Embryology of the Heart Development PDF

Embryology of the heart DEVELOPMENT OF THE HEART FORMATION OF THE HEART TUBE  Angiogenic cell clusters arise in the splanchnic layer of lateral plate mesoderm at the cephalic end of the embryonic disc.  They form endothelial blood vessels and islands which...

Embryology of the heart DEVELOPMENT OF THE HEART FORMATION OF THE HEART TUBE  Angiogenic cell clusters arise in the splanchnic layer of lateral plate mesoderm at the cephalic end of the embryonic disc.  They form endothelial blood vessels and islands which unite to form right and left endocardial heart tubes.  As a result of folding of the embryo in a transverse direction, the endocardial heart tubes fuse to form single median heart tube.  As the head fold develops (craniocaudal), the endocardial tube and pericardial cavity rotate on a transverse axis about 180 degrees, so that they come to lie ventral to the oesophagus and below developing mouth. S O SI ON E UD PS 11  Dorsal mesocardium suspending heart tube to the pericardial cavity disappears forming the transverse pericardial sinus with connection of both sides of the pericardial cavity  The splanchnic mesoderm (of pericardial cavity) which surrounds the endocardial heart tube forms a myoepicardial layer which differentiates into the myocardium (cardiac muscle) and the epicardium (visceral layer of the serous pericardium).  Therefore, the wall of heart tube consists of three layers, endocardium, myocardium, and epicardium. It has upper arterial end and lower venous end.  The heart undergoes differential expansion, so that several dilatations are formed from the arterial to the venous ends: truncus arteriosus, bulbus cordis, ventricle, atrium, and sinus venosus. Formation of the cardiac loop  At first the heart tube is straight then it bends due to the following causes:(exam) S 1. Bulbus cordis and ventricle elongate more rapidly than rest of tube. 2. Arterial and venous ends of heart tube are fixed by the pericardium. SI  The upper portion of the tube bends ventrally, downwards and to the right. O The lower atrial portion shifts dorsally, upwards and to the left (becomes ON U- shaped).  Then it becomes S-shaped as the atrium lying posterior to the ventricle.  The atrium expands transversely and appears on either side of the bulbus cordis. E UD PS 12  Fate of the sinus venosus  It consists of a small transverse portion and the right & left horns.  Each horn receives three veins: 1. The vitelline veins from the yolk sac. 2. The umbilical veins from the placenta. 3. The common cardinal veins from the body wall.  At first, junction between sinus and atrium (sinoatrial orifice) is wide in the middle then, it shifts to the right and becomes guarded by the right and left venous valves which fuse above to form septum spurium. S O SI ON Fig. (14): Fate of sinus venosus E UD PS 13  SUMMARY OF FATE OF SINUS VENOSUS (VIP) Right side Left side Right horn is absorbed into right atrium Left horn forms the coronary sinus. forms the smooth part and crista terminalis O p e n n i n g of the right vitelline vein Left umbilical and vitelline veins are forms the opening of inferior vena cava. obliterated at 5th week The right umbilical vein disappears Right common cardinal vein forms the Left common cardinal vein is lower part of superior vena cava. obliterated at 10th week and its remaining part forms the oblique vein of left atrium (Marshal). The lower part of the right venous valve Left venous valve and septum spurium forms the valves of I.V.C and coronary fuse with the interatrial septum. sinus. S O SI ON E UD PS 14  Fate of the primitive atrium At 4th week, it is divided into right and left atria as follows: 1. Division of the atrioventricular canal into right (tricuspid) and left (mitral) by formation of septum intermedium. Each canal is absorbed into the corresponding atrium. 2. Formation of the interatrial septum:  Septum primum grows from the roof of primitive atrium towards septum intermedium leaving an opening below; foramen primum.  With growth of the septum, the foramen primum is gradually obliterated.  Then, the upper part of septum primum degenerates forming a new opening called foramen secundum.  Septum secundum appears on the right side of the septum primum and bounds an opening called foramen ovale with a valvular mechanism which permits passage of blood from right to left atria through foramen ovale. S O SI ON E UD Fig (15): Formation of septum primum & septum secundum PS  Immediately after birth, foramen ovale is closed due to: a) A decrease in right atrial pressure due to occlusion of placental circulation. b) An increase in left atrial pressure due to increased pulmonary venous return. 15   Septum primum fuses with septum secundum to form the interatrial septum.  Lower margin of septum secundum forms limbus fossa ovalis.  Septum primum forms the floor of fossa ovalis. 3. Expansion of left atrium: by absorption of the pulmonary vein (opening into back of left atrium) and its two branches to each lung forming its large smooth posterior part with formation of 4 openings of pulmonary veins. Components of The Two Atria Right atrium Left atrium Right 1/2 of primitive atrium forms the Left 1/2 of primitive atrium forms the anterior rough part and the right auricle. rough auricular part Right horn of the sinus venous forms the Pulmonary vein and its branches forms the posterior smooth part. smooth part Upper part of the right atrio-ventricular Upper part of the left atrio-ventricular S canal forms the lower part. canal forms the lower part O SI ON E UD PS 16 Defects of the interatrial septum (ASD): 1. Patent foramen ovale: due to incomplete fusion of septa primum and secundum. 2. Foramen secundum defect: due to excessive degeneration of septum primum or defective development of septum secundum. 3. Common atrium (cortrilocular biventricular): (‫ )ﻗﻠﺐ ﺛﻼﺛﻲ ﺍﻟﻐﺮﻑ‬due to complete failure of development of the septa (primum and secundum). 4. Premature closure of foramen ovale. S  Defects of atrioventricular (A-V) septum (intermedium): SI 1. Persistent A-V canal: due to failure of formation of septum intermedium. 2. Tricuspid atresia (obliteration of the right A-V canal). O ON E UD PS 17  Fate of primitive ventricle and bulbus cordis  During the 4th week, a muscular part of interventricular septum extends upwards from floor of primitive ventricle leaving an interventricular foramen (IVF) between it and the septum intermedium.  Distal bulbar septum is formed in the distal part of the bulbus cordis and divides it into infundibulum of right ventricle and aortic vestibule of left ventricle and closes the upper part of IVF.  At the end of 7th week, right margin of septum intermedium extends downwards to close lower part of IVF.  Therefore, membranous part of interventricular septum is formed from: S 1. Distal bulbar septum (upper part). SI 2. Extension from the septum intermedium (lower part). Components of the two ventricles Right ventricle O Left ventricle ON Right part of the primitive ventricle Left part of the primitive ventricle forms forms the rough trabeculated part. the rough trabeculated part. UD Right of the bulbus cordis forms Left part of the bulbus cords forms the the upper smooth part (Infundibulum). upper smooth part (Aortic vestibule). E PS 18  Defects of interventricular septum (VSD) 1. Membranous VSD: due to failure of development of the membranous interventricular septum. 2. Muscular VSD: due to perforations in muscular interventricular septum. 3. Common ventricle: due to failure of development of both membranous and muscular interventricular septa. Fate of the truncus arteriosus S  At 5th week, two ridges develop in the truncus arteriosus in a spiral course and SI fuse forming a spiral aortico-pulmonary septum (by migrating neural crest cells)  It divides the truncus into pulmonary trunk and ascending aorta. O  Pulmonary trunk lies anterior and to right of aorta near heart and posterior and ON to left away from it due to spiral course of the septum. E UD PS 19  Defects of the aortico- pulmonary septum: 1. Persistent truncus arteriosus:due to failure of development of the spiral aortico-pulmonary septum. 2. Tetralogy of Fallot: Pulmonary stenosis, overriding of aorta, VSD and right ventricular hypertrophy. 3. Aortic valve stenosis and atresia. 4. Pulmonary valve stenosis and atresia. 5. Transposition of the great vessels due to reversed development of the spiral aortico-pulmonary septum. S SI O ON E UD PS 20