Formation of the atria
- The right and left atria undergo remodeling during weeks 4 and 5.
- The bilateral sinus horns fuse to form transverse sinus venosus, which then shifts from midline position to drain exclusively into right atrium through slit-like orifice.
- The orifice from sinus venosus is incorporated into the wall of the right atrium to form sinus venarum, creating the smooth walled portion of the posterior atrial chamber.
- The smooth sinus venarum is clearly distinguishable from the atrial appendage (auricle) and remainder of the right atrium, which have pectinate trabeculations that are functionally contractile.
- A ridge of tissue called the crista terminalis demarcates the line between the sinus venarum and remainder of right atrium. This tissue contains fibers that will later carry impulses from sinoatrial node to atrioventricular node allowing for spread of depolarizing electrical current and contraction of the heart.
- The orifice continues to enlarge to allow drainage of inferior vena cava and coronary sinus into right atrium.
- Tissue flaps develop on right and left sides of these orifices to form the venous valves opening to the inferior vena cava and coronary sinus.
- A single pulmonary vein originates from the dorsal mesocardium and initially lies in the midline, connecting future lungs to dorsal aspect of common atrium.
- The single large orifice is later divided into the four orifices of the four pulmonary veins.
- This pulmonary vein shifts toward the left and commits itself to the definitive left atrium then bifurcates twice to form two right and two left pulmonary veins.
- The first two pulmonary vein branches are incorporated into the left atrium, forming the smooth walled region of the left atrium.
- The trabeculated and contractile portion of left atrium is shifted toward the left and ventrally forming an appendage (auricle).
- The major events in heart septation occur between days 28 and 37 (Larsen) via differential growth and remodeling.
- Atrial septation begins on approximately day 26, when the bulbus cordis and truncus arteriosus push on the superior and exterior aspect of the common atrium, creating a depression along the midline.
- Complete septation requires formation of 2 separate structures:
- On day 28, a sickle-shaped, thin septum primum begins to grow from posterosuperior wall, extending toward atrioventricular canal, while a projection of cells originating from the dorsal mesocardium at the inferior pole grows up to meet and contribute to septal formation.
- A foramen in the septum primum, called the ostium primum, located near atrioventricular septum leaves a connection between right and left atria.
- The ostium primum is later obliterated by simultaneous fusion between concave edge of septum primum with dorsal and ventral endocardial cushion tissue, dividing the atrioventricular canal into right and left orifices.
- At the end of the 6th week, the septum primum fuses with the atrioventricular septum and eliminates the ostium primum.
- Before the ostium primum fully closes, fenestrations form in anteriosuperior portion of septum primum; these fenestrations coalesce to form second hole, referred to as ostium secundum near the roof of the left ventricle.
- The ostium secundum maintains a connection between left and right atria, allowing for continued shunting of blood.
- While the septum primum forms and lengthens, an additional sickle-shaped and thicker structure, the septum secundum, develops from anterosuperior wall of right ventricle.
- The septum secundum grows toward atrioventricular region, overlapping ostium secundum; however, stops before reaching the atrioventricular septum and thus leaving a hole near the floor of the right atrium, referred to as foramen ovale.
- During fetal development, blood is shunted through the staggered openings of the foramen ovale and ostium secundum.
- Septum primum acts as a one-way flutter valve, allowing blood in right atrium to enter left atrium.
- After birth, there is normally no shunting from the left atrium to right, as blood flow causes the thin septum primum to collapse back against the thicker septum secundum.
- Note that the foramen ovale remains patent in utero to allow oxygenated placental blood to pass into left ventricle, largely bypassing the lungs.
Realignment of primitive chambers
- Cardiac looping leaves a direct pathway between the primitive atrium and future left ventricle via the atrioventricular canal and connection between the future right ventricle and outflow tract that later forms the aorta and pulmonary trunk.
- Realignment between the atioventricular canals, ventricles, and cardiac outflow tract must occur before final septation of the atria, ventricles, and outflow tract and development of valves, coronary vasculature, and conduction system can occur.
- During the process of myocardialization, dorsal endocardial cushion tissue is replaced my invading myocardial cells and the outflow tract shifts toward the left, eventually lying over the atrioventricular canal.
- The dorsal and ventral endocardial cushion tissue simultaneously grows to divide the atrioventricular canal into right and left orifices which can then align with the right and left atria and ventricles, respectively.