Development of the heart October -08 -12 TUBE FORMATION • 15 days after conception 0 Begins as flat sheet of mesodermal cells (cells of the mesoderm layer in an embryo) • Day 18-20 0 The cells form a horse -shoe shape (cardiogenic region) 0 Angiogenic cell clusters coalesce to form right and left endocardial tubes • Day 21 0 Two endocardial tubes are completely fused • Day 22 0 Heart begins beating • Day 23 0 Heart tube forms constrictions prior to looping The first indication of heart development is around day 16- 19. Clusters of angiogenetic cells form a horseshoe- shaped” cluster anterior and lateral to the brain plate. Following cardiac mesoderm involution during gastrulation, the two endocardial tubes fuse along the embryonic midline. The heart starts to beat at day 22, but the circulation does not start until days 27 to 29. The single tubular heart develops many constrictions outlining future structures. Prior to looping the heart tubes forms constrictions defining separate components of the heart, these are (cranial to caudal): Bulbus Cordis , Ventricle and Atrium. The primitive atrium is still paired and onnects caudally to the paired sinus venosus. The heart tube will bend ventrally, caudally and slightly to the right. At this stage the paired sinus venosus extend laterally and give rise to the sinus horns. As the cardiac looping is developing the paired atria form a common chamber and move into the pericardial sac. New Section 1 Page 1 LOOPING • Day 21 0 Linear heart tube bends toward right side • Day 22 0 Chambers begin to form 0 Left ventricle is expands to a large size and is spherical ? Formed by most of the heart tube 0 Right ventricle is small ? Anterior to left ventricle • Day 26 As the heart continues to loop, the right and left ventricle arrange themselves so that they are beside one another New Section 1 Page 2 beside one another 0 0 Atria are arranged side by side on the posterior side ?
Begin to expand • Day 28 0 Ventricles grow downward ? Now they are anterior and inferior to the atria ATRIAL SEPTATION • Day 34 0 Two septa grow in order to separate the atria ? Septum primum grows from the ventral and posterior walls of the atrium ? A hole is formed in the septum primum, called the foramen (ostium) secundum ? Allows blood to go from right atrium to left atrium Septum secundum grows from the dorsal and posterior walls ? Thick and muscular, unlike the septum primum which is thin • Day 50 0 Septum primum fuses with atroventricular/endocardial cushions (cells located at the atroventricular border to separate the atria and the ventricles) 0 The two septums partially overlap 0 Foramen ovale (orifice between the two flaps) is kept open ? Allows maternal circulation to the heart ? When the child is born, the lungs inflate, applying pressure on the septum primum against the septum secundum to completely fuse the septa New Section 1 Page 3 he septum secundum to completely fuse the septa 0 0 0 New Section 1 Page 4 0 VENTRICULAR SEPTATION • Day 38 0 Cells that join the left and right ventricles grow upward • Day 46 0 Cells grow toward endocardial cushions ? Stops growing before the ventricles are completely separated ? Later in development, membranous septum is formed to connect the muscular septum to the endocardial cushions OUTFLOW TRACT SEPARATION • Day 35 0 Outflow tract (truncus arteriosus) is a single tube that is connected to both ventricles • Day 40 0 Outflow tract undergoes epithelial-mesenchymal transformation (EMT) ?
Forms internal outflow tract cushions (truncoconal swellings) that converge towards one another • Day 45 0 Base of outflow tract rotates clockwise ? Length of tract shortens 0 Truncoconal swellings rotate and fuse at midline • Day 56 0 Aorta and pulmonary outflow tracts are separated 0 Neural crest cels at base of tract helps in outflow tract septation 0 Result ? Two separate blood vessels 1) Aorta – connected to left ventricle 2) Pulmonary artery – connected to right ventricle New Section 1 Page 5 2) Pulmonary artery – connected to right ventricle • • • • • • Congenital Heart Defects Present at birth
Not necessarily hereditary Disrupts the functioning or structure of the heart Most defects cause the blood flow in the heart to flow abnormally Atrial septal defect 0 Occurs in 1/3 of cases of congenital heart disease in adults; occurs more often in women 0 Leakage of blood flow between the left and right atria via the interarterial septum 0 During development of the heart, foramen ovale allows blood to pass from the right atrium to the l eft atrium ? Allows blood to bypass the fetal lungs, because the fetus obtains oxygen from the placenta ? After birth, the foramen oval is supposed to close completely ?
In this case, it does not 0 right ventricle has to contract stronger to pump out more blood received • hypertrophy (enlargement) of right ventricle & pulmonary hypertension • Worsening effect ? coronary artery disease ? increase stiffness of left ventricle ? increase filling pressure of left ventricle ? hypertension • Ventricular septal defect 0 Most common in infants and children 0 Hole in wall that separates right and left ventricles 0 Symptoms: fast breathing, sweating, fast HR, inability to gain weight 0 25-40% of these defects will close spontaneously by the time the child is 2 years old ?
Of the remaining, 90% of them will closed spontaneously by 10 years old 0 When the left ventricle contracts, it ejects some blood into the aorta and some across the ventricular septal defect into the right ventricle and pulmonary artery New Section 1 Page 6 ventricular septal defect into the right ventricle and pulmonary artery ? • Patent ductus arteriosis 0 Blood in the pulmonary artery and aorta mix 0 During development, the ductus arteriosis is an open passageway between these two vessels ? Should close after birth 0 Symptoms: fatigue, rapid breathing • OBSTRUCTION DEFECTS 0 Obstructions are called stenoses Pulmonary stenosis ? Pulmonary valve between the right ventricle and pulmonary artery does not open properly ? Right ventricle must pump harder to overcome the obstruction ? Aortic stenosis ? Aortic valve between the left ventricle and aorta is narrowed ? Decreases blood supply to the body ? Most children have no symptoms, but may have chest pain, unusual tiring, dizziness, fainting ? Valve can be enlarged surgically or replaced with artificial one ? Coarctation of the aorta ? Aorta is constricted ? Symptoms: congestive heart failure, high blood pressyre ? Bicuspid aortic valve ? Normal arotic valve has 3 cusps Has only 2 cusps in this case ? Subaortic stenosis ? Constriction of left ventricle below the aortic valve ? Limits flow out of left ventricle ? Ebstein’s anomaly ? Downward displacement of tricuspid valve ? Usually associated with atrial septal defect • CYANOTIC DEFECTS 0 Blood pumped to body contains insufficient amount of oxygen 0 Skin appears blue ? Tetraology of Fallot ? Ventricular spetal defect (blood passed from right to left ventricle without going to l ung first), narrowing at or beneath pulmonary valve, right ventricle more muscular than normal, aorta lies directly ver ventricular septal defect New Section 1 Page 7