Advanced - Cardiac Septation 2
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Development of the Atria
Membranous tissue forming the septum primum grows from the roof of the atrium, dividing it into left and right halves. The septum primum originates from myocardium that differentiates from splanchnic mesoderm near the venous pole and approaches the endocardial cushions. The foramen primum refers to the decreasing communication between the septum primum and endocardial cushions. The junction of the septum primum and endocardial cushions becomes myocardialised by ingrowth of myocardial cells, although the centre is maintained as dense connective tissue and is referred to as Todaro’s tendon. Apoptosis-induced perforations appear in the centre of the septum primum to produce the foramen secundum. At this time, the strong, muscular septum secundum grows immediately to the right of the septum primum and gradually overlaps the foramen secundum during the fifth and sixth weeks of development. Part of the atrial septum and dorsal right atrium, as well as the septum secundum develop from left-sided mesenchyme. The incomplete partition of the atrium by the septum secundum forms the foramen ovale. Blood flows through the foramen ovale and foramen secundum to the left atrium. The remaining portion of the septum primum acts as the valve of the foramen ovale. Blood cannot flow in the opposite direction as the muscular strength of the septum secundum prevents prolapse of the septum primum.
Remodelling of the venous pole, including the further induction of myocardial cells, contributes to the development of the atria. In the mouse, myocardial differentiation occurs in the dorsal mesocardium and cells are then recruited to the venous pole. The development of two left to right shunts in the venous system leads to an increase in the right horn of the sinus venosus and consequentially a decrease in left horn by the end of the fourth week. The sinuatrial orifice correspondingly shifts to the right thus becomes located in the right atrium. Hence the right atrium receives the superior vena cava (SVC) and inferior vena cava (IVC) in the adult. In mice and chicks the left sinus horn develops as the left SVC, however this regresses to form the coronary sinus in humans. Thus the sinus venosus gradually becomes incorporated into the right atrium. It contributes to the smooth walled part of the adult right atrium, referred to as the sinus venarum. The trabeculated right atrium corresponds to the primordial atrium; the division between these structures is indicated by the inner crista terminalis and outer sulcus terminalis.
The primordial pulmonary vein develops in the dorsal wall of the left atrium. As the atrium increases in size it incorporates more of the branches of the pulmonary vein, culminating in its receiving the four pulmonary veins. The smooth wall of the adult left atrium originated from the primordial pulmonary vein, while the trabeculated wall represents the primordial atrium. (Click image to play on current page or Play video on new page).
Development of the Ventricles
Minor trabeculations appear during early development of the primordial ventricle. Following growth of the ventricles further trabeculations appear and grow as larger, muscular structures. Some authors tout the idea that as trabeculations grow they coalesce resulting in the formation of the ventricular septum. However, the more commonly described theory of septation begins with the appearance of a primordial muscular interventricular (IV) ridge developing in the floor of the ventricle near the apex. As either side of the ventricle grows and dilates, their medial walls fuse forming the prominent IV septum. The foramen located between the cranial portion of the IV septum and the endocardial cushions, the IV foramen, closes by the end of the seventh week as the bulbar ridges fuse with the endocardial cushions.
Factors Controlling Chamber Specification
A gradient of retinoic acid with the highest concentration in the caudal segment of the heart tube establishes atrial identity. It defines the region of the heart tube that becomes the atria and atrioventricular canal. Other similar gradients exist which includes the slow myosin heavy chain 3 (slowMyHC3) gene, while the vitamin D receptor represses expression of slowMyHC3 in the ventricle thus is present in an inverse gradient.
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