External form of the heart

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The arterial end of the primitive heart delivers blood to the paired dorsal aortae—longitudinal vessels on each side of the notochord. The vessels that connect the heart to the dorsal aortae bend around the foregut from ventral to dorsal. These aortic arch arteries pass through swellings in the neck region filled with mesenchyme known as pharyngeal arches, structures that will figure prominently in development of the head and neck.

The venous end of the primitive heart initially receives six vessels:

1–2: Left and right common cardinal veins
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Receive venous blood from the tissues of the embryo proper. The common cardinal veins are short trunks formed by the union of the anterior (from the head region) and posterior (from the trunk) cardinal veins.
3–4: Left and right vitelline veins
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Receive venous blood from the yolk sac.
5–6: Left and right umbilical veins
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Transmit oxygenated blood from the placenta to the embryo (originally, two umbilical veins course through the umbilical cord into the embryo—the right umbilical vein later degenerates).
Figure 32.4.
Figure 32.5. NETTER, ATLAS OF HUMAN EMBRYOLOGY, FIGURE 4.1.

Primitive heart chambers

Figure 32.6. LANGMAN’S MEDICAL EMBRYOLOGY, FIGURE 13.8.

The primitive heart tube resembles a beaded necklace, consisting of a series of constrictions and dilatations. Blood flows from venous end to arterial end in the primitive heart tube, so we will name the primitive heart chambers in that order:

Has two lateral expansions called the left and right sinus horns. Each sinus horn receives the common cardinal, vitelline, and umbilical veins from that side of the embryo. Blood vessels entering the sinus venosus pass through the septum transversum (future diaphragm).

Receives blood from the sinus venosus and communicates cranially with the primitive ventricle.

Receives blood from the primitive atrium via a narrow opening called the atrioventricular (AV) canal.

Located cranial to the primitive ventricle—the two chambers are separated externally by a deep groove called the bulboventricular sulcus. Two parts of the bulbus are recognized by embryologists based on their developmental fates—but these are not demarcated externally by any particular landmark.

    • Proximal bulbus cordis: Receives blood directly from the primitive ventricle.
    • Distal bulbus cordis (also referred to as the conus cordis): Passes blood from the proximal bulbus cranially to the truncus arteriosus.

This is the most cranial swelling of the primitive heart tube and it is the part recognized as the “arterial end of the heart.” It will eventually split to form the two outflow vessels of the definitive heart: the ascending aorta and the pulmonary trunk.

Cranial to the truncus arteriosus is a dilated vessel (not considered part of the primitive heart tube) that connects the heart tube to the aortic arches—this is called the aortic sac.

The simple heart tube becomes multi-layered

  • At the end of the 3rd week, splanchnic mesoderm invests the endothelial heart tube and differentiates into two layers: the myocardium (the heart muscle) and cardiac jelly, an acellular mass that separates the myocardium from the underlying endothelium. The jelly (a fine anatomic term!) forms a matrix for the development of structures called endocardial cushions as well as the primordial mitral and tricuspid valves (atrioventricular valves).
  • The outermost layer of the heart forms as the heart tube grows ventrally into the intra-embryonic coelom (remember the “bicycle handlebars”?). This invests the heart tube in a serous membrane that becomes the epicardium.
  • The original endothelium of the heart tube gives rise to the definitive endocardium.
  • As the heart tube grows into the coelom, it is initially suspended from the foregut by a double layer of splanchnic mesoderm called the dorsal mesocardium. Later the dorsal mesocardium ruptures, leaving the heart suspended in the pericardial cavity at its arterial and venous ends by the reflections of serous membrane from heart to pericardial sac. The passageway dorsal to the heart tube formed by breakdown of the mesocardium becomes the transverse sinus of the pericardial cavity.
Figure 32.7. DAVIES, J., HUMAN DEVELOPMENTAL ANATOMY, THE RONALD PRESS COMPANY, FIGURE 7-2.
Figure 32.8. DAVIES, J. , HUMAN DEVELOPMENTAL ANATOMY, THE RONALD PRESS COMPANY, FIGURE 7-2.

The heart tube is thrown for a loop

Figure 32.9. LANGMAN’S MEDICAL EMBRYOLOGY, FIGURE 13.8.

As the primitive heart tube grows within the space imposed by the pericardium, it buckles or bends upon itself at the junction of the primitive ventricle and bulbus cordis (bulboventricular sulcus). This results in the heart tube being “looped” within the pericardium, producing a sharp indentation at the bulboventricular sulcus and a convexity in the heart tube directed toward the RIGHT. On the interior, this sharp infolding produces a shelf of tissue called the bulboventricular flange, which partially separates the chambers of the primitive ventricle and proximal bulbus cordis.

Figure 32.10. LARSEN’S HUMAN EMBRYOLOGY, FIGURE 12.8.
Figure 32.11. LANGMAN’ S MEDICAL EMBRYOLOGY, FIGURE 13.10.

Consequences of heart looping

  • Positions the proximal bulbus cordis and primitive ventricle side-by-side,

    with the bulbus cordis on the right and primitive ventricle on the left.

  • Moves the primitive atrium posteriorly and superiorly.

  • Draws the sinus venosus out of the septum transversum.

    Now all of the primitive heart chambers are surrounded by the serous pericardium.

  • Kinks the heart tube and approximates its arterial and venous ends,

    like the ends of a rope brought together.

  • The convexity of the loop is on the right,

    placing the future apex of the heart on the left. The future heart chambers are in their correct definitive spatial orientation = ventricles inferior and ventral to the atria.

Clinical correlation

Looping of the heart to the LEFT produces dextrocardia. Note that the apex of this heart projects to the RIGHT side. Dextrocardia can be associated with mirror image reversal of all visceral organs compared to normal—a rare condition called situs inversus (1 in 10,000 births). When this is the case, cardiac defects may be mild. However, isolated dextrocardia (without situs inversus) usually produces severe cardiovascular anomalies.

The proximal bulbus cordis and primitive ventricle are side-by-side in the “looped” heart tube. Let’s now refer to this composite space as the bulboventricular chamber. This is important since later we will learn that the inflow portions of the definitive left and right ventricles are derived from this composite chamber.

Remodeling of the inflow and outflow vessels

Figure 32.12.

During the 3rd week, the inflow/outflow blood vessels associated with the heart have perfect symmetry: there are two dorsal aortae, each receiving blood from the left and right aortic arch arteries and two common cardinal veins draining the left and right sides of the embryo and delivering their blood to the left and right sinus horns. Now we have to go and ruin things!!

The paired dorsal aortae fuse below the head to form a single dorsal aorta and the aortic arch arteries are remodeled to form the definitive arterial pattern in the mediastinum and neck. More on this later.

A complex remodeling job is done on the inflow veins producing two left-to-right channels for venous blood = the final result being that all venous blood from the embryo and placenta is shifted to the right atrium via two newly formed vessels: the superior and inferior venae cavae.

Figure 32.13. Summary diagrams of venous remodeling. Posterior views. DRAWING BY PHILLIP COWMEY, M.D., UNIVERSITY OF WASHINGTON. USED BY PERMISSION.

This occurs in the septum transversum, where the liver is developing. A venous connection is established between the left umbilical and vitelline veins on the one hand, and the right vitelline vein on the other. This newly formed connection is the ductus venosus. The ductus venosus moves blood around high resistance immature vessels in the developing liver.

 The remodeled blood flow pattern is:

Left umbilical vein and Left vitelline vein Ductus venosus Right vitelline vein

Consequences of the caudal left-to-right venous remodeling

  • As blood is directed from left-to-right into the ductus venosus, blood flow is reduced to the proximal portions of the left umbilical,posterior cardinal, and vitelline veins (the parts between septum transversum and heart). Consequently, the proximal parts of these vessels obliterate.
  • The entire right umbilical vein degenerates; the distal part of the left umbilical vein persists to become THE umbilical vein (there is now only one).
  • Distal portions of the right and left vitelline veins are remodeled to become the portal vein.
  • Ductus venosus closes and degenerates after birth, becoming the vestigial ligamentum venosum of the definitive liver.
  • Umbilical vein closes and degenerates after birth, becoming the round ligament of the liver.


See the summary diagram, Figure 32.13.

A new vessel develops cranial to the heart, connecting the anterior cardinal veins. This is the anterior cardinal anastomosis. Blood from the left side of the embryo’s upper body is redirected to the right via the anastomosis.

Consequences of the cranial left-to-right venous remodeling

  • The anterior cardinal anastomosis becomes the left brachiocephalic vein.
  • The proximal portion of the right anterior cardinal vein + the right common cardinal vein become the superior vena cava.
  • The right posterior cardinal vein persists—in the adult, it become the azygos vein.

 
See the summary diagram, Figure 32.13.

 

Fates of the sinus horns:

  • Due to reconfiguration of veins both caudal and cranial to the heart, the left-to- right redirection of blood impacts the two horns of the sinus venosus differently:
    • The left sinus horn shrinks. In the definitive heart it becomes the coronary sinus.
    • The right sinus horn enlarges and is absorbed into the posterior wall of the developing right atrium, becoming the smooth part of the definitive atrium: the sinus venarum.


See the summary diagram, Figure 32.13.

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Internal form of the heart