Embryological Development of the Heart PDF

EMBERYOLOGICAL DEVELOPMENT OF THE HEART DR. ASHRF, BZIEZ HEART DEVELOPMENT The primitive heart is a single tube which soon shows grooves demarcating the sinus venosus, atrium, ventricle and bulbus cordis from behind forwards. As this tube enlarges it kinks so that its caudal end, receiving v...

EMBERYOLOGICAL DEVELOPMENT OF THE HEART DR. ASHRF, BZIEZ HEART DEVELOPMENT The primitive heart is a single tube which soon shows grooves demarcating the sinus venosus, atrium, ventricle and bulbus cordis from behind forwards. As this tube enlarges it kinks so that its caudal end, receiving venous blood, comes to lie behind its cephalic end with its emerging arteries. The sinus venosus later absorbs into the atrium and the bulbus becomes incorporated into the ventricle so that, in the fully developed heart, the atria and great veins come to lie posterior to the ventricles and the roots of the great arteries. The boundary tissue between the primitive single atrial cavity and single ventricle grows out as a dorsal and a ventral endocardial cushion which meet in the midline, thus dividing the common atrio-ventricular orifice into a right (tricuspid) and left (mitral) orifice ILLUSTRATION DEVELO. OF THE HEART The division of the primitive atrium into two is a complicated process but an important one in the understanding of congenital septal defects. A partition, the septum primum, grows downwards from the posterior and superior walls of the primitive common atrium to fuse with the endocardial cushions. Before fusion is complete, however, a hole appears in the upper part of this septum which is termed the foramen secundum in the septum primum. A second membrane, the septum secundum, then develops to the right of the primum but this is never complete; it has a free lower edge which does, however, extend low enough for this new septum to overlap the foramen secundum in the septum primum and hence to close it. DEVELO. OF HEART The two overlapping defects in the septa form the valve-like foramen ovale which shunts blood from the right to left heart in the fetus.. After birth, this foramen usually becomes completely fused leaving only the fossa ovalis on the septal wall of the right atrium as its memorial. In about 10% of adult subjects, however, a probe can still be insinuated through an anatomically patent, although functionally sealed foramen. DEVELO. OF Division of the ventricle is commenced by the upgrowth of a fleshy septum from the apex of the heart towards the HEART endocardial cushions. This stops short of dividing the ventricle completely and thus it has an upper free border, forming a temporary interventricular foramen. At the same time, the single truncus arteriosus is divided into aorta and pulmonary trunk by a spiral septum (hence the spiral relations of these two vessels), which grows downwards to the ventricle and fuses accurately with the upper free border of the ventricular septum. This contributes the small pars membranacea septi, which completes the separation of the ventricle in such a way that blood on the left of the septum flows into the aorta and on the right into the pulmonary trunk. SINUS The primitive sinus venosus absorbs into the right atrium so that the venae cavae draining into the VENOSUS sinus come to open separately into this atrium. The smooth-walled part of the adult atrium represents the contri- bution of the sinus venosus, the pectinate part represents the portion derived from the primitive atrium. Rather similarly, the adult left atrium has a double origin. The original single pulmonary venous trunk entering the left atrium becomes absorbed into it, and donates the smooth-walled part of this chamber with the pulmonary veins entering as four separate openings; the trabeculated part of the definitive left atrium is the remains of the original atrial wall.  Emerging from the bulbus cordis is a common arterial trunk termed the truncus arteriosus, from which arise six pairs of aortic arches, equivalent to the arteries supplying the gill clefts of the fish. These arteries curve dorsally around the pharynx on either side and join to form two longitudinally placed dorsal THE aortae which fuse distally into the descending aorta. DEVELOPMENT The 1st and 2nd arches disappear; the 3rd arches become the carotids. The 4th arch on the right becomes the brachiocephalic and right subclavian artery; on OF THE AORTIC the left, it differentiates into the definitive aortic arch, gives off the left subclavian artery and links up distally with the descending aorta. ARCHES AND The 5th arch artery is rudimentary and disappears. THEIR When the truncus arteriosus splits longitudinally to form the ascending aorta and pulmonary trunk, the 6th arch, unlike the others, remains linked with the DERIVATIVES latter and forms the right and left pulmonary arteries. On the left side this arch retains its connection with the dorsal aorta to form the ductus arteriosus (the ligamentum arteriosum of adult anatomy). AOTIC ARCHES AORTIC This asymmetrical development of the aortic arches accounts for the different course taken by the recurrent laryngeal nerve ARCHES on each side. In the early fetus the vagus nerve lies lateral to the primitive pharynx, separated from it by the aortic arches. What are to become the recurrent laryngeal nerves pass medially, caudal to the aortic arches, to supply the developing larynx. With elongation of the neck and caudal migration of the heart, the recurrent nerves are caught up and dragged down by the descending aortic arches. On the right side the 5th and distal part of the 6th arch absorb, leaving the nerve to hook round the 4th arch (i.e. the right subclavian artery). On the left side, the nerve remains looped around the persisting distal part the 6th arch (the ligamentum arteriosum) which is overlapped and dwarfed by the arch of the aorta. THE FETAL CIRCULATION The circulation of the blood in the embryo is a remarkable example of economy in nature and results in the shunting of well-oxygenated blood from the placenta to the brain and the heart, leaving relatively desaturated blood for less essential structures. Blood is returned from the placenta by the umbilical vein to the inferior vena cava and thence the right atrium, most of it by-passing the liver in the ductus venosus. Relatively little mixing of oxygenated and deoxygenated blood occurs in the right atrium since the valve overlying the orifice of the inferior vena cava serves to direct the flow of oxygenated blood from that vessel through the foramen ovale into the left atrium, while the deoxygenated stream from the superior vena cava is directed through the tricuspid valve into the right ventricle. From the left atrium the oxygenated blood (together with a small amount of deoxygenated blood from the lungs) passes into the left ventricle and hence into the ascending aorta for the supply of the brain and heart via the vertebral, carotid and coronary arteries. FETAL CIRCULATION "Dieses Foto" von Unbekannter Autor ist lizenziert gemäß CC BY-SA "Dieses Foto" von Unbekannter Autor ist lizenziert gemäß CC BY-SA FETAL CIRCULATION As the lungs of the fetus are inactive, most of the deoxygenated blood from the right ventricle is short-circuited by way of the ductus arteriosus from the pulmonary trunk into the descending aorta. This blood supplies the abdominal viscera and the lower limbs and is shunted to the placenta, for oxygenation, along the umbilical arteries arising from the internal iliac arteries. At birth, expansion of the lungs leads to an increased blood flow in the pulmonary arteries; the resulting pressure changes in the two atria bring the overlapping septum primum and septum secundum into apposition which effectively closes off the foramen ovale. At the same time active contraction of the muscular wall of the ductus arteriosus results in a functional closure of this arterial shunt and, in the course of the next 2–3 months, its complete obliteration. Similarly, ligature of the umbilical cord is followed by thrombosis and obliteration of the umbilical vessels. CONGENITAL ABNORMALITIES OF THE HEART AND GREAT VESSELS The complex development of the heart and major arteries accounts for the multitude of congenital abnormalities which may affect these structures, either alone or in combination. Dextro-rotation of the heart means that this organ and its emerging vessels lie as a mirror- image to the normal anatomy. It may be associated with reversal of all the intra-abdominal organs ( dextro- Cardia). SEPTAL DEFECTS At birth, the septum primum and septum secundum are forced together, closing the flap valve of the foramen ovale. Fusion usually takes place about 3 months after birth. In about 10% of subjects, this fusion may be incomplete. However, the two septa overlap and this patency of the foramen ovale is of no functional significance. If the septum secundum is too short to cover the foramen secundum in the septum primum, an atrial septal defect persists after the septum primum and septum secundum are pressed together at birth. This results in an ostium secundum defect, which allows shunting of blood from the left to the right atrium. This defect lies high up in the atrial wall and is relatively easy to close surgically. A more serious atrial septal defect results if the septum primum fails to fuse with the endocardial cushions. This ostium primum defect lies immediately above the atrioventricular boundary and may be associated with a defect of the pars membranacea septi of the ventricular septum. In such a case, the child is born with both an atrial and ventricular septal defect. Occasionally the ventricular septal defect is so huge that the ventricles form a single cavity, giving a trilocular heart. CONGENITAL PULMONARY STENOSIS may affect the trunk of the pulmonary artery, its valve or the infundibulum of the right ventricle. If stenosis occurs in conjunction with a septal defect, the compensatory hypertrophy of the right ventricle (developed to force blood through the pulmonary obstruction) develops a sufficiently high pressure to shunt blood through the defect into the left heart; this mixing of the deoxygenated right heart blood with the oxygenated left-sided blood results in the child being cyanosed at birth. FALLOT’S TETRALOGY The commonest combination of congenital abnormalities causing cyanosis is Fallot’s tetralogy. This results from unequal division of the truncus arteriosus by the spinal septum, resulting in a stenosed pulmonary trunk and a wide aorta which overrides the orifices of both the ventricles. The displaced septum is unable to close the interventricular septum, which results in a ventricular septal defect. Right ventricular hypertrophy develops as a consequence of the pulmonary stenosis. Cyanosis results from the shunting of large amounts of unsaturated blood from the right ventricle through the ventricular septal defect into the left ventricle and also directly into the aorta. TETRALOGY OF FALLOT "Dieses Foto" von Unbekannter Autor ist lizenziert gemäß CC BY-SA PERSISTENT DUCTUS ARTERIOSUS A persistent ductus arteriosus is a relatively common congenital defect. If left uncorrected, it causes progressive work hypertrophy of the left heart and pulmonary hypertension. AORTIC COARCTATION Aortic coarctation is thought to be due to an abnormality of the obliterative process which normally occludes the ductus arteriosus. There may be an extensive obstruction of the aorta from the left subclavian artery to the ductus, which is widely patent and maintains the circulation to the lower parts of the body. often there are multiple other defects and frequently infants so afflicted die at an early age. More commonly there is a short segment involved in the region of the ligamentum arteriosum or still patent ductus. In these cases, circulation to the lower limb is maintained via collateral arteries around the scapula anastomosing with the intercostal arteries, and via the link-up between the internal thoracic and inferior epigastric arteries. COARACTION OF THE ORTA THE SUPERIOR MEDIASTINUM This is bounded in front by the manubrium sterni and behind the first four thoracic vertebrae (Fig. 22). Above, it is in direct continuity with the root of the neck and below it is continuous with the three compartments of the inferior mediastinum. Its principal contents are: the great vessels, trachea, oesophagus, thymus—mainly replaced by fatty tissue in the adult, thoracic duct, vagi, left recurrent laryngeal nerve and the phrenic nerves (Fig. 17). The arch of the aorta is directed anteroposteriorly, its three great branches, the brachiocephalic, left carotid and left subclavian arteries, ascend to the thoracic inlet, the first two forming a V around the trachea. The brachio- cephalic veins lie in front of the arteries, the left running almost horizontally across the superior mediastinum and the right vertically downwards; the two unite to form the superior vena cava. Posteriorly lies the trachea with the oesophagus immediately behind it lying against the vertebral column THE THORACIC DUCT The cisterna chyli lies between the abdominal aorta and right crus of the diaphragm. It drains lymphatics from the abdomen and the lower limbs, then passes upwards through the aortic opening to become the thoracic duct. This ascends behind the oesophagus, inclines to the left of the oesoph- agus at the level of T5, then runs upwards behind the carotid sheath, descends over the subclavian artery and drains into the commencement of the left brachiocephalic vein (see Fig. 213). The left jugular, subclavian and mediastinal lymph trunks, draining the left side of the head and neck, upper limb and thorax respectively, usually join the thoracic duct, although they may open directly into the adjacent large veins at the root of the neck.  COURSE OF THE THORACIC DUCT "Dieses Foto" von Unbekannter Autor ist lizenziert gemäß CC BY-SA "Dieses Foto" von Unbekannter Autor ist lizenziert gemäß CC BY-SA THORACIC DUCT The thoracic duct thus usually drains the whole lymphatic field below the diaphragm and the left half of the lymphatics above it. On the right side, the right subclavian, jugular and mediastinal trunks may open independently into the great veins. Usually the subclavian and jugular trunks first join into a right lymphatic duct and this may be joined by the mediastinal trunk so that all three then have a common opening into the origin of the right brachiocephalic vein. CLINICAL FEATURES 1The lymphatics may become blocked by infection and fibrosis due to the Microfilaria bancrofti. This usually results in lymphoedema of the legs and scrotum but occasional involvement of the main channels of the trunk and thorax is followed by chylous ascites, chyluria and chylous pleural effusion. 2The thoracic duct may be damaged during block dissection of the neck. If noticed at operation, the injured duct should be ligated; lymph then finds its way into the venous system by anastomosing channels. If the accident is missed, there follows an unpleasant chylous fistula in the neck. 3Tears of the thoracic duct have also been reported as a complication of fractures of the thoracic vertebrae to which, in its lower part, the duct is closely related. Such injuries are followed by a chylothorax