Take a deep breath and relax. Before we consider the development of the heart’s internal anatomy, let’s take stock of a few things we already know. That will give us the confidence to move on!
So far in our heart development discussion,
we have considered a hollow primitive heart tube with simple chambers connected in series to one another: sinus venosus primitive atrium bulboventricular chamber (primitive ventricle + proximal bulbus cordis) distal bulbus cordis truncus arteriosus. Blood flows from the venous end of the heart tube to the arterial end through these chambers.
What about the definitive (adult) heart?
Is it a simple tube with a series of interconnected chambers? NO! The definitive heart has completely separate left and right sides—essentially it is two hearts stuck together side-by-side.
Idea!
It seems what we need to do in order to change the embryonic situation to match that in the definitive heart is to partition (divide) the chambers in the primitive heart so that each has a separate left and right side. That’s the ticket! Here’s a preview of things to come:
The single embryonic primitive atrium will give rise to the left and right atria in the definitive heart—so we will build a septum to divide it (interatrial septum).
The single embryonic bulboventricular chamber (primitive ventricle + proximal bulbus cordis) will give rise to the inflow portions of the definitive left and right ventricles (the parts inferior to the mitral and tricuspid valves). The embryonic distal bulbus cordis will give rise to the outflow portions of the definitive left and right ventricles (the parts just inferior to the aortic and pulmonary valves). We will partition both inflow and outflow portions of the ventricles by building an interventricular septum that will subdivide both the bulboventricular chamber and distal bulbus cordis.
The single embryonic truncus arteriosus will give rise to the definitive ascending aorta and pulmonary trunk—so we will build a septum to divide it (trunco-conal septum).
What about the sinus venosus, you say? Remember that the left-to-right venous shunting in the embryo enlarged the right sinus horn and caused the left sinus horn to shrink. The right sinus horn is the only part of the sinus venosus to figure into internal heart development. It is absorbed into the wall of the common primitive atrium and will ultimately become the smooth part (sinus venarum) of the internal right atrium.
All of this of course is happening simultaneously in the developing embryonic heart—but to keep us sane, we will discuss each event as if it occurs in sequence.
Remodeling of the primitive (common) atrium
Recall that the left and right atria in the definitive heart both have smooth and rough parts.
The original internal texture of the entire primitive atrium is rough—it has ridges called trabeculae. However, the definitive atria are not entirely rough—they also have smooth parts. How does this come about? The answer is that nearby veins (having smooth inner linings) are absorbed into the walls of the future right and left atria, via a process called intussusception = the taking up of one structure within another, kind of like folding up an old-fashioned telescope.
Right side of the atrium
Left side of the atrium
All this left-to-right venous remodeling enlarges the right sinus horn. It is absorbed into the right posterior wall of the atrium, becoming the smooth part of the definitive right atrium (sinus venarum). The left sinus horn gives rise to the coronary sinus. Since the sinus horns are connected, the coronary sinus will empty into the future right atrium. Also, since the SVC and IVC connect to the right sinus horn, they too will empty into the definitive right atrium.
Intussusception of the right sinus horn into the posterior right side of the common atrium displaces the original rough (trabeculated) part. It is pushed around the heart to become the definitive right auricle. It is thus visible on the external heart from an anterior view. This is why the auricle has rough internal texture = the pectinate muscle.
In the definitive right atrium, an internal ridge indicates where the smooth part and trabeculated part have fused—this is the crista terminalis. This structure is significant because it contains fibers of the heart’s conducting system that carry impulses from the pacemaker (SA node) to the AV node.
Intussusception of the right sinus horn also produces a flap of tissue adjacent to the internal orifice of the inferior vena cava. This is called the valve of the IVC. This is important to the embryo since it directs the flow of blood entering the right atrium via the umbilical vein toward a hole in the interatrial septum (foramen ovale). This oxygen-rich blood from the placenta thus bypasses the right atrium and enters the left atrium (discussed later).
Figure 32.14. THE DEVELOPING HUMAN, FIGURE 13.16.
During the 4th week, a primitive pulmonary vein grows away from the left side of the primitive atrium, ultimately dividing into left and right branches that grow into the developing lungs. The left and right veins subsequently divide into superior and inferior branches. Now there are four primitive pulmonary veins.
As happened on the right side, a process of intussusception absorbs the primitive pulmonary veins into the posterior left wall of the primitive atrium. This process absorbs the left and right veins up to the point where each bifurcates into superior and inferior portions, so that four pulmonary orifices are created in the left atrial wall. This process alsocreates the smooth part of the definitive left atrium.
Similar to the right side, the absorption process displaces the trabeculated (rough) part of the left side of the primitive atrium ventrally, so that it wraps around the truncus arteriosus to form the definitive left auricle.
Partitioning of the AV canal and common atrium
Recall that the atrioventricular (AV) canal connects the primitive atrium to the bulboventricular chamber (primitive ventricle + proximal bulbus cordis). Prior to partitioning of the atrium, the AV canal is divided into right and left portions. Two masses of endothelial tissue, the endocardial cushions, grow from the dorsal and ventral walls of the AV canal toward each other. When they meet, they divide the common AV canal into right and left atrioventricular canals. Now two AV canals connect the primitive atrium to the primitive ventricle.
Figure 32.15. DAVIES, J. , HUMAN DEVELOPMENTAL ANATOMY, THE RONALD PRESS COMPANY, FIGURE 7-6.
Figure 32.16. DAVIES, J. , HUMAN DEVELOPMENTAL ANATOMY, THE RONALD PRESS COMPANY, FIGURE 7-9.
Two overlapping septa will give rise to the definitive interatrial septum:
The septum primum, a crescent-shaped wedge of endothelium grows from the roof of the common atrium caudally toward the fused endocardial cushions that divided the atrioventricular canal.
Figure 32.17. LARSEN’ S HUMAN EMBRYOLOGY, FIGURE 12.21.
The gap between the caudal edge of the growing septum primum and fused endocardial cushions is the ostium primum. This eventually is obliterated as the septum primum meets and fuses with the endocardial cushions.
Figure 32.18. LARSEN’ S HUMAN EMBRYOLOGY, FIGURE 12.22.
The septum primum perforates in several places in its upper region through programmed cell death. When these holes coalesce, another opening is formed: the ostium secundum.
A second (more robust) septum develops to the right of the thin septum primum = the septum secundum. Its caudal curved edge is called the limbus fossae ovalis. Like the septum primum, the septum secundum grows caudally from the roof of the common atrium. Growth of the septum secundum is halted before it can fuse with the endocardial cushions, leaving a gap, the foramen ovale, near the base of the developing interatrial septum.
Two staggered non-overlapping openings (foramen ovale and ostium secundum) are within the primordial interatrial septum. See Figure 32.20. Until the birth of the baby, blood isshunted from the right atrium to the left atrium throughthe two staggered openings. The primary reason is that the immature lung vasculature provides too much resistance to blood flow. Also, having separate systemic and pulmonary circulations is not practical for the fetus, since oxygenated blood is already entering its body through the umbilical vein.
The septum primum provides a handy flap valve preventing backflow of blood from left-to-right through the foramen ovale. See Figure 32.20.
The valve of the IVC (Eustachian valve) aims oxygen-rich blood entering the right atrium from the placenta (via the umbilical vein and IVC) into the foramen ovale.
Figure 32.19. LARSEN’ S HUMAN EMBRYOLOGY, FIGURE 12.23.
After birth and the opening of the lung vasculature, the pressure in the left atrium rises, pressing the septum primum against the septum secundum. This produces a functionally intact interatrial septum. Proliferation of tissue usually completely seals the two septa.
The internal anatomy of the definitive right atrium includes a depression where the foramen ovale once existed = the fossa ovalis. Above the fossa is a ridge = the limbus fossae ovalis, the former caudal edge of the septum secundum.
Question
What embryonic structure gives rise to the floor of the fossa ovalis (as viewed from the right atrium)?
Figure 32.20. DAVIES, J. , HUMAN DEVELOPMENTAL ANATOMY, THE RONALD PRESS COMPANY, FIGURE 7-19.
Clinical correlation: Atrial Septal Defects
Due to the complex development of the interatrial septum just described, atrial septal defects (ASDs) can occur. Depending on the size of the defect, problems can range from non-existent (up to 25% of the population may have a tiny hole in their interatrial septum called a probe patent foramen ovale) to severe left-to-right shunting of blood and right- sided heart overload with pulmonary hypertension. There are various types of ASDs depending on which part of the primordial septum has the malformation.
The most common type of ASD is the ostium secundum defect. These result from abnormal resorption of septum primum tissue (producing an abnormally large ostium secundum overlapping a normal-sized foramen ovale) or incomplete formation of the septum secundum leading to an abnormally large foramen ovale that overlaps an normal-sized ostium secundum. Whatever the case, an appreciable hole in the middle of the interatrial septum results.
Another type of ASD is an ostium primum defect. This results when the septum primum fails to fuse properly with the endocardial cushions, leaving a gap below the septum primum that overlaps the foramen ovale. This produces a hole in the lower part of the interatrial septum.
side Note
Not all the blood entering the right side of the heart is shunted to the left atrium via the foramen ovale. After all four heart chambers have developed, some fetal blood does reach the right ventricle.
This is deoxygenated blood from the head and neck entering the heart via the superior vena cava and aimed straight down into the right ventricle. This blood is pumped into the pulmonary trunk but does not reach the lungs because of the presence of the ductus arteriosus, a vessel that connects the pulmonary trunk to the arch of the aorta. Shortly after birth, the ductus arteriosus closes to form the fibrous ligamentum arteriosum.
Figure 32.21. DR. JON MALLATT, UNIVERSITY OF IDAHO WWAMI MEDICAL PROGRAM. USED WITH PERMISSION.
The common atrium has been partitioned . . . now on to the ventricles
The inflow parts of the definitive ventricles will come from the bulboventricular chamber (primitive ventricle + proximal bulbus cordis) and their outflow parts from the distal bulbus cordis (conus cordis).
The definitive atrioventricular orifices (the openings connecting the atria and ventricles—surrounded by the tricuspid valve cusps on the right and the mitral valve cusps on the left) are derived from the embryonic left and right atrioventricular canals—which we divided previously when the endocardial cushions fused.
Figure 32.22. LARSEN’ S HUMAN EMBRYOLOGY, FIGURE 12-24.
Seems straightforward enough—but there are TWO problems. Note in Figure 32.22A that the left and right AV canals BOTH empty into the left side of the bulboventricular chamber. If we grow a septum in the middle of the BV chamber to partition it, blood from BOTH atria will flow only into the left side. Uh oh—no blood into the right ventricle! Also note that the distalbulbus cordis—the source of the future outflow parts of the ventricles—is located above ONLY the right side of the BV chamber—so growing a septum would preclude the inflow part of the left ventricle from lining up with its respective outflow part. The problems are solved by:
1
Migrating the left and right AV canals to the right, so they are centered in the BV chamber. See Figure 32.22B.
2
Widening and moving the distal bulbus cordis to the left so it is centered over the BV chamber and thus straddles the inflow parts of BOTH future ventricles.
3
The obliteration of the bulboventricular flange and remodeling of the bulboventricular walls facilitates both these processes. (See the finished product in Figure 32.22C.)
Development of the left and right ventricles
Developmentally and functionally, each ventricle is a composite chamber consisting of two parts:
The inflow portion
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The part receiving blood from the atria
The outflow portion
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The part transmitting blood to the great arteries
Here’s a summary of the embryonic origins of each part:
Partitioning of the bulboventricular chamber
At the caudal bulboventricular junction, inward growth of the myocardium proceeds cranially toward the fused endocardial cushions (see Figure 32.22C). This occurs at about the same time as obliteration of the bulboventricular flange. This growing wall of myocardial tissue, the muscular part of the interventricular septum, has a crescent-shaped cranial edge. The space between it and the fused endocardial cushions is termed the interventricular foramen. The process just described partitions the inflow portions of the two ventricles. Note on the left of the muscular IV septum is the primitive ventricle, on the right is the proximal portion of the bulbus cordis.
Partitioning of the outflow tracts of the ventricles and the arterial end of the heart
At this point, the inflow portions of the developing left and right ventricles communicate with the distal bulbus cordis cranially and with each other via the interventricular foramen. If we left things as is, both oxygenated and de-oxygenated blood would mix before being sent out of the heart in the great arteries. What is needed now is further partitioning of the ventricles, specifically their outflow portions, and partitioning of the truncus arteriosus into an ascending aorta (to the systemic circulation) and a pulmonary trunk (to the pulmonary circulation). This is a complex and highly important process—as such, it is not surprising that many heart defects occur in this region.
It is debatable exactly how the outflow portions of the ventricles and the truncus arteriosus are partitioned. Some authors suggest that as many as three separate pairs of swellings contribute to this process. Let’s simplify it this way:
Mesenchymal ridges appear in both the distal bulbus cordis (conus cordis) and in the truncus arteriosus. These are the left and right bulbar ridges and the left and right truncal ridges. Both sets of ridges expand and grow toward the center of the heart chamber as well as towards one another, until they fuse. The completed partition containing both truncus and conus components is referred to as the trunco-conal septum.Instead of being oriented in a strictly coronal plane, the trunco-conal septum twistsinaspiralarrangement. This composite septum partitions both the distal bulbus cordis and the truncus arteriosus.
In the definitive heart the outflow portion of the right ventricle (known as the infundibulum) transmits blood to the pulmonary trunk.
In the definitive heart the outflow portion of the left ventricle (known as the aortic vestibule) is situated dorsal to the outflow portion of the right ventricle (infundibulum) and transmits blood to the ascending aorta.
Figure 32.23. MOORE ET AL., THE DEVELOPING HUMAN, FIGURE 13-21.
Note the spiral arrangement: The outflow portions of the ventricles and the great arteries twist (“do a tango”) around one another.
At the level of the outflow portions of the ventricles, the aortic vestibule is located posteriorto the infundibulum (section 3 in Figure 32.23).
Above this, at the level of the two great arteries, the ascending aorta lies at first to the rightof the pulmonary trunk (section 2 in Figure 32.23) and then anteriorto the pulmonary trunk(see section 1 in Figure 32.23). Look at the figures in your Atlas to confirm this.
Completion of the interventricular septum
The gap created by the interventricular foramen is later filled in by tissue mainly derived from the fused endocardial cushions, with a contribution from the lower part of the trunco-conal septum. These tissue sources merge with the muscular part of the interventricular septum, completing the definitive interventricular septum. See Figure 32.24.
The cranial portion of the IV septum, formed from the fused endocardial cushions and trunco-conal septum is the membranous part of the IV septum—i.e., it contains little myocardial tissue.
Because of the spiraling of the ventricle’s outflow tracts, the outflow part of the left ventricle (aortic vestibule) is twisted posterior and to the right so that it lies ever so slightly above the right AV orifice—i.e., where the septal cusp of the tricuspid valve is attached. The small portion of the interventricular septum here is termed the atrioventricular portion of the IV septum (because it separates the right atrium from the outflow part of the left ventricle).
Parts of the trunco-conal septum are derived from cells of the neural crest. They form connective tissue and muscular elements of the trunco-conal septum and also form the parasympathetic postganglionic neurons of the heart.
Figure 32.24. MOORE ET AL., THE DEVELOPING HUMAN, FIGURE 13-18.
Clinical correlation
Failure of the muscular and membranous parts of the IV septum to fuse is one mechanism for producing a ventricular septal defect (VSD). VSDs cause blood to be shunted from left-to-right, possibly leading to pulmonary hypertension (too much blood in the pulmonary vessels). Surgical intervention to cover the VSD with a patch is needed to reduce the pulmonary blood pressure and prevent hypertrophy of the right ventricle.
Clinical correlation
Failure of neural crest to migrate and/or properly form ectomesenchyme has been linked to congenital abnormalities of the heart, especially those involving septation (division) of the truncus arteriosus. These defects are part of the larger category of cyanotic cardiac anomalies—those involving abnormal communication between the systemic and pulmonary circulations – causing poorly oxygenated blood to be circulated in the systemic circuit to the body’s tissues. Several of the more important cyanotic cardiac developmental defects are listed below.
Transposition of the great vessels
In this case, the trunco-conal septum has formed but has not “tangoed”—that is, no spiral has developed. The left ventricle (blood rich in oxygen) empties into the pulmonary trunk and the right ventricle (containing poorly oxygenated blood) into the aorta. The infant can survive only if a patent foramen ovale or patent ductus arteriosus is present to allow mixing of blood—although cyanosis is still a symptom. Surgery is required to detach and reattach the arteries to their correct ventricles.
Figure 32.25. Transposition of the great vessels.
Figure 32.26. Persistent truncus arteriosus.
A rare condition in which the trunco-conal septum fails to form, resulting in a VSD and blood from both ventricles mixing in an undivided truncus arteriosus. This produces severe cyanosisand requires complicated surgery to repair the VSD, convert the truncus into an aorta, and install a shunt from the right ventricle to the pulmonary arteries.
Tetralogy of Fallot
Four co-occurring heart defects arise from the same malformation = misalignment of the trunco-conal septum, producing uneven division of the distal bulbus cordis and truncus arteriosus.
Figure 32.27.
VSD
Narrowed outflow tract of the right ventricle (pulmonary stenosis).
An aorta that straddles the interventricular septum (overriding aorta), allowing oxygen-poor blood from the right side of the heart to enter the systemic circulation.
An enlarged muscle mass in the wall of the right ventricle (right ventricular hypertrophy).
Surgery in the neonate is always required. This involves repairing the VSD with a patch and canalizing the stenotic pulmonary trunk or installing a shunt from the right atrium to thepulmonary arteries. Although the abnormalities are complex, the post-operative prognosis is good.
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Hand-drawn Conley-gram. (tap to open; use your Apple Pencil to draw and make notes)
