LESSON 8 DEVELOPMENT OF THE CARDIOVASCULAR SYSTEM Objectives By the end of this lesson, you should be able to: 1. Describe the origin of the CVS 2. Describe the formation of the heart tubes 3. Describe the partitioning of the atrium, ventricles and bulbus cordis and truncus atriosus 4. Describe the development of the sinus venosus, atria and ventricles 5. Describe the development of the aortic arches 6. Describe fetal circulation and changes that occur immediately after birth INTRODUCTION The cardiovascular system is the first system to function in the embryo. In the middle of the third week, the cells of the splanchnic mesoderm proliferate to form the angiogenetic clusters. The ventral portion of a horseshoe-shaped plexus of blood vessels forms the cardiogenic area, while the lateral portions form the dorsal aortae. Initially, the cardiogenic area is located anterior to the prechordal plate and the neural plate. Human embryo - day 18 1. Neural plate 2. Primitive node 3. Primitive streak 4. Cut edge of amnion 5. Angiogenetic cell clusters FORMATION OF PAIRED HEART TUBES The growth of the central nervous system pulls the cardiogenic area ventrally and caudally to the buccopharyngeal (oropharyngeal) membrane. The canalization of cardiogenic clusters in the splanchnic mesoderm results in the formation of the paired heart tubes. Heart tubes 1. Epimyocardial cells 2. Heart tube 3. Endoderm 4. Endocardial cells 5. Notochord 6. Dorsal aorta 7. Neural crest 8. Neural fold DEVELOPMENT OF SINGLE HEART TUBE • "These heart tubes approach each other and fuse to form a single heart tube, temporarily attached to the dorsal side of the pericardial cavity by the dorsal mesocardium. • The splanchnic mesoderm around the heart tube (future endocardium) thickens and forms the myoepicardial mantle (future myocardium and epicardium). Fusion of the heart tubes 1. Foregut 2. Intraembryonic coelom 3. Heart tubes 4. Dorsal mesocardium 5. Epimyocardium 6. Neural groove 7. Neural crest 8. Notochord 9. Dorsal aorta FURTHER DEVELOPMENT OF THE PRIMITIVE HEART The heart tube elongates and develops dilatations and constrictions: the truncus arteriosus, bulbus cordis, ventricle, atrium and sinus venosus. The heart tube bends upon itself, forming a U-shaped bulboventricular loop. The primitive heart tube has only one atrium and one ventricle. Their partitioning begins around the middle of the 4th week and is completed by the end of the 5th week. Fusion of the heart tubes 1. Sinus venosus 2. Truncus arteriosus 3. Bulbus cordis 4. Ventricle 5. Atrium 6. Cardinal veins Partitioning of the atrium, ventricle, bulbus cordis and truncus arteriosus Atrium At the end of the fourth week, a crest (septum primum) grows from the dorsocranial wall of the primitive atrium towards the endocardial cushions. The temporary opening (ostium primum) between the lower rim of the septum primum and the endocardial cushions gradually closes. Perforations in the upper part of the septum primum form the ostium secundum. Subsequently, another crest (septum secundum) grows from the right side of the septum primum and gradually covers the foramen secundum. The oval opening left by the septum secundum is called foramen ovale. Atrial and ventricular septae 1. Venous valves 2. Septum primum 3. Ostium secundum 4. Septum secundum 5. Pulmonary veins 6. Endocardial cushions 7. Interventricular foramen 8. Muscular interventricular septum 9. Foramen ovale Ventricle By the end of the 4th week, the two primitive ventricles begin to expand and gradually fuse together, thus forming the muscular interventricular septum. The interventricular foramen found above the muscular interventricular septum subsequently closes by the outgrowth of endocardial cushions which fuse with the caudal part of the aorticopulmonary septum. After the complete closure, the interventricular foramen becomes the membranous interventricular septum. Atrial and ventricular septae 1. Septum primum 2. Septum secundum 3. Pulmonary veins 4. Ostium secundum 5. Foramen ovale 6. Membranous interventricular septum 7. Muscular interventricular septum 8. Endocardial cushions Bulbus cordis and truncus arteriosus • During the 5th week, a pair of opposing ridges appear in the walls of the bulbus cordis and truncus arteriosus. • These ridges twist around each other, forming a spiral course of the aorticopulmonary septum. • This septum divides the bulbus cordis and truncus arteriosus into two channels, the aorta and the pulmonary artery. • It also participates in the closure of the interventricular foramen Formation of aorticopulmonary septum 1. Aorta 2. Left pulmonary artery 3. Pulmonary trunk 4. Muscular interventricular septum 5. Right ventricle 6. Membranous interventricular septum • Development of aorticopulmonary septum 1. Aorticopulmonary septum 2. Pulmonary valve 3. Pulmonary artery 4. Aortic valve 5. Aorta Formation of the sinus venosus, atria and ventricles • The sinus venosus receives blood from the umbilical, vitelline and common cardinal veins. • Initially it is a separate chamber which opens into the caudal wall of the right atrium. • The primitive right atrium enlarges by the incorporation of the right sinus horn (sinus venarum) into its wall. • A remnant of the primitive right atrium becomes trabeculated, while the smooth-walled part of right adult atrium originates from the right sinus horn. Formation of the atria 1. Superior vena cava 2. Pulmonary veins 3. Left atrium 4. Right atrium 5. Septum primum 6. Septum secundum 7. Primitive left atrium 8. Primitive right atrium 9. Valve of inferior vena cava 10. Valve of coronary sinus 11. Sinus venarum Development of the atrium cont… • In the posterior wall of the primitive left atrium develops a pulmonary vein, which progressively branches. • During further development, the left atrium expands greatly and incorporates branches of the pulmonary vein (the smooth-walled part of left adult atrium). • The bulbus cordis is gradually incorporated into the walls of the ventricles. • It is represented in the right ventricle by the conus arteriosus, while in the left ventricle the bulbus cordis forms the aortic vestibule. Formation of the ventricles 1. Membranous interventricular septum 2. Muscular interventricular septum 3. Endocardial cushions 4. Left ventricle 5. Right ventricle 6. Ventricular wall Partitioning of the atrioventricular canal • Endocardial cushions develop in the dorsal and ventral walls of the heart in the region of the atrioventricular canal. • They grow towards each other and fuse, thus dividing the atrioventricular canal into right and left orifices. • In addition, the lateral interventricular cushions appear on the borders of the canal. • These localized proliferations of the mesenchymal tissue subsequently differentiate into bicuspid and tricuspid atrioventricular valves. Partitioning of the atrioventricular canal 1. Lateral endocardial cushion 2. Ventral endocardial cushion 3. Dorsal endocardial cushion 4. Left atrioventricular canal 5. Right atrioventricular canal Partitioning of the atrioventricular canal 1. Left atrioventricular canal 2. Right atrioventricular canal 3. Tricuspid valves 4. Bicuspid valves AORTIC ARCHES • During the 4th week of development, the branchial (pharyngeal) arches receive arteries from the heart. • Aortic arches (arteries) arise from the truncus arteriosus and terminate in the dorsal aortae. • During the 6th to 8th week, the primitive aortic arch pattern is transformed into the basic adult arterial arrangement. • The first, second and fifth pairs of the aortic arches disappear, while the third, fourth and sixth pairs of aortic arches have their derivatives. Aortic arch system 1. Internal carotid artery 2. External carotid artery 3. Common carotid artery 4. Right subclavian artery 5. Arch of aorta 6. Brachiocephalic artery 7. Ductus arteriosus 8. 7th intersegmental artery 9. Pulmonary artery 10. Carotid duct 11. Obliterated right dorsal aorta FETAL AND NEONATAL CIRCULATION • The fetal circulation is designed to serve prenatal needs. Very little blood goes to the lungs before birth because they are nonfunctional. • The well-oxygenated blood returns from the placenta into the umbilical vein, goes through the ductus venosus and vena cava inferior (which also receives deoxygenated blood from the portal vein) to the right atrium. • Most of the blood goes through the foramen ovale into the left atrium and mixes with the blood from the pulmonary veins. • The blood passes into the left ventricle and leaves via the ascending aorta. • A small amount of blood goes from the right atrium into the right ventricle, leaves by the pulmonary trunk and through the ductus arteriosus into the aorta. • The mixed blood from the descending aorta returns to the placenta through umbilical arteries. Circulation before birth 1. Liver 2. Portal vein 3. Umbilical vein 4. Ductus venosus 5. Right atrium 6. Right ventricle 7. Vena cava superior 8. Foramen ovale 9. Vena cava inferior 10. Left atrium 11. Left ventricle 12. Ductus arteriosus 13. Lung 14. Pulmonary vein 15. Pulmonary artery 16. Descending aorta 17. Umbilical arteries 18. Ascending aorta 19. Interventricular septum 20. Ventricural wall Circulation after birth • At birth, the circulation of the fetal blood through the placenta ceases and the lungs begin to function. • The foramen ovale, ductus arteriosus, ductus venosus and umbilical vessels subsequently close or transform into corresponding ligaments as indicated below: - 1. Foramen ovale becomes fossa ovalis 2. Ductus arteriosus becomes ligament arteriosus 3. Ductus venosus becomes ligament venosus 4. Umbilical vein becomes round ligament 5. Umbilical arteries become umbilical ligaments Conclusion Origin of the CVS Formation of the heart tubes Partitioning of the atrium, ventricles and bulbus cordis and truncus atriosus Development of the sinus venosus, atria and ventricles Development of the aortic arches Fetal circulation and changes that occur immediately after birth .