Intermediate - Atrial Ventricular Septation
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All of the partitioning of the primitive heart occurs between the middle of the fourth week and the end of the fifth week. The following animation shows the processes involved in the division of the atrioventricular canal, atria and ventricles. These are described in more detail in the text below.
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Division of the Atrioventricular (AV) Canal
Two endocardial cushions form on the dorsal and ventral surfaces of the AV canal, referred to as the superior and inferior cushions respectively. The cardiac jelly in this region expands while mesenchymal cells from the endocardium invade the cushions, allowing them to grow and fuse. This fusion divides the common AV canal into the right and left canals, hence partially separating the primitive atrium and ventricle. Two smaller endocardial cushions also form on the lateral walls of the AV canal, which later help to form the mitral and tricuspid heart valves.
Septation of the Atria
Membranous tissue forming the septum primum grows from the roof of the atrium, dividing it into left and right halves. The space between the septum primum and the endocardial cushions is referred to as the foramen primum. 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. The incomplete partition of the atrium by the septum secundum forms the foramen ovale.
Blood flows from the right atrium through the foramen ovale and foramen secundum to the left atrium, forming a right-to-left shunt. 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.
The flow of blood throughout the septated atria can be seen below:
Remodelling of the Inflow tract and Atria
The development of two left-to-right shunts in the venous system leads to an increase in size of the right horn of the sinus venosus and consequently a decrease in left horn by the end of the fourth week. The sinuatrial orifice correspondingly shifts to the right and thus becomes located in the right atrium. Hence the right atrium receives the superior vena cava and inferior vena cava in the adult. The left sinus horn 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 LA. 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 LA originated from the primordial pulmonary vein while the trabeculated wall represents the primordial atrium.
Septation 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 researchers believe that as the 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 (see next section) fuse with the endocardial cushions.
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Apex: Anatomical term referring to the most inferior, left, downwards pointing part of the heart.
Atrioventricular canal: Junction between the primitive atrium and primitive ventricle in the embryo. This canal splits to later form two atrioventricular canals which consequently form the valves of the adult heart.
Bulbar ridges: Endocardial cushion tissue located in the bulbus cordis extending into the truncus arteriosus thus forming ridges. These fuse together to form the aorticopulmonary septum.
Coronary sinus: a venous sinus emptying into the right atrium that collects blood from the myocardium of the heart.
Endocardial cushions: Swellings of migrated cells on the inner lining of the heart located in the atrioventricular canal.
Foramen ovale: Shunt allowing blood to enter the left atrium from the right atrium. It is located in the septum secundum.
Foramen primum: Original space between the septum primum and the fused endocardial cushions as the septum primum grows towards the cushions.
Foramen secundum: Refers to the coalesced perforations in the septum primum after it has fused with the endocardial cushions.
Inferior vena cava (IVC): Large vein which carries deoxygenated blood from the lower half of the body to the right atrium.
Interventricular septum: Wall of muscular tissue growing from the base of the heart dividing the primitive ventricle into the left and right ventricles.
Interventricular foramen: Space between the interventricular septum and the fused endocardial cushions. The foramen closes when the septum fuses with the endocardial cushions and bulbar ridges.
Mitral valve (bicuspid valve): Two leaflet valve located on the left side of the heart i.e. between the left atrium and ventricle.
Pulmonary veins: Four veins that allow oxygenated blood from the lungs to empty into the left atrium.
Septum primum: Original structure growing from the roof of the heart towards the endocardial cushions dividing the primitive atrium.
Septum secundum: Second structure growing to the left of the septum primum dividing the primitive atrium.
Sinuatrial orifice: The opening between the sinus venosus and right atrium which has two valve leaflets to prevent backflow of blood.
Sinus venarum: Smooth-walled portion of the adult right atrium; originally the left horn of the sinus venosus.
Superior vena cava (SVC): Short vein which carries deoxygenated blood from the upper half of the body to the right atrium.
Trabeculae (trabeculations): Muscular beams located within the ventricles and parts of the atria of the heart.
Tricuspid valve: Three leaflet valve located in the right atrioventricular canal i.e. between the right atrium and ventricle.