Atrial Septation & Differentiation
Notes
Atrial septation and differentiation
- The mature atrial septum comprises two embryological structures that, during development, facilitate the right to left shunt that bypasses the non-ventilated lungs throughout gestation.
- The septum primum arises first; its foramen primum and, later, foramen secondum, facilitate blood shunting.
- The septum secundum arises to the right of the septum primum, and facilitates shunting via the foramen ovale.
- The atrioventricular canal (aka, AV canal), which is the connection between the atria and ventricle, is divided into right and left canals when dorsal and ventral endocardial cushions fuse.
30 Days
- Common atrium and ventricle, connected via single atrioventricular canal.
- Single pulmonary vein drains to smooth area of the left atrial region.
- Superior and inferior venae cavae drain into the right area of the atrium via the orifices of the right sinus horn.
- The coronary sinus drains via the orifice of the left sinus horn.
- The septum spurium, which is continuous with the valves of the orifices.
- the sinus venarum is the smooth portion of the right area of the atrium; recall that this area reflects the incorporation of the sinus venosus.
Over the next few days, new structures arise to divide the common atrium and AV canal.
33 Days
- The muscular ventricular septum is forming, which we'll address in more detail, elsewhere.
- Singular pulmonary vein is now four separate vessels, each with their own orifices into the left atrium. Recall that, in the mature form, each lung is drained by right and left pulmonary veins.
- Sinus venarum surrounds the definitive venous valves and orifices of the superior and inferior venae cavae and coronary sinus; the crista terminalis (aka, terminal crest) is a ridge where the smooth sinus venarum meets the rougher trabeculated area of the right atrium.
- Endocardial cushions arise in the tissue surrounding the atrioventricular canal. As mentioned in the introduction, the endocardial cushions invade the singular canal to create two openings.
- Dorsal, ventral, left and right.
- The septum primum arises as a sickle-shaped crest from the dorso-cranial atrial wall and grows towards the atrioventricular endocardial cushions; the dorsal mesenchymal cap lies at its edge.
- The dorsal mesenchymal protrusion lies between the septum and the endocardial cushions. This area of proliferating mesenchyme arises from cells of the second heart field (be aware that this structure may be referred to as the spina vestibuli in older literature).
As the septum primum grows towards the dorsal mesenchymal protrusion, the foramen primum (aka, ostium primum) maintains blood flow between the right and left sides of the atrium.
Day 40
- Dorsal and ventral endocardial cushions fuse, creating left and right AV canals.
Endocardial cushion defects lead to incomplete or absent septa; as a result, blood shunting and mixing occurs, and, if severe, can lead to heart failure.
- Extension of the septum primum is complete.
- The dorsal mesenchymal cap, dorsal mesenchymal protrusion, and atrioventricular septum comprise the atrioventricular mesenchymal complex.
- Before the complex completely closes off the foramen primum, the foramen secundum (aka, ostium secundum) begins to form in the septum primum to maintain blood flow through the atria.
- The septum secundum, which is thick and muscular, arises to the right of the septum primum.
Day 43
- Both septi are complete.
- Blood drains from the venae cavae and coronary sinus, passes through the foramen ovale of the fully formed septum secundum, then through the foramen secundum of the septum primum.
At birth, pressure changes in the atria push the septa together, closing off the opening in the AV septum and eventually fusing. The remnant of the foramen ovale, now called the fossa ovalis, can be felt in the right atrial side of the interatrial septum.
However, in some individuals (~25%), the septa do not fuse; despite widespread literature regarding the potential for clot to pass through the patent foramen ovale (PFO) from the right side of the heart to the left and pump into the cerebrovascular system and cause stroke (or migraine) – modern evidence doesn't support this pathophysiology, and, surgical closure of PFO is an extremely contentious topic!