DEVELOPMENT OF HEART Prof. Mujahid Khan

Objectives Describe heart tubes List dilation and constrictions of heart tube Describe endocardial cushion Describe atrial septum Describe ventricular septum Describe oval fossa اسم ورقم المقرر – Course Name and No. 7/19/2019

Development of Heart Primordium of heart is first evident at 18 days in the cardiogenic area Mesenchymal cells aggregate side by side to form 2 cardiac primordia, Angioblastic cords These cords canalize to form two thin walled endocardial heart tubes

Development of Heart The endocardial heart tubes approach each other and fuse to form a single heart tube after lateral folding Fusion of tubes begins at the cranial end of the developing heart and extends caudally

Primordial Myocardium As the heart tubes fuse, an external layer of heart the primordial myocardium is formed from mesoderm At this stage the developing heart is composed of a thin endothelial tube, separated from thick muscular tube by cardiac jelly

Endocardium The endothelial tube becomes the internal endothelial lining of the heart, called endocardium The primordial myocardium becomes the muscular wall of the heart or myocardium The visceral pericardium or epicardium is derived from mesothelial cells and spread over the myocardium

After Folding As folding of head region occurs The heart and pericardial cavity come to lie ventral (anterior) to the foregut and caudal (inferior) to the oropharyngeal membrane

Fate of Heart Tubes The tubular heart elongates and develops alternate dilations and constrictions: Truncus Arteriosus Bulbus Cordis Ventricle Atrium Sinus venosus اسم ورقم المقرر – Course Name and No. 7/19/2019

Fate of Heart Tubes As the developing heart elongates and bends, it gradually invaginates into the pericardial cavity Heart is now attached only at its cranial and caudal ends

Partitioning of Heart Partitioning of the atrioventricular canal, primordial atrium, and primordial ventricle begins around 4th week Completed by the end of the 5th week

Partitioning of Atrioventricular Canal Toward the end of the 4th week Endocardial cushions form on the dorsal (posterior) and ventral (anterior) walls of the AV canal The AV endocardial cushions approach each other and fuse Dividing the AV canal into right & left canals These canals partially separate the primordial atrium from the ventricle Endocardial cushion works as AV valves

Endocardial Cushion It develops from a specialized extracellular matrix (ECM) or cardiac jelly The transformed endocardial cushions contribute to the formation of the valves and membranous septa of the heart اسم ورقم المقرر – Course Name and No. 7/19/2019

Partitioning of Primordial Atrium Beginning at the end of the 4th week Is divided into right & left atria by the formation and subsequent modification and fusion of two septa: Septum primum Septum secondum

Septum Primum It’s a thin crescent shaped membrane Grows from the roof of the primordial atrium towards the fusing endocardial cushions Dividing the common atrium into left & right halves

Foramen Primum A large opening forms in the growing septum primum between its crescentic free edge and the endocardial cushion It serves as the shunt, enabling oxygenated blood to pass from right to the left atrium Becomes progressively smaller and disappears as the septum primum fuses with the fused endocardial cushions to form a primordial AV septum

Foramen Secondum Perforation appears in the central part of the septum primum before the foramen primum disappears Perforation coalesce to form another opening, the foramen secondum Concurrently, the free edge of the septum primum fuses with the left side of the fused endocardial cushions Foramen secondum ensures a continuous flow of oxygenated blood from right to the left atrium

Septum Secondum A crescentic muscular membrane Grows from ventrocranial wall of atrium Immediately to the right of the septum primum It is thick and grows during 5th & 6th weeks Gradually overlaps the foramen secondum in the septum primum

Septum Secondum Its forms an incomplete partition between the atria Consequently an oval foramen forms Cranial part of the septum primum attached to the roof of it disappears gradually Remaining part of septum primum (attached to endocardial cushion) forms a flap like valve of oval foramen

Oval Foramen Before birth it allows most of the oxygenated blood entering the right atrium from IVC to pass into the left atrium Prevents the blood flow in opposite direction After birth it normally closes and the valve of the oval foramen fuses with septum primum The interatrial septum becomes a complete partition between the atria

Oval Fossa An oval depression in the lower part of the interatrial septum of the right atrium known as oval fossa It’s a vestige of the oval foramen اسم ورقم المقرر – Course Name and No. 7/19/2019

Partitioning of Primordial Ventricle Division of primordial ventricle is first indicated by a median muscular ridge, the primordial interventricular septum Is a thick crescentic fold has a concave free edge Initially most of its height results from dilation of the ventricles on each side of the IV septum

Partitioning of Primordial Ventricle Medial walls of the enlarging ventricles approach each other and fuse to form the primordium of the muscular part of the IV septum Active proliferation of myoblasts in the septum increase its size Until the 7th week there is a crescent shaped interventricular foramen between the free edge of IV septum and the fused endocardial cushion

Interventricular Foramen The IV foramen permits communication between the right and the left ventricles It usually closes by the end of the 7th week

Interventricular Septum The membranous part of the IV septum is derived from an extension of tissue from the right side of the endocardial cushion to the muscular part of the IV septum

Interventricular Septum After closure of the IV foramen and formation of the membranous part of the IV septum, the pulmonary trunk is in communication with the right ventricle and the aorta with the left ventricle

THE END اسم ورقم المقرر – Course Name and No. 7/19/2019