MD121_Cardiovascular_L6_Embryology and septal formation_2024_GD.pdf

Lecture 6 Introduction to Heart Embryology Duffy 2024 Learning Objectives At the end of this lecture, you should: Know the main points of early cardiac development Know the derivatives of the main early vessels Be able to label the early heart at specific stages of development Know the main aspects...

Lecture 6 Introduction to Heart Embryology Duffy 2024 Learning Objectives At the end of this lecture, you should: Know the main points of early cardiac development Know the derivatives of the main early vessels Be able to label the early heart at specific stages of development Know the main aspects of heart folding Describe formation of the great vessels Describe atrial and ventricular septum formation Know the difference between the foetal and neonatal circulation and be able to list the blood-shunts Cardiovascular System The first major system to function in the embryo. The primitive heart and vascular system appear in the middle of the third week of development The heart begins to beat at day 22 Early stages: week 3 Paired angioblastic cords appear in mesoderm They canalize and form 2 heart tubes These fuse together late in week 3 B. Through level of heart primordium C. Through entire embryo, from front to back Heart tubes: 2 become 1 During lateral folding, the 2 heart tubes fuse to form one (craniocaudally) The primordial myocardium forms from splanchnic mesoderm Oropharyngeal membrane = future mouth and pharynx Septum transversum = central tendon of the diaphragm The sinus venosus receives the paired veins The venous and arterial ends of the heart are fixed by the pharyngeal arches and septum transversum respectively. The bulbus cordis and the ventricle grow faster and the heart bends upon itself, forming a Ushaped bulboventricular loop. Changes to the Heart Tube Elongation and dilatation produces -from above down: Truncus arteriosus (continuous with the aortic sac which lies above); Bulbus cordis; Ventricle; Atrium, Sinus venosus Differential Growth As the primitive heart bends, the atrium and sinus venosus end up behind the bulbus cordis, truncus arteriosus and ventricle. Around this time, the sinus venosus has developed two horns The heart gradually invaginates the pericardial cavity 24 Days Pattern of Circulation Circulation now is as follows: >Blood in from the paired veins to sinus venosus >Passes to atrioventricular canal into primordial ventricle >Ventricle contracts, blood forced through bulbus cordis and truncus arteriosus to aortic sac >Distribution to body. *Partitions of the heart develop around 4th week, essentially finished by end of 5th (35 days). 1 Truncus arteriosus 2 Bulbus cordis 3 Material for atria 4 Material for ventricles 5 Sinus venosus 6 Left umbilical vein 7 Left omphalomesenteric vein 8 Pericardium 9 Interventricular sulcus 1 Truncus arteriosus 2 Bulbus cordis 3 Material for atria 4 Material for ventricles 5 Sinus venosus 6 Left umbilical vein 7 Left omphalomesenteric vein 8 Pericardium 9 Interventricular sulcus 1 Truncus arteriosus 2 Bulbus cordis 3 Material for atria 4 Material for ventricles 5 Sinus venosus 6 Left umbilical vein 7 Left omphalomesenteric vein 8 Pericardium 9 Interventricular sulcus 1 Truncus arteriosus 2 Bulbus cordis 3 Material for atria 4 Material for ventricles 5 Sinus venosus 6 Left umbilical vein 7 Left omphalomesenteric vein 8 Pericardium 9 Interventricular sulcus Embryonic Arteries These form a large network During development they coalesce to form the Aorta and other major vessels Related to Head & Neck Embryology (pharyngeal arches - cranially) Each arch receives an artery Not all are present simultaneously Many Obliterate 1st arch disappears 2nd arch disappears 3rd arch – Common carotid – Proximal internal carotid – External branches of the 3rd arch 4th arch – Left becomes arch of the aorta – Right becomes subclavian artery 5th arch disappears 6th arch – Pulmonary trunk – Left – ductus arteriosus Truncus Septation Blood exits the left ventricle through the aorta and exits the right ventricle through the pulmonary artery. Fusion of the outflow tract cushions results in separation of the blood flow. Truncus Arteriosous Changes… The truncus arteriosus will divide It forms: – The aorta – The pulmonary trunk. Persistent Truncus Arteriosus – Single outflow tract remains – Always associated with a Ventricular Septal Defect (membranous part of the septum fails to form) – Cyanosis, systolic murmur Transposition of the Great Vessels – Conotruncal septum fails to spiral – Cyanosis – usually a systolic murmur, not compatible with life unless another shunt is present Intra-atrial Septum Formation Septation Right atrial Entrance of sinus venosus has shifted over to right Atrium enlarges in size Some of the new wall is formed from the sinus venosus (smooth wall of right atrium) Right venous valve Crista terminalis Valve of IVC Valve of coronary sinus Left Atrium Now no venous inflow... Bud / outgrowth develops Grows posteriorly Pulmonary veins Divides / grows & connects with developing lung buds Atrium enlarges in size Some of new wall is formed from these pulmonary veins (smooth) https://embryology.med.unsw.edu.au/embryology/index.php/Heart_ Atrial_Septation_Movie Development of the Atrial septum Septum primum Sickel shaped Grows from roof of atrium into lumen Ostium primum Opening at lower free edge of septum primum Cell death / perforations in upper septum Before ostium primum closes Form ostium secundum Septum primum Fuses with endocardial cushions Ostium Secundum Septum secundum Grows from roof of atria Never becomes a complete septum Lower free edge - Foramen Ovale Pre-natal Lungs deflated High Pulmonary Vascular Resistance (PVR) & arterial pressures Blood flows from right to left Post-natal Lungs inflate Pulmonary (& right heart) pressures drop Pressures now LA > RA Patent Foramen Ovale (PFO) Usually asymptomatic 10 – 15% with ECHO Atrial Septal Defects Left-to-right shunting of blood Exercise intolerance in older children/adults Acyanotic heart disease May be asymptomatic Auscultation: -Ejection systolic murmur Ostium Primum defect Septum primum fails to close ostium primum Just above AV valves Risk of associated valve defects Ostium secundum defects Most common defect 70% of ASDs Occur in region of fossa ovalis Septum primum & secondum fail to fuse Two main causes Excessive resorption of septum primum Underdevelopment or absence of septum secundum Interventricular Septum Muscular part Majority of septum Formed by medial walls of expanding ventricles Membranous part Superiorly an interventicular foramen exists Tissue from inferior endocardial cushion closes this foramen Ventricular septal defect 2 main forms Muscular vs. membranous Spontaneous closure may occur RA Closure Physiological, then anatomical Closure of Ductus Arteriosus Pre-natal Blood flows from right to left Post-natal Bradykinin released Smooth muscle contracts Physiological, then anatomical closure -Ligamentum arteriosum Closure of ductus arteriosus Patent ductus arteriosus (PDA) Postnatal blood flow left to right Pulmonary hypertension “machine-like” murmur (systole & diastole) Summary Acyanotic Left to right shunt Cyanotic Right to left shunt -Reduced flow to pulmonary circulation (patent truncus, Tetrology of Fallot) Tetralogy of Fallot Unequal division of conus cordis -(Septum too anterior) Pulmonary stenosis -Narrow right ventricular outflow Large VSD -Ventricular septal defect Overriding aorta -From directly above septal defect Hypertrophy of the right ventricle -Raised right interventricular pressure