The heart develops from mesodermal cardiogenic fields on either side of the embryo that fuse to form a primitive heart tube. This tube undergoes looping to the right, forming the chambers. Partitioning forms the atrial and ventricular septa, separating the chambers. In fetal circulation, blood is shunted away from the lungs through openings like the foramen ovale and ductus arteriosus. At birth, these close as the lungs become functional and circulation assumes the postnatal pattern. Defects can occur if septation or closure of passages is incomplete, leading to congenital heart diseases.

1. Development of the Heart
in a few lines.
“No flow, No grow.”

2. Background
• The earliest steps
– Three germ layers—ectoderm, endoderm, and mesoderm
• Cardiogenic field on two sides
– Mesenchymal cell at anterior part
• Cardiogenic field meet in midline to form cardic
• Cardiogenic fields can be subdivided into two groups
– First heart field[Anterior)-future RT ventricle
– Second heart field[Posterior] –future left ventricle
• The two sides of the cardiac crescent fuse along the midline to form the primitive
heart tube
• The primitive heart tube can itself be subdivided into regions along the caudal to
rostral axis
– Sinus venosus
– Primitive atria
– Primitive ventricle
– Bulbus cordis (conus)
– Truncus arteriosus
• Heart beat at 5 weeks’ gestation

3. Earliest Development
Cardiovascular system makes its first appearance while the
embryo is still flat. Clusters of mesodermal cells specialise
to form blood cells. Mesodermal cells around these flatten
to form endothelium of blood vessels. These clusters are
called blood islands of angiogenic (“blood vessel-forming”)
cell clusters.
In the accompanying diagram note that these form a curve
reaching well beyond the neural plate and the notochord. A
mass of mesoderm, called cardiogenic area, near the head
end (H) will give rise to the heart.
The sagittal section below illustrates the three germ layers,
prochordal plate and the cardiogenic area.
H
Prochordal plate
Cardiogenic area

4. Head Fold
With the formation of the head fold (shown in the blue
circle), note how the cardiogenic area changes its
position. Also observe that the endoderm (yellow)
is beginning to form the gut tube. At this stage only
the head and tail ends of the digestive tube are
recognisable.
In the lowest picture, the gut tube is better seen and
the heart is in fact in the form of a tube (red).
Heart Tube

5. Heart
Liver
The Heart Tube
In the picture on the left the relationships of the
heart, the gut tube and the liver are clearer.
In the magnified picture of the heart tube, the tail
end is the venous end and the cranial end is the
arterial end. The changing shape of the tube
also makes it possible to recognise the primitive
chambers of the tube.
Remember that the tube is not partitioned at this
stage.
Hereafter, for descriptive convenience, we shall
view this tube in the vertical position, with the
caudal (venous) end below and the cranial
(arterial) end at the top as shown below.

7. The Tube Bends
This picture shows three successive stages in the growth of the tube. The tube, as it grows,
cannot be accommodated within the pericardial cavity and undergoes bending.
The primitive chambers of the heart are recognisable, and are labelled in the last picture.
SV – sinus venosus (receives veins from the body), A – atrium, V – ventricle. The ventricle
continues into the ‘bulbus cordis’which in turn leads to the arterial end.
Two terms are used somewhat confusingly for the parts at the arterial end. These are
conus arteriosus and truncus arteriosus. In our discussion we shall simply say ‘arterial
end’ of the heart.
SV
A
V
BV D

8. Cardiac Looping
• Folds on itself and twists –looping
• Mechanism
– Differential ballooning out of the chambers
• D Loop- the looping occurs to the right
• The first visible sign of left-right
asymmetry
• Looping sets up the relationship between
the inflow tract, the outflow tract, and the
ventricular septum of the right ventricle

9. The Chambers
Recognise the chambers in these two views. In the view from the left side, the sinus
venosus is partly hidden. Note that with the bending of the tube the atrium is now dorsal
and the loop formed by the ventricle and the bulbus cordis (bulbo-ventricular loop) is
ventral.
In the next slide we shall examine the interior of the unpartitioned heart.
A
A A
V
B-V Loop
Left view Front view

10. The Interior
A portion of the ventral wall of the bulbo-
ventricular loop is removed to show the
interior.
Since there is no partition, there is a single
passage from the atrium to the ventricle.
This passage is the atrioventricular canal.
Note the direction of blood flow through
the bulboventricular loop.
Also note that the single vessel leading out of
the heart has given rise to what are called
aortic arches.
RA LA
AVC
A-Ar

11. Left – Right Partitioning
• Interatrial septum
• Interventricular septum
• Spiral (aortico-pulomonary) septum
• Endocardial cushions (A-V cushions)
• Functional requirements
• There must always be a right to left passage!

12. Interatrial septum
• Partitioning
• Right to left passage
• Mechanism for closing the passage

13. A
V

14. Septum Primum
• This is a sagittal
section seen from the
right.
V
AVC

15. Foramen Primum
• Foramen primum :
Between
the septum and
the AV Cushions

16. Passage is a Must!
• Foramen secundum
• Foramen primum
about to disappear

17. Septum Secundum
• To the right of primum
• Foramen primum has
disappeared

18. Foramen ovale
• F. Ovale –
• In septum secundum
• Further…

19. The ‘Valve’
• Two septa
• Two foramina

20. Sinus Venosus
• Originally a symmetrical structure
• Venous return more to the right
• Left horn becomes smaller
• Opening shifts to the right
• Later – part of right atrium

21. Left Atrium
• Four pulmonary veins
• Common opening
• “Absorption” of veins into atrium
• Rough part - auricle

22. The Ventricular Septum
Three Parts
– Interventricular septum
– AV Cushions
– Spiral Septum

23. Ventricular septum
• The primitive right ventricle is more anterior
• The flow of blood comes into the left ventricle, then goes across the
bulboventricular foramen to the right ventricle and out the as-yet-undivided
outflow tract. As development progresses, inflow becomes more directed
toward both ventricles
• Failure of this process can result in a double-inflow left ventricle [DILV]—a
situation much more common than double-inflow right ventricle
• The ventricular septum begins to grow toward the AV canal and outflow tract
from the apical and inferior portion of the junction between the primitive right
and left ventricle- muscular part of the interventricular septum
• Septation of the ventricle is complete when the muscular septum meets the
canal septum between the AV valves and the conal septum just below the
now separate outflow tracts
• The area at which these structures meet, there is the thinner membranous
septum.

24. Ventricular Septum
R
Membranous
Muscular Spiral
(Aorticopulmonary)

25. VEINS

26. Arch

27. Foetal Circulation
• Very little pulmonary flow
• Placental Circulation
• Right to Left Passages

28. Postnatal vs Foetal Circulation
• Postnatal
Body  RA  RV  Lungs  LA  LV  Body
• Foetal
Body  RA  RV Lungs LA  LV  Body
The basic difference between postnatal and foetal circulation is that foetal lungs are
nonfunctional. Effectively, blood from the right side of the heart has nowhere to go
and needs to be ‘shunted’ to the left. Such a shunting passage exists between the
right and the left atria. However, if no blood flows through the right ventricle, that
chamber will fail to develop. Thus some blood does pass to the RV. As it is pumped
into the pulmonary artery, it needs to be shunted again, this time to the aorta. This
illustrated below.
But we are jumping too far ahead! This was mentioned as one of the basic principles of
the development of the heart…let us begin at the beginning.

29. • IVC :
Blood from
placenta
– Ductus
venosus
• F. ovale
• Ductus
arteriosus

30. Changes At Birth
• Closure of interatrial septum
• Closure of ductus arteriosus
• Closure of ductus venosus

31. Congenital Heart Disease
• Septal Defects – Atrial and Ventricular
• Endocardial cushion defects
• Aorticopulmonary defects
• PDA
• Others

32. Thank You