1. LECTURE NOTES
Dr Nusrat Zareen
Associate professor Anatomy
MBBS; FCPS
Heart Development I, II, III
Learning Objectives:
1. Recall the source of heart.
2. Describe the development of the cardiogenic region
correlating the process of gastrulation with the changes that
occur to form a single heart tube in the thorax, and the role
that embryonic folding plays in this process.
3. Describe the looping and folding of the heart tube that gives
rise to the adult location of the developing heart chambers.
4. Describe the development of the inflow tract.
5. Describe the development of the atria and the interatrial
septum.
6. Describe the development of the atrioventricular valves and
the aortic and pulmonary valves.
7. Describe the development of the ventricles and the
interventricular septum.
8. Describe the partitioning of the outflow tract and the
contribution of neural crest cells to this process.

2. THE THREE LAYERS OF THE HEART
THE ENDOCARDIUM. (Mesodermal- splanchnic mesoderm)
THE MYOCARDIUM. (Mesodermal- splanchnic mesoderm)
THE EPICARDIUM. (Mesodermal)- it is derived from the mesothelial cells from the region of SINUS
VENOSUS migrating over the heart.
The 5 stages to heart development are:
1. Specification of cardiac precursor cells.
2. Migration of cardiac precursor cells and fusion of the primordia.
3. Heart looping .
4. Heart chamber formation.
5. Septation and valve formation
There are 02 types of cardiac precursors
Splanchnic mesoderm from primitive streak. (Cardiogenic mesoderm at the
cranial end of embryonic disc.
Out flow portions of the heart from NCC
The cardiogenic cells ingression from the primitive streak & migrate cranially rostral to the neural
plate to form a horse shoe shaped cardiogenic area.
Horse shoe shaped ---- CARDIOGENIC
FIELD

3. ROLE OF NEURAL CREST CELLS
The contribution of NCC: Migration and positioning
The cardiac neural crest migrating into the heart region is responsible for forming the entire
musculoconnective tissue wall of the large arteries emerging from the heart, the membranous portion
of the ventricular septum, and the septum between the aorta and pulmonary artery.
STAGES OF HEART TUBE FORMATION
The first sign of heart development is the formation of ENDOCARDIAL TUBES/cardiogenic cords.
These tubes fuse to form a single heart tube. This tube then loops and folds and remodel to
finalize the chambering of heart.

4. Notice the formation of the paired endocardial tubes, derived from the splanchnic mesoderm.
These tubes then fuse to form the single heart tube, suspended in the pericardial cavity. All
encircled.
THE THREE Histological LAYERS OF THE HEART, all mesodernal
THE ENDOCARDIUM. (Angioblastic Mesoderm derived from splanhnic mesoderm)
THE MYOCARDIUM.(Splanchnic Mesoderm)
THE EPICARDIUM. (Mesoderm derived from the mesothelial cells on the surface of septum trnsversum
migrating over the heart.)
What is cardiac jelly? How is it derive

5. Buccopharyngeal / oropharyngeal membrane
As the heart develops, it
changes its relation to the
buccopharyngeal membrane.
What is this changing relation
and what is the cause of this?
The changed relation is that originally the heart is anterior / cranial to the membrane then it
gradually comes in____________________________ relation to the membrane, and sinks into
the thoracic cavity. (see the figure and fill in the blank.)
The causes of this changing relation are:
1. Closure of Neural tube.
2. Formation of brain vesicle
3. FOLDING OF THE EMBRYO

6. SUMMARY OF EVENTS - DEVELOPMENT OF PRIMITIVE HEARTTHE
• The cardiogenic cells in EPIBAST (immediately lateral to Primitive streak) migrate
through the PRIMITIVE STREAK, proceed cranially and reside in SPLACHNIC layer of
Lateral Plate mesoderm.
• The Pharyngeal endoderm beneath the mesoderm induces them to become cardiac
myoblasts and vascoulognesis also starts.
• Paired Endothelial strands (THE ANGIOBLASTIC CORDS) form.
• These cords CANALIZE to form ENDOCARDIAL HEART TUBES (mid 3rd wk)
• Paired ENDOCARDIAL HEART TUBES fuse to form TUBULAR HEART (late in the 3rd week)
• HEART BEGINS TO BEAT BY DAY 22- 23 (BEGINNING OF 4RTH WK)

7. Development time line of heart

8. Exercise: Summarize the events of heart formation

9. THE HEART TUBE
Cephalic end
Caudal end
Looping of heart tube
The fused heart/ endocardial tube begin
to loop at day 23 and complete at day 28.
Observe:
• The cephalic portion moving ventrally, caudally and to the right.
And
• The caudal portion looping dorsally, cranially and to the left.

10. WHAT FORMS FROM WHAT
EMBRYONIC DILATATION ADULT STRUCTURE
TRUNCUS ARTERIOSUS Aorta
Pulmonary trunk
BULBOUS CORDIS Smooth part of right ventricle
Smooth part of left ventricle
PRIMITIVE VENTRICLE Trabeculated part of right ventricle
Trabeculated part of left ventricle
PRIMITIVE ATRIUM Trabeculated part of right Atrium
Trabeculated part of left Atrium
SINUS VENOSUS Smooth part of right Atrium
Coronary vein
Oblique vein of left Atrium

11. Circulation route through the heart Tube.
Blood enters the “venous end” (sinus venosus) and leaves through the arterial
end (truncus arterious)
UNIDIRECTIIONAL BLOOD FLOW
Blood enters the Sinus venosus - common atrium -AV canal - Common Ventricle -
Bulbous cordis - Truncus arteriosus - Aortic sac - Aortic arches and into Dorsal
aorta.
Compare the primordia of looped heart and the
respective adult structures formed

12. After the looping of the heart tube, internal specifications start taking form. We
shall see the following:
Fate of Sinus venosus.
Septae formation / Partitioning of fused endocardial tube:
Partitioning of common atrium.
Partitioning of Atrioventricular canal.
Partitioning of common ventricle.
Partitioning of common out flow tract (truncus arteriosus)
The fate of sinus venosus- what forms from it? Observe
• Absorption of left
sided veins
• Formation of oblique
vein & coronary sinus
on left
• Incorporation of right
horn into the right
atrium.

13. Notice the sino artial junction and the veins of the right sinus horn opening into the right atrium
after incorporation of the right horn into it.
The atrioventricular canal septates (DIVIDES):
The common atrioventricular canal is wide communication between the undivided atrium and
ventricle. But due the formation of the endocardial cushions and their fusion (see figure above),
this common AV canal is divided into right and left AV canals. These canals will provide routes
of blood from right and left atria to right and left ventricles after their formations.

14. THE INTERATRIAL SEPTA - (Wall between the atria)
Following is the shape of the looped heart (tube). Notice the position of the atrium and the
ventricle. Both these chambers are undivided originally with an atrioventricular canal between
them. Later septa develop inside them dividing them into right and left chambers separated by
right and left AV Canals
Common atrium
Position of the Av CANAL where
endocardial cushion form, dividing it into
right & left halves
Common ventricle
Notice the position of the AV canal (ENDOCARDIAL CUSHIONS) and the position of septae in the
atrium and the ventricle respectively
The interatrial septum develops from the roof of the
atrium and grows towards the endocardial cushions in
the AV canal. The interventricular septum develops
from the floor of the ventricle and grows towards the
endocardial cushions in the AV canal.

15. The steps of interatrial septum formation:
1. During the 4th week, the septum primum grows from the roof of the primitive
atrium toward the endocardial cushions. The lower end does not completely descend to
the cushions leaving a space called osteum primum
2. Later the osteum primum closes off. Vacuoles develop In the septum primum which
coalesce to form the osteum secundum.
3. Another septum, the septum secundum grows from the roof of the primitive
atrium toward the endocardial cushions, curtaining off the osteum secundum. This
septum also leaves a gap – the foramen ovale.

16. 4. The septum primum forms the valve for the foramen ovale which shunts blood from the right to
the left atrium.
5. This completes the formation of interatrial septa with a defect, the foramen ovale. At birth
pressure in the left atrium increases and the septum primum flaps close the foramen ovale. So
the development of interatrial septa completes at birth.
ASD: Atrial septum defect. (Patent foramen ovale)

17. Partition of the conotruncal region- Conus cordis and truncus arteriosus (the out
flow tract) & FORMATION OF AORTICO PULMONARY SEPTUM
• Neural crest cells migrate into the outflow tract populating the wall and
participating/inducing the formation of the septum.
• The first indication of a developing septum is the appearance of two ridges
projecting into the outflow tract from opposite sides. Curiously, these ridges
spiral in a counter-clockwise direction up the developing outflow tract.

18. 1. Pair of ridges growing from the opposite walls of the outflow tract.
2. The two ridges grow toward each other spirally and eventually fuse resulting in the
formation conotruncal sptum (Aortico pulmonary septum) that divides
the tract into aorta and the pulmonary trunk.

19. The spiral pattern results in the apparent curve of the aorta up, over, and behind the
pulmonary trunk seen in the adult.
INTERVENTRICULAR SEPTUM FORMATION
As the interatrial septa are forming, the primitive ventricle is also partitioning.

20. There are 2 portion s of Intrerventricular septum;
Membranous
Muscular
1. The forming interventricular septum is initially a muscular partition defining an interventricular
foramen between its upper border and the fused endocardial cushions.
The muscular ridge- IV
septum
2. A portion of the muscular interventricular septum will eventually fuse to the endocardial
cushions contributing to the formation of a membranous portion of the interventricular
septum.

21. IV septum is contributed by:
• Endocardial cushions.
• Interventricular septum
• Aortico pulmonary septum
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FOR cardiac anomalies consult LANGMAN’S EMBRYOLOGY