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Anatomy and
development of the atrial
septum.
By Manoja
6th semester.

Introduction to development of
the heart.
It develops early in the
middle of 3rd week , from
aggregation of splanchnic
mesodermal cells, in
cardiogenic area ,ventral
to pericardial coelom, and
dorsal to yolk sac.
They form 2 angioblastic
cords that canalize to form
2 endocardial heart tubes.

The Heart Tube
• The two endocardial
tubes fuse to form Single
heart tube.

The heart tube is
differentiated into:
1-truncus arteriosus.
2-bulbus cordis.
3-primitive ventricle.
4-primitive atrium.
5-sinus venosus.

Folding of the heart tube
During the 4th week, the folding
of the heart tube takes place.
The formation of the atrio-
ventricular canal and the
endocardial cushions also take
place around the same time.

Formation of the
Interatrial septum.
The septum is developed from three sources:
1. Septum primum
2. Septum intermedium
3. Septum secundum

• Septum primum: It grows
from the roof and dorsal
wall of primitive atrium, and
passes sagittally
downwards. The lower
margin of the septum is free
and concave. Anterior and
posterior hons of the
septum fuse respectively
with the ventral and dorsal
endocardial cushions of the
primitive atrioventricular
canal.

• Septum intermedium: The
ventral and dorsal cushions of
the atrioventricular canal fuse
to form a broad
anterioposterior partition, which
divide the canal into right and
left atrioventricular orifices.
A foramen known as ostium
primum is formed between the
upper surface of septum
intermedium and lower border of
septum primum. Later, ostium
primum is closed by the fusion of
two septa. Associated with the
closure of ostium primum, the
upper and dorsal part of septum
disintegrates forming a foramen
known as ostium secundum.

Septum secundum: It arises on
the right side of septum primum
from the space of the right atrium
which is interval between septum
primum and septum spurium. The
septum secundum incorporates
the whole of left venous valve,
and extends vertically
downwards. The lower margin
grows sufficiently to overlap the
upper margin of the septum
primum. The valvular opening
formed between the lower margin
of the septum secundum and
upper margin of the septum
primum is called foramen ovale.
The valvular foramen ovale is
created to regulate the blood flow
from the right to left atrium.

In the fetus (before birth), the
pressure is higher in the right atrium
than in the left and highly oxygenated
blood flows directly from right atrium to
left atrium through open foramen ovale.
After birth, when the circulation of
the lungs begins & the blood pressure
in left atrium rises, the upper edge of
septum primum is pressed against the
upper limb of septum secundum. This
will close the foramen ovale
functionally, forming a complete
partition between the two atria.
Later anatomical closure occurs by
fusion of the margins of septum
primum and secundum.
An oval depression in the lower part
of interatrial septum of the right atrium,
the fossa ovalis is the remnant of the
foramen ovale.

Foramen Ovale
Behaves like flap valve.
If it is present after the
birth i.e. if the
anatomical closure
does not take
place(25% cases), it is
present as a remnant of
fetal circulation, it
opens during increased
intra-thoracic pressure.

Requirement of the passage from
the right atrium to the left atrium in
the foetus.
 The source of oxygenated blood is not from the lung but the
placenta.
 Oxygenated blood from the placenta reaches the heart
through the inferior vena cava(umbilical vein left branch of
portal veininferior vena cava).
 It enters the right atrium from where it is directed towards the
foramen ovale by the valve of inferior vena cava.
 From the foramen ovale the blood enters the left atrium then
left ventricle, from where it enters the systemic circulation.

Features of the
interatrial septum.
 On the right side:
1. Fossa ovalis: Oval depression in the lower part of the septum, and the
floor is formed by the septum primum.
2. Limbus fossa ovalis: It is a sickle shaped fold that surrounds the upper,
anterior and posterior margins of the fossa ovalis. It represents lower free
margins of the septum secundum.
3. Foramina venarium minimarium: Venae cordis minimi open through these
foramina.
4. Atrio-ventricular node: It is situated in the lower part of the septum above
the opening of coronary sinus, and in between the annular and looped
fibres of the musculature of the right atrium.

 On the left side:
1. Presence of the semilunar fold with the concavity directed
upwards; it is a remnant of the upper margin of the septum
primum.
2. Lunate impression above the fold is formed by septum
secundum.
3. Foramina venarium minimarium.

Congenital septal
defect.
 If the closure of the interatrial septum does not
take place, it results in congenital atrial septal
defect(ASD).
 ASDs account for about 10-15% of all
congenital cardiac anomalies.
Types of ASDs:
1.Ostium secundum defect→70% of ASDs.
2.Ostum primum defect→20% of ASDs.
(with endocardial cushion defect- split in the mitral
valve.)
3.sinus venosus defect.→10%of ASDs.
4.coronary sinus septal defect→ < 1% of ASDs .

Ostium
secundum
Ostium primum Sinus venosus

Other congenital amomalies
 Probe patency of foramen ovale: It takes place when the
foramen is closed functionally, but remains patent
anatomically. These subjects are considered as normal.
 Biventricular mono-atrial heart: This is due to complete failure
of the septation of primitive atrium.
 Pre-natal closure of foramen ovale: This is a rare anomaly.

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