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LATE DEVELOPMENT AND PARTITIONING OF THE HEART LEARNING OBJECTIVES • By the end of this lecture the students should be able to understand: • The development of heart tube its division and its rotation • Development of Four Chambered Heart • Interatrial and Interventricular septum formation • Formation of Heart Valves • Formation of Conduction System THE HEART TUBE • The heart tube is formed by the fusion of two endocardial tubes in the cardiogenic area Site of Development of Heart * Development of heart begins in cardiogenic area, located at the cranial edge of the trilaminar germ disc Pos itio n of Hea rt Tube * Folding of head region of embryo brings the heart and pericardial cavity ventral to the foregut and caudal to the oropharyngeal membrane The Divisions Heart Tube • The tubular heart develops 5 demarcations: • Trunsus Arteriosus • • • • Bulbus Cordis Ventricle Atrium Sinus Venosus Folding of Heart Tube • The heart tube folds back on itself forming an “S” shaped curve. • The bulbus cordis and ventricle grow more rapidly forming a “U” shaped Bulboventricular loop • Bulboventricular loop shifts the region of heart to the right and ventrally Division of Bulbus Cordis • Proximal dilated portion is incorporated into ventricle • Mid-part called conus cordis, will form the outflow tracts of both ventricles. • Distal part of the bulbus cordis along with truncus arteriosus, will form the roots and proximal portion of the aorta and pulmonary artery Atrioventricular Canal • Both atrium and ventricle are connected by means of single tube, the common atrioventricular canal Development of Right atrium • The sinus venosus incorporates into the dorsal heart wall. • The right sinus horn and veins enlarge greatly and become the only communication between the original sinus venosus and the atrium. Development of Right atrium • The right auricle and the rough portion of the right atrium that contain pectinate muscles represent remnants of the original embryonic right atrium. • The left horn of the sinus venosus become the coronary sinus • The proximal part of the left common cardinal vein become oblique vein of the left atrium Development of Left atrium • The original embryonic left atrium becomes the trabeculated left auricle. • The smooth left atrium develops from the primitive pulmonary vein as its main branches incorporate into the wall. Dorsal Mesocardium • The heart tube sinks into the pericardial cavity and becomes dorsomedially suspended by the dorsal mesocardium which is formed from the right and left epimyocardial layers and later disappears, except at its cephalic and caudal ends Septum formation of the Heart • Development of the primitive heart with a single atrium and ventricle, into the typical four-chambered structure occurs between the fourth and seventh weeks by formation of interatrial and interventricular septa • Many congenital heart problems can develop during this crucial time Atrioventricular septum • Toward the end of the fourth week, dorsal and ventral endocardial cushions are developed in the walls of the common atrioventricular canal at junction of atrium and ventricle Endocardial cushions • Endocardial cushions grow toward each other and, during the sixth week, meet and fuse, dividing the common atrioventricular canal into right (tricuspid) and left (mitral, or bicuspid) atrioventricular canals The Atrial Septum • The atrial septum is responsible for the division of primitive atrium into Right and Left Atria • The process involves the formation of: – Septum primum – Septum secondum Septum Primum • Septum primum first forms during the fourth week as a partition in the dorsocephalic wall of the primitive atrium • It grows towards the Atrioventricular septum Foramen primum Foramen Primum • The space between the free edge of septum primum and the developing endocardial cushions • It gets obliterated when the septum primum fuses completely with AV septum Foramen Secondum • It is formed due to obliteration of tissue in the center of septum primum Septum secondum • Grows alongside the Right edge of Septum primum • It fuses with the remaining portion of Septum primum to cover the foramen secondum, forming the Interatrial Septum Foramen Ovale • Defect remaining in the septum secondum even after its fusion with septum primum • Foramen ovale allows Right to Left shunt Ventricular Septum • Ventricular septum is formed as: – Interventricular septum – AV cushions – Spiral (Aorticopulmonary) septum • Interventricular septum is formed as – Muscular septum – Membranous septum The Interventricular Septum Muscular septum • Grows upwards from the base of the primitive ventricle towards the AV septum • The resulting gap is called the Interventricular foramen The Interventricular Septum Membranous septum • Grows downwards from the AV cushions and fuses with the muscular septum • The interventricular foramen is thus obliterated Spiral (Aorticopulmonary) Septum • Neural crest cells migrate in the conotruncal ridges forming Aorticopulmonary septum • As the septum descends, it undergoes spiral twisting • Truncus arteriosus is thus separated into Aorta on the left and Pulmonary trunk on the right Development of Aorticopulmonary Septum The Semilunar Valves • The semilunar valves begin to develop from three swellings of subendocardial tissue around the orifices of the aorta and pulmonary trunk • These swellings are hollowed out and reshaped to form three thin-walled cusps The AV valves The AV valves (tricuspid and mitral valves) develop similarly from localized proliferations of tissue around the AV canals. Conduction System • The SA node develops during the fifth week. It is originally in the right wall of the sinus venosus, but it is incorporated into the wall of the right atrium with the sinus venosus Conduction System After incorporation of the sinus venosus, cells from its left wall are found in the base of the interatrial septum just anterior to the opening of the coronary sinus. Together with cells from the AV region, they form the AV node and bundle • The fibers arising from the AV bundle pass from the atrium into the ventricle and split into right and left bundle branches REFERENCES Langman’s embryology KLM introduction to embryology ***********************************************************