EMBRYOLOGY: Development of Body Cavity, Serous Membranes and Gut (I) Recommended reading: Larsen (3rd ed.) , pp. 133-143, 235-239 Syllabus, 9 pages Animations: http://www.med.uc.edu/embryology/chapter6/animations/contents.htm OBJECTIVES: Following this lecture and having read the assigned text, the student should be able to: 1. Explain the formation of the coelomic cavity (intra-embryonic coelom) and the role of embryonic folding in this process. 2. Explain the origin of the septum transversum and how it assumes its definitive position. 3. Correlate the 3 principal body cavities of the adult with various regions of the embryonic coelomic cavity. 4. Account for separation of the pleural and pericardial cavities. 5. Account for separation of the thoracic and abdominal cavities by development of the diaphragm. 6. Explain the formation of the primitive gut tube, its division into foregut, midgut, and hindgut, and its relationship to primitive mesenteries. 7. List the adult derivatives of the 3 regions of the primitive gut. 8. Define: pericardioperitoneal canal, pleuropericardial fold and membrane, pleuroperitoneal fold and membrane, dorsal mesentery, ventral mesentery, mesentery of esophagus. I. Formation of the intra-embryonic coelom A. The intra-embryonic mesoderm (3rd germ layer) is established during gastrulation. 1. At the end of week 3, the mesoderm on each side of the midline differentiates into 3 continuous parts: a. Paraxial mesoderm b. Intermediate mesoderm c. Lateral plate mesoderm 2. Intercellular clefts form in the lateral plate mesoderm and divide it into two layers: a. Somatic mesoderm layer b. Splanchnic mesoderm layer-continuous with the mesoderm covering the wall of the yolk sac Adapted from: Langman's Medical Embryology B. The intra-embryonic coelom is the space bordered by the somatic and splanchnic mesoderm layers. 1. The intra-embryonic coelom is initially open into the extra-embryonic coelom on both sides. 2. The intra-embryonic coelom becomes closed off during week 4 by a folding of the originally flat embryo in both lateral and cephalo-caudal directions. a. The intra-embryonic coelomic cavity extends from the thoracic to the pelvic regions of the embryo. b. The mesoderm lining the coelomic cavity forms a serous membrane or mesothelium. (1) Somatic mesoderm forms the parietal layer of mesothelium, which lines the body wall surfaces of the pleural, pericardial, and peritoneal cavities. (2) Splanchnic mesoderm forms the visceral layer of mesothelium, which lines the surfaces of the lungs, heart, and abdominal organs. (3) At sites of contact between the body wall and organs, the parietal and visceral layers of mesothelium are continuous. (4) Two-sided elongations of mesothelium between that lining the body wall and that lining the abdominal organs are called mesenteries. Adapted from: Langman's Medical Embryology II. Division of the coelom A. The diaphragm, the partition that separates the thoracic and abdominal cavities in the adult, develops from the following four embryonic structures: 1. Septum transversum a. It is the first structure that subdivides (partially) the coelom into primitive thoracic and abdominal cavities. b. It is formed by mesoderm cranial to the pericardial cavity at the end of week 3. c. As the head folds under during week 4, it is relocated into a ventral position between the primitive pericardial cavity and the yolk stalk. d. It then fuses with mesenchyme ventral to the esophagus. e. It does not completely separate the thoracic and abdominal cavities, since there are two large posterior openings, known as the pericardioperitoneal canals, on either side of the foregut. f. It becomes the central tendon of the adult diaphragm. 2. Pleuroperitoneal membranes a. These begin as crescent-shaped pleuroperitoneal folds at the caudal ends of the pleural cavities. b. The folds grow medially and ventrally across the pericardioperitoneal canals and by week 7 fuse with: (1) The dorsal portions of the septum transversum (2) The dorsal mesentery of the esophagus (dorsal mesoesophagus) c. After fusion, the folds become the pleuroperitoneal membranes, which close off the pericardioperitoneal canals and complete the separation of thoracic and abdominal cavities. 3. Dorsal mesoesophagus a. It suspends the esophagus from the dorsal body wall. b. After fusion with the pleuroperitoneal folds, it forms the median portion of the diaphragm. c. Muscle grows into the dorsal mesoesophagus to form the crura of the adult diaphragm. 4. Lateral and dorsal body wall a. From weeks 9-12, the pleural cavities enlarge into the body wall. b. Muscle tissue from the body wall is excavated by the expanding pleural cavities and becomes the peripheral muscle of the adult diaphragm. Embryo of approximately 5 weeks. Adapted from: Langman's Medical Embryolo gy B. Innervation of diaphragm 1. Phrenic nerves-provide motor and central sensory innervation a. Arise from 3rd, 4th and 5th cervical segments supplying septum transversum in its original cervical location. b. Pass through the pleuropericardial folds, which become the fibrous pericardium of the adult. 2. Lower thoracic nerves-provide sensory innervation to peripheral diaphragm C. Division of the thoracic cavity 1. Pleuropericardial membranes a. Ridges of mesenchyme (containing the veins returning systemic blood to the heart) form pleuropericardial folds. b. The folds elongate medially to form membranes, which partially separate pericardial and pleural cavities. 2. Within the pericardioperitoneal canals, the lung buds rapidly grow and excavate the mesenchyme of the body wall in dorsal, lateral, and ventral directions, thus enlarging the pleural cavities. 3. The root of each developing lung fuses with the free edge of each pleuropericardial membrane, thereby completing the separation of the pericardial cavity from the two pleural cavities. Adapted from: Langman's Medical Embryology D. Diaphragmatic hernia 1. Usually congenital (Bochdalek’s hernia) and if extensive may be associated with severe hypoplasia of lungs that is fatal at birth 2. More common on the left side because the pericardioperitoneal canal tends to close later on the left. III. Development of the gut and mesenteries (part I) A. The general structural plan of the definitive gut 1. A continuous series of tubular segments with associated organs that began as diverticula 2. Lined by simple columnar epithelium 3. Reinforced externally by splanchnic (visceral) mesoderm 4. Suspended in the peritoneal cavity by dorsal and (in some segments) ventral mesenteries Adapted from: Langman's Medical Embryology B. Formation of primitive gut tube from endoderm 1. The endoderm originally lines a cavity (not the body cavity), which forms the primitive gut, yolk sac, and allantois. a. During the cephalocaudal and lateral folding of the embryo, part of the endoderm lined cavity is incorporated into the embryo to form the primitive gut. b. The yolk sac and allantois temporarily remain outside the embryo, but retain their continuity with the gut. 2. The primitive gut acquires blind ending tubes in the cephalic and caudal ends of the embryo, but in the middle remains open into the yolk sac. a. Foregut-the cephalic tube extending from the buccopharyngeal membrane to the anterior intestinal portal (the opening of foregut into midgut) b. Hindgut-the caudal tube extending from the posterior intestinal portal (the opening of midgut into hindgut) to the cloacal membrane c. Midgut-the central region, which opens into the yolk sac via the vitelline duct or yolk stalk C. Formation of primitive mesenteries 1. The foregut, midgut, and hindgut are initially in direct contact with the dorsal body wall. 2. The developing abdominal cavity enclosing the gut is lined with mesothelium, which in the abdominal cavity is called peritoneum. 3. The space lined by peritoneum is called the peritoneal cavity. 4. The dorsal tissue between the foregut. midgut, and most of the hindgut narrows until the tube is suspended from the body wall by a dorsal mesentery. a. The mesentery consists of two layers of peritoneum, between which are blood vessels, lymphatics, nerves, and fat. b. The peritoneum on each side of the mesentery connects parietal peritoneum on the body wall with visceral peritoneum on the gut or gut-associated organ. 5. A ventral mesentery is transiently present, but persists only in the foregut region. D. Adult derivatives of primitive gut regions 1. The primitive foregut (supplied by the celiac artery) gives rise to the: a. Pharynx and lower respiratory system b. Esophagus c. Stomach d. Cranial portion (upper half) of the duodenum, with 3 appendages: (1) Liver (2) Gall bladder (3) Pancreas 2. The primitive midgut (supplied by the superior mesenteric artery) gives rise to the: a. Caudal portion (lower half) of the duodenum b. Jejunum c. Ileum d. Ascending colon and two appendages: (1) Cecum (2) Appendix e. Proximal 2/3 of the transverse colon 3. The primitive hindgut (supplied by the inferior mesenteric artery) gives rise to the: a. Distal 1/3 of the transverse colon b. Descending colon c. Rectum d. Upper 2/3 of the anal canal (1) To the level of the cloacal membrane (2) In adults, to the pectinate line e. Allantois f. Urogenital sinus .