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Embryology of the Heart
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Cardiogenic area derived from
Intra-embryonic mesoderm
- mid 3rd week
Figures: Sadler 2004
Two cardiogenic cords
develop within cardiogenic
area. Cords become
canalized to give rise to two
endocardial heart tubes.
Figure: Larsen 1993
Figure: Sadler 2004
Late 3rd week
Figures: Sadler 2004
Late 3rd week
Figures: Sadler 2004
Figure: Larsen 1993
Endocardial heart
tubes come to lie
side by side ventral
to foregut.
Figure: Larsen 1993
•At 21 days, two endocardial heart tubes begin to fuse into one tube.
•Caudal end of heart tube in anchored in septum transversum.
•Cranial end is developing connections to paired dorsal aortae.
•Fusion of tubes is complete by 23 days.
Figure: Gilbert 1989
Figure: Moore&Persaud 1998
On day 22, tube begins to elongate, and develops alternate
dilations and constrictions. Begin to see primitive heart chambers:
•Bulbus cordis
•Ventricle
•Atrium
Figure: Larsen 1993
Truncus
arteriosus
•On day 23, common venous collection chamber added: sinus
•venosus, and common origin for aortic arches: truncus arteriosus.
•As heart tubes elongates, loop forms to right: bulboventricular loop.
Figures: Sadler 2004
Figure: Larsen 1993
Day 23
By day 22 or 23,
contractions of
myogenic origin
begin. Occur in
peristaltic waves
beginning at
venous end.
Figure: Sadler 2004
Day 24
Figure: Sadler 2004
Figure: Moore&Persaud 1998
Figure: Gilbert 1989
Figure: Larsen 1993
4 week heart
Figure: Sadler 2004
4 – 5 week heart
Figure: Sadler 2004
4 week heart
Externally, ventricle and atrium appear to have right and left sides.
Figure: Sadler 2004
4 week heart
Internally, there is still unidirectional blood flow: Sinus venosus  RA 
LA  Atrioventricular Canal  LV  RV(bulbus cordis)  Conus cordis
(bulbus cordis)  Truncus arteriosus
Figure: Gilbert 1989
Late 4th week – swellings develop in dorsal and ventral walls of
atrioventricular canal: AV endocardial cushions.
Figure: Gilbert 1989
Late 4th week:
View from the
right side.
Figure: Gilbert 1989
Figure: Gilbert 1989
During 5th week
endocardial
cushions fuse to
divide AV canal
into right and
left AV canals.
Figure: Gilbert 1989
Division of atrium
into right and left
atria also begins
in the 5th week.
Figure: Gilbert 1989
Division of atrium begins with the appearance of a crescent-shaped
membrane that appears in roof and grows ventrally toward the
endocardial cushtions: septum primum.
Figure: Gilbert 1989
The opening between the growing septum primum and the
endocardial cushions, which allows blood flow from right to left, is
called the foramen primum.
Figure: Gilbert 1989
As septum primum grows, foramen primum gets smaller eventually
disappears. New foramen develops high up in septum: foramen
secundum.
Figure: Gilbert 1989
Toward end of 5th week, new crescent-shaped ridge appears in roof of
atrum to right of septum primum: septum secundum. More muscular
than first.
Figure: Gilbert 1989
As septum secundum grows (into 6th week), covers foramen secundum.
Opening made by inferior margin of septum secundum called foramen
ovale. Foramen ovale persists throughout fetal development.
Figure: Gilbert 1989
Figure: Larsen 1993
Figure: Larsen 1993
Close approximation between opening of inferior vena cava and
foramen ovale causes blood from the IVC to pass from RA to LA,
and into LV and out aorta.
Because septum secundum is rather
rigid while septum primum is more
flexible, combination of septa and
formena work as a unidirectional flutter
valve.
Figures: Gilbert 1989: Moore&Persaud 1998
As atrium is being divided, part of
sinus venosus is drawn Into wall
of RA giving rise to smooth-walled
portion of RA known as the sinus
venarum. Another part of sinus
venosus is incorporated into the
LA, and gives rise to a pulmonary
vein, the stem of which is also
drawn into wall of the LA.
Figures: Larsen 1993
Figure: Moore&Persaud 1998
Figure: Sadler 2004
Figure: Moore&Persaud 1998
Day 35
Figure: Sadler 2004
Completion of the interventricular septum is problematic because
truncus arteriosus is a right sided structure, and blood from LV
must pass through IV foramen to exit out the aorta.
Figure: Langebartel&Ullrich 1977
If the truncus had been positioned in the midline, then the ventricles could
be separated by the spiral septum merging with the original IV septum.
Figure: Langebartel&Ullrich 1977
Figure: Langebartel&Ullrich 1977
But since the truncus is on the right, the ventricles are separated by the
addition of an oblique septum derived from bulbar ridges, truncal ridges,
and endocardial cushions, that partition off part of the bulbus cordis to
be incorporated into the LV as the aortic vestibule.
To divide the ventricle while maintaining an outflow track for the LV, must
divide the bulbus cordis so that part is incorporated into the LV for its
outflow track.
Figures: FitzGerald 1978
Beginning in 5th week, bulbar ridges (neural crest cells) form in right
posterior and left anterior walls of bulbus cordis. Endocardial cusion
area also begins to proliferate.
Figures: FitzGerald 1978
The septum created By the
fused bulbar ridges fuses to
the aorticopulmonary septum
superiorly.
Inferiorly, the bulbar septum
joins with original interventricular septum and with
material from endocardial
cushions to complete
division of the ventricle.
Figure: FitzGerald 1978
Figure: Sadler 2004
Completion of Interventricular Septum
• Original IV septum becomes thick-walled
portion of adult IV septum
• Part of septum derived from bulbar ridges,
truncal ridges and endocardial cushions
becomes membranous portion of adult IV
septum
• Right anterior portion of bulbus cordis
incorporated into RV as conus arteriosus or
infundibulum
• Left posterior portion of bulbus cordis
incorporated into LV as aortic vestibule
By 8 weeks all major components of
heart development are complete
• As septa are developing, internal
features such a trabeculae carneae
within the ventricles are developing
• Atrioventricular valves form on R & L
• Semilunar valves form in aorta and
pulmonary trunk
• Conducting system develops
Cardiac Malformations
• Because development is complex,
malformations are relatively common
– Approximately 20% of congenital
malformations involve heart and/or great
arteries
– Estimated to occur in about 1% live births
and ten times more frequently in still births
(Sadler 2004).
• Following list are fairly common defects
amenable to surgery
Atrial Septal Defects (ASD)
• Interatrial septum fails to form properly
• More common in females than males
(3:1)
• If large, will cause interatrial shunting of
blood and hypertrophy of RV and
pulmonary trunk
• Large ASD: 6 out of 10,000 births
(0.06%)
Atrial Septal Defects (ASD)
Figure: Moore&Persaud 1998
Atrial Septal Defects (ASD)
Incomplete fusion of septum primum and septum secundum large
enough for a probe to pass through: Probe patent foramen
ovale. Estimated to occur In about 25% of population.
Figure: Moore&Persaud 1998
Atrioventricular Septal Defect
Figure: Moore&Persaud 1998
Ventricular Septal Defect (VSD)
• Interventricular septum incomplete and
fails to fully divide ventricles
• Most common of all congenital cardiac
defects:
– About 25% of cardiac abnormalities
documented in live births include VSDs
– Isolated defects occur in 10-12 of 10,000
births (0.10%)
– More common in males than females
Normal complete interventricular septum
•Most VSD occur in membranous portion of septum
•Less commonly, VSD occur in muscular part of septum, probably
due to excessive resorption of myocardial tissue
Figure: Sadler 2004
Figure: Sadler 2004
Dextrocardia
Figure: Moore&Persaud 1998; Larsen 1993
Abnormal Division of Truncus Arteriosus
Persistent truncus arteriosus
•Failure of aorticopulmonary septum to form
•Approximately 1 in 10,000 births (0.01%)
•Necessarily includes VSD
•Results in mixing of blood from R&L sides of heart
•Can be fatal if untreated
Figure: Sadler 2004
Abnormal Division of Truncus Arteriosus
Transposition of Great Arteries
•Aorticopulmonary septum does not spiral
•Approx. 2 in 10,000 births (0.02%); more common in infants of
diabetic mothers; more common in males than females (3:1)
•Usually mixing of blood through patent foramen ovale or ductus
arteriosus
Figure: Sadler 2004
Abnormal Division of Truncus Arteriosus
Unequal Division of Truncus Arteriosus
•One artery large, other small (stenotic)
•Ventricle on stenotic side must work harder, typically hypertrophies
•Often aorticopulmonary septum not aligned with interventricular
septum and also have VSD
Figure: Sadler 2004
Tetralogy of Fallot
Combination of defects, seen in approx. 10 in 10,000births (0.10%).
1. Pulmonary stenosis
2. VSD
3. Overriding aorta
4. Hypertrophy of right ventricle
Figure: Sadler 2004
Figure References
• FitzGerald, M. J. T. (1977) Human Embryology: A Regional
Approach. New York: Harper & Row Publ.
• Gilbert, S. G. (1989) Pictorial Human Embryology. Seattle: Univ.
Washington Press
• Langebartel, D. A. & Ullrich, R. H. Jr. (1977) The Anatomical
Primer: An Embryological Explanation of Human Gross
Morphology. Baltimore: Univ. Park Press.
• Larsen, W. J. (1993) Human Embryology. New York: Churchill
Livingstone.
• Moore, K. L. & Persaud, T. V. N. (1998) The Developing Human:
Clinically Oriented Embryology. Philadelphia: W. B. Saunders
Co.
• Sadler, T. W. (2004) Langman’s Medical Embryology, 9th Ed.
Philadelphia: Lippincott Williams & Wilkins.