As the lateral mesoderm of the early embryo splits, the somatic portions fold and merge, ventrally. The space between the somatic and the splanchnic layers of mesoderm becomes the common intraembryonic coelom or intraembryonic (body) cavity. The same folding process that results in the completion of the ventral body wall and the separation of the intraembryonic coelom from the extraembryonic coelom also brings the two layers of splanchnic mesoderm together around the newly formed gut as the primary or common mesentery. The primary mesentery suspends the gut from the dorsal body wall as the dorsal mesentery and also attaches the gut to the ventral body wall as the ventral mesentery. This placement divides the coelom into right and left components. Soon, however, most of the ventral mesentery breaks down and causes a convergence of the right and left halves of the coelom. In the region of the developing stomach and liver ventral mesentery persists, forming the ventral mesogastrium and the falciform ligament of the liver. Cranially, the tubular primordium of the heart is similarly supported by a dorsal mesocardium and briefly by a ventral mesocardium, which soon breaks down. Serous Membranes Cells of the somatic mesoderm lining the intraembryonic cavity become mesothelial and form the parietal layer of the serous membranes. The parietal layer lines the peritoneal, pleural, and pericardial cavities. Cells of the splanchnic mesoderm form the visceral layer of the serous membranes covering the abdominal organs, lungs, and heart. Parietal and visceral layers are continuous with each other at the dorsal mesentery. Formation of the Septum Transversum and Pleural Canals A major component in division of the common coelom into thoracic and abdominal components is the septum transversum. The septum grows from the body wall as a semicircular shelf, separating the heart from the developing liver. During its early development, a major portion of the liver is embedded in the septum transversum. Eventually, the septum transversum constitutes a significant component of the diaphragm, namely its central tendon. The expanding septum transversum serves partly separates the pericardial and peritoneal portions of the coelom. For a while, two short channels located on either side of the foregut will connect the coeloms above and below the septum transversum. 2 Initially, these canals, the pleural (pericardioperitoneal) canals, represent the spaces into which the developing lungs grow. The pleural canals enlarge greatly as the lungs increase in size and ultimately form the pleural cavities. The pleural canals are partially bounded by two paired folds of tissue: the pleuropericardial and pleuroperitoneal folds. The pleuropericardial folds are ridges of tissue associated with the common cardinal veins, which bulge into the dorsolateral wall of the coelom. The common cardinal veins are continuous with the sinus venosus of the heart. Extensions of the folds are the pleuropericardial membranes which, in the adult, form the fibrous (parietal) layer of the pericardium. Associated with the pleuropericardial folds are the phrenic nerves. These arise from joined branches of cervical roots 3, 4, and 5 and supply muscles of the diaphragm. At the caudal ends of the pleural canals the pleuroperitoneal folds become prominent as the expanding lungs push into the mesoderm of the body wall. The pleuroperitoneal folds fuse with the septum transversum and mesentery of the esophagus. This process obliterates the pleural canal, hence, eliminating the connection between the abdominal and thoracic cavity. Formation of the Diaphragm The diaphragm, which separates the thoracic from the abdominal cavity in adults, is a composite structure derived from four embryonic components. The large ventral element of the diaphragm arises from the (1) septum transversum, which fuses with the ventral part of the esophageal mesentery and forms the central tendon of the diaphragm. Converging on the esophageal mesentery from the dorsolateral sides are the (2) pleuroperitoneal folds. The septum transversum and pleuroperitoneal folds form the bulk of the diaphragm. As the lungs continue to grow, their caudal tips excavate additional space in the body wall. (3) Mesenchyme that is separated from the body wall by this process becomes another component of the diaphragm, forming a thin rim of tissue along the diaphragm's dorsolateral borders. (4) Dorsal mesentery of the esophagus will develop into the crura of the diaphragm. Except for the central tendon, the rest of the diaphragm is predominately muscular. Clinical Correlations Cleft sternum – This results from lack of fusion of bilateral bars of mesoderm responsible for this defect. In some cases the heart protrudes through the defect to produce ectopia cordis. Omphalocele – This is herniation of abdominal viscera through an enlarged umbilical ring and the protruding organs are covered by parietal peritoneum and amnion. The omphalocele occurs when the organs fail to return into the abdominal cavity at about 10 weeks of gestation. Gastroschisis – This is herniation of abdominal contents through the body wall directly into the amniotic cavity. Here, the abdominal contents are not covered by parietal peritoneum or amnion. Usually, it occurs to the right side of the umbilicus. Unlike the omphalocele, gastroschisis is not associated with chromosome abnormalities or other 3 severe defects and survival rates are excellent. Both omphalocele and gastroschisis result in elevated α-fetoprotein in amniotic fluid. Diaphragmatic hernias – A congenital diaphragmatic hernia is one of the more common malformations in the newborn (1/2000). It is most frequently caused by failure of one or both the pleuroperitoneal membranes, or folds, to close the pericardioperitoneal canals. The abdominal viscera enter into the chest and compress the heart and lungs. Esophageal hernia – This is thought to be due to a congenital shortness of the esophagus. The stomach gets constricted by the diaphragm. Questions 1. The ________ extends from the liver to the mid part of the transverse colon. a. foregut b. midgut c. hindgut c. vitelline gut 2. Splanchnic mesoderm is continuous with the wall of the ____________. a. amnion b. chorionic plate c. epiblast d. yolk sac 3. The falciform ligament is derived from the ________. a. ventral mesentery b. buccopharyngeal membrane c. dorsal mesentery d. cloacal membrane 4. Somatic mesoderm will form the _________. a. visceral layers of serous membranes b. septum transversum c. parietal layers of serous membranes d. ventral mesocardium 5. The __________ separates the thoracic coelom form the abdominal coelom. a. visceral layers of serous membranes b. septum transversum c. parietal layers of serous membranes d. ventral mesocardium 4 6. Two short canals on either side of the foregut temporarily connecting the thoracic and abdominal coeloms are the _____________, spaces for the developing lungs. a. pericardioperitoneal canals b. umbilical ring c. pleuropericardial folds d. allantois 7. The common cardinal veins are located in the __________________. a. pericardioperitoenal canals b. umbilical ring c. pleuropericardial folds d. allantois 8. The pleuropericardial folds contain the ___________ and eventually fuse to enclose the heart in the fibrous pericardium. a. lungs b. phrenic nerves c. ribs d. stomach 9. The central tendon of the diaphragm is mostly derived from the ______________. a. pleuroperitoneal b. body wall mesoderm c. septum transversum d. pleuropericardial folds 10. Herniation of abdominal viscera not covered by amnion or peritoneal membranes is called _________. a. ectopia cordis b. omphalocele c. gastroschisis d. diaphragmatic hernia