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Chapter 7 The Gut Tube and the Body Cavities A tube on top of a tube • During 3rd & 4th weeks: neural tube dorsally + gut tube ventrally= tube on top of a tube Formation of the body cavity • At the end of the third week, • Intraembryonic mesoderm differentiates into: • Paraxial mesoderm , that forms somitomeres and somites; • Intermediate mesoderm , that contributes to the urogenital system; and • Lateral plate mesoderm that is involved in forming the body cavity • Lateral plate mesoderm split into two: • (a) the parietal (somatic) layer adjacent to the surface ectoderm • (b) the visceral (splanchnic) layer adjacent to endoderm forming the gut tube • The space created between the two layers of lateral plate mesoderm constitutes the primitive body cavity . Formation of body wall • During the fourth week, the sides of the embryo begin to grow ventrally forming two lateral body wall folds (lateral folding). • These folds consist of the parietal layer of lateral plate mesoderm, overlying ectoderm, and cells from adjacent somites that migrate into this mesoderm across the lateral somitic frontier. • As these folds progress, the endoderm layer also folds ventrally and closes to form the gut tube. • By the end of the fourth week, the lateral body wall folds meet in the midline and fuse to close the ventral body wall. Formation of body wall • This closure is aided by head and tail folds that cause the embryo to curve into the fetal position (cephalocaudal folding). Cephalocaudal folding Lateral folding • Closure of the ventral body wall is complete except in the region of the connecting stalk. • Similarly, closure of the gut tube is complete except for a connection from the midgut region to the yolk sac that forms the vitelline (yolk sac) duct. This duct is incorporated into the umbilical cord, becomes very narrow, and degenerates later. Serous membranes • Parietal layer of lateral plate mesoderm forms the parietal layer of the serous membranes lining the outside of the peritoneal, pleural , and pericardial cavities . • visceral layer of lateral plate mesoderm forms the visceral layer of the serous membranes covering the abdominal organs, lungs, and heart. Visceral and parietal layers are continuous with each other as the dorsal mesentery, which suspends the gut tube from the posterior body wall into the peritoneal cavity. Dorsal mesentery extends continuously from the caudal limit of the foregut to the end of the hindgut. Ventral mesentery: exists only from the caudal foregut to the upper portion of the duodenum and results from thinning of mesoderm of the septum transversum. These mesenteries are double layers of peritoneum that provide a pathway for blood vessels, nerves, and lymphatics to the organs. Dorsal & ventral mesentry Clinical Correlates Ventral Body Wall Defects : • Occur in the thorax, abdomen, and pelvis and involve the heart (ectopia cordis) , abdominal viscera (gastroschisis), and/or urogenital organs (bladder or cloacal exstrophy), depending upon the location and size of the abnormality. • The malformations are due to a failure of the ventral body wall to close. Ectopia cordis Body wall folds fail to close the midline in the thoracic region causing the heart to lie outside the body cavity. Gastroschisis • • • • Occurs when body wall closure fails in the abdominal region. As a result, intestinal loops herniate into the amniotic cavity through the defect, which usually lies to the right of the umbilicus. The malformation is not associated with chromosome abnormalities. is associated with elevated alpha fetoprotein (AFP) concentrations. Bladder and cloacal exstrophy • results from abnormal body wall closure in the pelvic region. • Bladder exstrophy represents a less severe closure defect in this region and only the bladder is exposed; in males, the penis may be involved and epispadius [a split in the dorsum of the penis; is common). • Cloacal exstrophy results from a more severe failure of body wall closure in the pelvis such that the bladder and rectum, which are derived from the cloaca, are exposed. Omphalocele • A ventral body wall defect • Does not arise from a failure in body wall closure. • Originates when portions of the gut tube (the midgut), that normally herniates into the umbilical cord during the 6th to 10th weeks (physiological umbilical herniation) , fails to return to the abdominal cavity. Subsequently, loops of bowel, and other viscera, including the liver may herniate into the defect. Since the umbilical cord is covered by a reflection of the amnion, the defect is covered by this epithelial layer. (In contrast, loops of bowel in gastroschisis are not covered by amnion because they herniate through the abdominal wall directly into the amniotic cavity.) • Omphalocele is associated with high mortality rates and severe malformations, including cardiac abnormalities and neural tube defects. • Chromosome abnormalities are present in 15% of cases. • Omphaloceles is associated with elevated alpha fetoprotein (afp) concentrations. Diaphragm and thoracic cavity The septum transversum is a thick plate of mesodermal tissue occupying the space between t he thoracic cavity and the stalk of the yolk sac. This septum does not separate the thoracic and abdominal cavities completely but leaves large openings, the pericardio-peritoneal canals, on each side of the foregut. • • • When lung buds begin to grow, they expand within the pericardio-peritoneal canals. As a result of the rapid growth of the lungs, the pericardioperitoneal canals become too small, & the lungs begin to expand into the mesenchyme of the body wall. Ventral and lateral expansion is posterior to the pleuroperi-cardial folds. • With expansion of the lungs, mesoderm of the body wall splits into 2 components: (a) the definitive wall of the thorax and (b) the Pleuro-pericardial membranes, which are extensions of the pleuropericardial folds that contain the common cardinal veins and phrenic nerves . • Descent of the heart and positional changes of the sinus venosus shift the common cardinal veins toward the midline, and the pleuropericardial membranes are drawn out in mesentery-like fashion. • Finally, they fuse with each other and with the root of the lungs, and the thoracic cavity is divided into the definitive pericardial cavity and two pleural cavities. • In the adult, the pleuropericardial membranes form the fibrous pericardium. Formation of the diaphragm • Formation of septum transversum • During further development, the pericardioperitoneal canals are closed by growth of the pleuroperitoneal folds which will form the pleuroperitoneal membranes. • Further expansion of the pleural cavities relative to mesenchyme of the body wall adds a peripheral rim to the pleuroperitoneal membranes. Once this rim is established, myoblasts originating from somites at cervical segments three to five(C 3-5) penetrate the membranes to form the muscular part of the diaphragm. • Thus, the diaphragm is derived from the following structures: the septum transversum, which forms the central tendon of the diaphragm; the two pleuroperitoneal membranes; muscular components from somites at cervical segments three to five; and the mesentery of the esophagus, in which the crura of the diaphragm The phrenic nerves • • • During the fourth week, the septum transversum lies opposite cervical somites, and nerve components of the 3, 4, and 5 cervical segments of the spinal cord grow into the septum. At first, the nerves, known as phrenic nerves , pass into the septum through the pleuropericardial folds. This explains why further expansion of the lungs and descent of the septum shift the phrenic nerves that innervate the diaphragm into the fibrous pericardium .Although the septum transversum lies opposite cervical segments during the fourth week, by the sixth week, the developing diaphragm is at the level of thoracic somites. • The repositioning of the diaphragm is caused by rapid growth of the dorsal part of the embryo (vertebral column), compared with that of the ventral part. By the beginning of the third month, some of the dorsal bands of the diaphragm originate at the level of the first lumbar vertebra. • The phrenic nerves supply the diaphragm with its motor and sensory innervation . • Since the most peripheral part of the diaphragm is derived from mesenchyme of the thoracic wall, it is generally accepted that some of the lower intercostal (thoracic) nerves contribute sensory fibers to the peripheral part of the diaphragm. Clinical Correlates • • • • Diaphragmatic Hernias A congenital diaphragmatic hernia: – failure of one or both of the pleuroperitoneal membranes to close the pericardioperitoneal canals, – usualy on the left side. – The abdominal viscera in the chest push the heart anteriorly and compress the lungs, which are commonly hypoplastic. – A large defect is associated with a high rate of mortality Parasternal hernia: a small part of the muscular fibers of the diaphragm fails to develop. Esophageal hernia, is due to congenital shortness of the esophagus. Upper portions of the stomach are retained in the thorax, and the stomach is constricted at the level of the diaphragm. Congenital Diaphragm atic Hernia