Development of the Hear t
Development of primitive 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
After lateral folding of
embryo, 2 endocard.tubes
fuse to form…. Single heart
tube (C,D) T.S of 21,22 days.
This heart tube lies inside
the pericardial cavity , its
dorsal wall is connected to
foregut by dorsal
mesocardium (D,22 days).
The central part of dorsal
,forming transverse passage
dorsal to heart ,called
transverse sinus of
(E,F) schematic & T.S of 28
The layers of primitive
heart wall :
T.S in D, at 22 days and in F at 28-days ,
Thin endothelial tube becomes…
internal endothelial lining of the heart or
Splanchnic mesoderm surrounding the
pericardial coelom becomes…..
primordial myocardium (muscular wall of
Thin endothelial tube is separated from
thick muscular tube (myocardium) by
gelatinous connective tissue (cardiac
jelly)…. Forming AV septum & valves.
After head folding of embryonic disc :
A,B,long. sections as the head
fold develops (during 4th week) ,
heart tube & pericardial cavity lie
ventral to foregut and caudal to
The position of heart tube is
reversed ,it lies dorsal to
C,Long. Section, during 4th week
showing : complete head folding
and reversion of heart tube ,
pericardium & septum transversum
(future central tendon of diaphragm).
Note also the heart tube lies inside
the pericardial cavity.
The primitive heart tube
elongates and develops
alternate dilatations and
Truncus arteriosus is
Ventral veiw ,By the end of 4th week
continous cranially with aortic
Sinus venosus has right
sac ,from which aortic arches
& left hornes .
Each horn receives umbilical,
vitelline ,& common cardinal veins
from the chorion, yolk sac &
Bulbus cordis &
ventricle grow faster
than other regions, so
the heart bends upon
(by the end of 4th week).
The atrium & sinus
venosus also come to lie
dorsal to truncus
arteriosus, bulbus cordis
Blood Flow through
the Primitive Heart :
By the end of 4th week,
unidirectional blood flow begins
at sinus venosus by peristalsislike waves.
A,sagittal section of primordial
heart(24 days),showing blood flow.
B,dorsal view of heart (26 days)
,illustrating horns of sinus venosus
– Note also dorsal location of
primordial atrium & sinus venosus.
Blood passes through sinuatrial
valves into atrium…
Atrioventricular canal … ventricle..
Bulbus cordis… Truncus
arteriosus… aortic sac… aortic
arches (arterial channels) …
2 dorsal aortae… into body of
embryo, yolk sac , and placenta.
C,ventral view of
heart (35 days),Note
the aortic arches
arising from aortic sac
and terminate in the
Partitioning of the primitive Heart
Dividing of A-V canal , primitive atrium & primitive
ventricle….. Begins at the middle or end of 4th week.
It is completed by the end of 5th week.
These processes occur concurrently.
Atrioventricular Canal :
At the end of 4th week, 2 endocardial
cushions on dorsal & ventral walls of
atrioventricular canal , develop from
mesenchymal cells of cardiac jelly. (B)
During 5th week, the AV- endocardial
cushions meet and unite in the middle line to
form a septum and divide the common A-V
canal into right & left A-V canals. (C,D)
Endocardial cushions also form the AVvalves + membranous septa of
Note in D,coronal section ,begining of
development of interatrial & intervent.
primordial Atrium :
It begins at the end of 4th week by
development of 2 septa.
1-Septum primum : a thin crescentshaped membrane grows from the roof
of common atrium into the fusing
endocardial cushions dividing common
primitive atrium into right & left halves.
-Foramen primum is formed to pass
oxyg.blood from righ to left atrium. It
disapears as septum primum fuses with
A1 to D1… coronal sections
A to D… views of interatrial
septum from right side.
Before closure of foramen primum ,
perforations appear in central part of
septum primium… coalesce to form
Foramen Secundum (C1-D1).
2-Septum secundum :
a crescentic muscular
memb.grows and descends from
roof of atrium during 5th week. It
overlaps foramen secondum in
septum primum .
The gap between the lower free
border of S.secundum and the
upper edge of S.primum… form
Cranial part of S.primum
disappears and remaining part of
S.primum which attached to
endocardial cushions… forms
flaplike valve of the foramen
In the fetus (before birth) … the
pressure is higher in right atrium than in the left
and highly oxygenayed blood flows directly
from right atrium to left atrium through open
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 ,forming a complete partition between
the 2 atria.
An oval depression in the lower part of
interatrial septum of right atrium…. The fossa
ovalis is a remnant of the foramen ovale.
Left side embryonic cardiovascular
system (26 days) 4 –week embryo :
Changes in Sinus venosus :
It consists of body and 2 hornes,right & left.each horn
receives 3 veins
1- Vitelline vein from yolk sac.
2- Umbilical vein from placenta,
Common cardinal vein from body of embryo.
Changes on left side :
1- left horn & body of sinus venosus form the
2-left common cardinal vein becomes small to
form oblique vein of left atrium.
3- left vitelline & left umbilical veins,
Changes on right side:
1- The right horn becomes absorbed into
right atrium to form its smooth part
2- Right common cardinal vein enlarges to
3- Right vitelline vein becomes IVC.
4- Right umbilical vein disapears.
What happen to Sinus
Venosus to share in
formation of Right Atrium?
1- left horn becoms the coronary sinus.
2- right horn becomes incorporated
into wall of right atrium to form the
smooth part (sinus venarum)… B, 8weeks
3- The remainder of the wall of right
atrium + conical muscular pouch (auricle)
have rough trabeculated area
and derived from primordial atrium.
4- The smooth part , (sinus
venarum ) & rough part
(primordial atrium) are
demarcated internally by a
ridge, crista terminalis.
-crista terminalis + valves of
IVC + valves of coronary
sinus are derived from right
sinuatrial valve. / But left
sinuatrial valve fuses with
incorporated with it into
Primordial pulmonary vein &
Development of left atrium :
At first, a single common
pulmonary vein is seen opening in
left atrium ,just to left of S.primum.
Most smooth part of left atrium
is derived from incorporation of
the single common primordial
pulm. vein at 5th week, (A & B).
then absorption of the 2pulm.veins at 6th week , (C).
lastly , aborption of the 4pulm.veins into left atrium , with
separate orifices at 8th week. (D).
Left auricle is derived from
primordial left atrium.
Embryological origin of the
1-Its rough part + auricle
from Right ½ of primitive
1- Its rough part +auricle
from left ½ of primitive
2-right ½ of A-V canal.
2- left ½ of A-V canal.
3- Its smooth part from
Absorbed right horn of
3- Its smooth part from
Absorbed part of
Development of muscular
part of interventricular
interventricular( IV )septum
arises in the floor of ventricle ,
as thick crescentic fold with
concave free edge.
A-sagittal section 5th week.
Coronal section.6th week.
This septum subdivides the
original ventricular cavity
incompletely into right & left
ventricles that communicate
together through IV foramen.
This foramen closes by the end
of 7th week as the 2 bulbar ridges
fuse with the endocadial cushion.
Closure of IV foramen & formation
of membranous part of IV septum
result from fusion of the following :
1-right bulbar ridge.
2-left bulbuar rige.
A,sagittal s.at 5th w.
B, coronal s.at 6th w.after
incorporation of the proximal part of
bulbus cordis into the ventricles.
C,5th w.,showing the bulbar ridges
& fused endocardial cushions.
D,6th w., proliferation of endocardial
cushions to diminish I V foramen.
E,7th w.,fusion of bulbar ridges +
extensions of endocardial cushions
upward with aortico-pulmonary septum
and down with muscular I V septum to
Cavitation of Ventricular Walls
Leads to formation of
spongy muscular bundles
These bundles become
the papillary muscles &
(attached to the cusps of
tricuspid & mitral valves).
Partitioning of distal part of the
Bulbus Cordis & Truncus Arteriosus :
During 5th w. firstly , a right & left
bulbar ridges are developed in the lower
Another ant.& post. Bulbar ridges in
the middle part.
Right & left truncal ridges are
developed in the upper part.
Bulbar & truncal ridges are developed
from proliferation of mesenchymal cells
of their wall.
They are also derived from neural crest
mesenchyme by passing through the
Partitioning of distal part of the
Bulbus Cordis & Truncus Arteriosus :
as development proceeds, the ridges
fuse together following a spiral course,
forming aortico-pulmonary septum
which has a spiral shape at the 6th week
, (as in G).
This septum divides bulbus cordid &
truncus arteriosus into aorta &
Because of spiraling of aorticopulmonary septum, pulm.trunk twists
around the aorta. Firstly pulm.trunk lies
ant.& to right of the aorta near the
ventricles, then upward,it lies post. & to
left of aorta.
Development of Atrioventricular Valves
A,5thw.,showing right & left AV
canals and begining of valve
swellings due to proliferations of
tissue (subendocardial tissue)
around AV canals.
C,7th w. complete development of
tricuspid & mitral valves….. Note
also development of complete
Development of aortic & pulmonary valves :
Results after development of
bulbar & truncal ridges and
formation of aorticopulmonary
3 Semilunar valves begin
to develop from 3 swellings
of subendocardial tissue
around aortic & pulmonary
These swellings are
hollowed out to form the
thin walled semilunar cusps.
Development of aortic & pulmonary
A, long. Section showing
bulbar & truncal ridges.
B, transverse section of
C,fusion of bulbar ridges.
D,formation of walls & valves
of aorta & pulmonary trunk.
E, rotation of the vessels &
F, long.sections showing
hollowing & thinning of valve
swelling to form the cusps.
Development of conducting system :
Sinuatrial (SA) node
begins to develop during 5th
w.as it is present in right wall
of sinus venosus.
SA-node is incorporated into
wall of right atrium with sinus
venosus. SA-node is located
high in the right atrium ,near
entrance of SVC.
Right sinuatrial valve
(cranial part)…. Forms crista
terminalis,but the caudal part
…forms the valves of IVC &
Development of conducting system :
Left sinuatrial valve is
incorporated into the interatrial
septum forming AV-node & bundle
,which are located superior to
Right & left bundle branches
arising from AV-bundle , pass from
atrium into the ventricular
A band of C.t. grows in from the
epicardium and separates the
muscle of atria from that of
ventricles to form the cardiac
skeleton (fibrous skeleton of
Atrial Septal defects (ASD)
There are 4 types of clinically significant
types of ASD :
1-ostium secundum defect.(with patent oval foramen).
2-endocardial cushion defect.(with
ostium primum defect).
3-sinus venosus defect.
4-common atrium…. Rare cardiac defect ,in which the
interatrial septum is absent due to failure of septum primum &
septum secundum to develop.
Atrial septal defect (ASD):(ostium secundum defect)
A probe patent oval foramen :
A, normal postnatal,
right veiw of interatrial
septum after adhesion of
septum primum to
A1, interatrial septum,
illustrating development of
oval fossa in right atrium.
B and B1, note incomplete
adhesion of septum primum
TO septum secundum and
development of a probe
patent oval foramen.
Various Types of Atrial Septal Defect (ASD) in the
right aspect of interatrial septum :
E, a deficiency of fusion of
endocardial cushions with
septum primum and AV septal
defect results and leads to a
patent foramen primum
-Ostium primum defect….
F, sinus venosus ASDs
(high ASDs) in the superior
part of interatrial septum close
to entry of SVC…. Rare type,
results from incomplete
absorption of sinus venosus
into right atrium and/or
abnormal development of
Tetralogy of Fallot :
It contains 4 cardiac
1- Pulmonary stenosis
(obstruction of right
2- Ventricular Septal Defect
3- Dextroposition of aorta
4- Right ventricular
cyanosis is one of the
obvious signs of tetralogy .
Ventricular Septal Defects (VSDs):
Membranous VSD …. Is the most common type.
Results from incomplete
closure of IV foramen due to
failure of development of memb.
part of IV septum.
Large VSDs with excessive
pulmonary blood flow &
pulm.hypertension result in
dyspnea (difficult breathing) +
Muscular VSD :
Due to excessive cavitation of the
muscular part of the interventricular
septum….. Producing multiple small
defects (Swiss Cheese VSD).
Or absence of the IV septum--Single
ventricle + Transposition of aorta &
Complication: heart failure and
This diagram showing transposition
of great arteries (TGA) which leads to
cyanosis. VSD+ASD allow mixing
arterial & venous blood.
Transposition results from that the
aortico-pulmonary septum descends
straight (instead of spiral).
The Aortic Arches Derivatives :
During the 4th week, as the
pharyngeal arches develop,
they are supplied by the
Aortic arches arise from the
aortic sac and terminate in
the dorsal aorta.
There are 6 pairs of aortic
arches, but they are never
present at the same time.
During 8th w.,the primitive
aortic arch pattern is
transformed into final fetal
The aortic Arches :
A, left sided-embryo (26days) showing the
B, schematic drawing
showing left aortic arches
arising from the aortic sac.
C, an embryo (37days),
showing the single dorsal
aorta and degeneration of
most of the first two pairs
of aortic arches.
Derivatives of 1st & 2nd pairs of aortic arches :
The 1st aortic arches
largely disappear. small
parts persist to form the
The 2nd aortic arches
disappear leaving small
parts forming the stapedial
artereis (run through the
ring of the stapes, a small
bone in middle ear).
Derivatives of 3rd & 4th pairs of aortic arches :
The 3rd arch artery persists forming
the common carotid artery and proximal
part of internal carotid artery (on both
sides), it joins with the dorsal aorta to
form the distal part of int.c.artery.
The ext.c.artery develops as a new
branch from 3rd arch.
The 4th arch forms the main part of
the arch of aorta… on left side,
and forms the proximal part of right
subclavian artery … on the right side.
Proximal part of the arch of
aorta develops from the aortic
The distal part of Rt.subclavian artery
and the distal part develops from the right dorsal aorta &
from left dorsal aorta.
right 7th intersegmental artery.
The left subclavian artery …. is not
derived from aortic arch but from the
Derivatives of 5th & 6th pairs of aortic arches :
The portion of dorsal aorta
connecting the 3rd & 4th arches
disappears on both sides.
The 5th arch artery disappears in
50% and in the other 50% of the
embryos, these arteries do not
The dorsal aorta on the
right side caudal to 4th arch
disappears down to the single
dorsal aorta, while persists on
left side to form descending
The 6th arch artery :
a- proximal part on both sides …
forms the pulmonary artery.
b- distal part of left artery : forms
ductus arteriosus which connects left
pulmonary artery with arch of aorta.
C- distal part of right artery :
Development of the final fetal arterial pattern :
A, aortic arches at 6 weeks, note
largely disappearance of the first two
pairs of aortic arches.
B,aortic arches at 7 weeks, showing
normal degeneration of aortic arches
and dorsal aortae.
C, final arterial arrangement at 8
weeks, note open ductus arteriosus.
D, 6-month-old infant, note the final
arrangement of the vessels - and that
the ascending aorta & pulmonary
arteries are smaller in C than in D.
Note also, obliterated & fibrosed ductus
arteriosus forming … ligamentum
arteriosum within few days after birth.
1- its proximal part
develops from left part
of distal part of aortic sac
(right part of aortic sac
forms brachio -cephalic
Proximal part of aortic
sac forms the pulmonary
2- its main middle part
develops from left 4th
3- its distal part
develops from the left
dorsal aorta between 4th &
6th aortic arches.
Final development of the arteries from
the aortic arches arteries :
Coarctation of Aorta :
In postductal coarctation
, the constriction is distal
(below) to ductus arteriosus.
This permits development of
a collateral circulation during
the fetal period to assist
passage of blood to lower
part of the body.
In preductal coarctation ,
the constriction is proximal
or above the ductus
arteriosus which remains
open and maintain the
circulation (below the
narrowing) to the lower part
of the body.