THORACIC EMBRYOLOGY I. Development of the Thoracic Cavity, Diaphragm, Trachea, and Lungs By 26 days of human embryonic development three well-defined body cavities have formed. These include 1) the pericardial cavity, 2) the peritoneal cavity, and 3) two small pericardio-peritoneal canals which connect the primitive pericardial and peritoneal cavities. As gestation continues, partitions form concurrently in each pericardioperitoneal canal and separate the pericardial cavity from the pleural cavities and the pleural cavities from the peritoneal cavity. 1. Pleuropericardial membranes form into the fibrous pericardium from the lateral walls of the pericardioperitoneal canals a division between the pleural and pericardial cavities the parietal pleura Right! No. The first three foils are correct. The parietal pleura which is a membrane covering of the inner thoracic wall forms from somatic mesoderm and not from the pleuropericardial membranes. 2. The pleural and peritoneal cavities are separated by the diaphragm which develops from the septum transversum pleuroperitoneal membranes dorsal mesentery of the esophagus lateral embryonic body wall Correct! No. All foils are correct. The diaphragm develops from all of these structures. The lower respiratory system (larynx, trachea, bronchi, and lungs) first develop as the laryngotracheal groove in the floor of the primitive pharynx. The ridges of this groove expand and fuse to form an outpocketing called the laryngotracheal diverticulum. Continued growth of this structure (forming the laryngopharyngeal tube) results in formation of the larynx, trachea, bronchi, and lungs. 3. Visceral pleura of the lungs develop from splanchnic mesenchyme surrounding the laryngotracheal tube. True False Correct! No, the answer is A. II. Development of the Heart The human heart first develops during day 18 or 19 of embryonic development as paired tubes in the septum transversum. These two tubes fuse at about 21 days of development. Elongation of this primitive heart tube results in the tube bending on itself such that it forms an S shape when observed from a lateral view. Subsequent fusion and ingrowth of the walls of the convoluted heart tube results in formation of a four chambered heart. 5. The walls of the heart develop from splanchnic mesenchyme which forms the primitive heart tube which in turn develops into the endocardium (inner lining of the heart) the myoblasts which develop into the myocardium (middle layer of the heart which is comprised of cardiac muscle) the thin mesothelial cells which form the outer layer of the heart called the epicardium or visceral pericardium none of the above Correct! No. The first three foils are correct. Partition of the heart into four chambers occurs at one time. This involves partition of the atrium from the ventricle and division of each of these structures into a right and left half. 6. Partition of the atrium from the ventricle results from ingrowth and fusion of the dorsal and ventral truncus arteriosus dorsal and ventral sinus venosus dorsal and ventral bulbus cordis dorsal and ventral endocardial cushions Very good! No. Only the fourth foil is correct. Ingrowth of the walls of the atrioventricular canal results in formation of the atrioventricular endocardial cushions which fuse at the fifth week of development. Fusion of the endocardial cushions divides the atrioventricular canal into a right and left atrioventricular canal (future tricuspid and mitral valves, respectively). 7. Partition of the primitive atrium involves formation of the septum primum formation of the foramen primum formation of the septum secundum formation of the foramen secundum Very good! No. All foils are correct. All structures are involved in the partition of the primitive atrium into a right and left atrium. A remnant of these structures is the fossa ovale of the adult atrial septum. 8. Partitioning of the primitive ventricle involves ingrowth of the interventricular septum from the floor (apex) of the ventricle fusion of the interventricular septum with the right bulbar ridges fusion of the interventricular septum with the endocardial cushions fusion of the interventricular septum with the left bulbar ridges Very good! No. All foils are correct. All structures are involved in partition of the ventricle into a right and left half. 9. The left horn of the sinus venosus forms the coronary sinus. True False Correct! No, the answer is A. 10. The right horn of the sinus venosus forms the sinus venosum of the right atrium. Correct! No, the answer is A. 11. Partition of the bulbus cordis and truncus arteriosus forms the aorta and pulmonary trunk. True False Very good! No, the answer is A. III. Formation and Derivatives of the Aortic Arches During the fourth week of human development, the aortic arches develop from the aortic sac and terminate in the right and left dorsal aorta after passing through the brachial arches. Although six pairs of arches develop, some degenrate while the remaining arches form the great vessels leaving the heart. 12. The first and second pair of aortic arches largely degenerate except for portions of the former which develop into the maxillary arteries and of the latter which contribute to the formation of the stapedial arteries. True False Right! No, the answer is A. 13. The third pair of aortic arches form the common carotid arteries subclavian arteries internal carotid arteries pulmonary arteries Right! No. The first and third foils are correct. The proximal portion of the right and left third aortic arch forms the common carotid arteries and the distal portion forms the internal carotid arteries. 14. The fourth pair of aortic arches form the pulmonary arteries right subclavian artery left subclavian artery part of the arch of the aorta Very good! No. The second and fourth foils are correct. The left fourth aortic arch forms part of the arch of the aorta and the right fourth aortic arch forms the proximal portion of the right subclavian artery. The left subclavian artery develops from the left seventh intersegmental artery. 15. The fifth pair of aortic arches degenerate or never form. True False Right! No, the answer is A. 16. The sixth pair of aortic arches form the ductus arteriosus which forms the ligamentum arteriosum in the adult the pulmonary veins the right and left pulmonary arteries the coronary sinus of the heart Very good! No. The first and third foils are correct. The left sixth aortic arch forms the left pulmonary artery and ductus arteriosus. The right sixth aortic arch forms the right pulmonary artery.