Development of the Heart

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					Development of the Heart
 Development of primitive heart tube
                                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.

B,transverse   C,longitudinal
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
mesocardium degenerates
,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 , showing :
Thin endothelial tube becomes…
internal endothelial lining of the heart or
Splanchnic mesoderm surrounding
the pericardial coelom becomes…..
primordial myocardium (muscular wall
of heart).
Thin endothelial tube is separated from
thick muscular tube (myocardium) by
gelatinous C.T. (cardiac jelly)….
Forming AV septum & valves.
 Visceral pericardium is derived from
mesothelial cells and forms the
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
                  oropharyngeal membrane.
                   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 transverse
                  (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
                                           constrictions :
                                           1-truncus arteriosus.
                                           2-bulbus cordis.
                                           3-primitive ventricle.
                                           4-primitive atrium.
                                           5-sinus venosus.
                                           Truncus arteriosus is
                                           continous cranially with aortic sac
                                           ,from which aortic arches develop.
                                            Sinus venosus has right
                                           & left hornes .
Ventral veiw ,By the end of   4th   week
                                           Each horn receives umbilical,
Heart tube bends upon                     vitelline ,& common cardinal veins
itself,giving rise an s-shaped             from the chorion, yolk sac &
heart,then u-shaped.                       embryo, respectively.
Bulbus cordis &
ventricle grow faster
than other regions, so
the heart bends upon
itself,forming U-shaped
bulboventricular loop
(by the end of 4th week).
The atrium & sinus
venosus also come to lie
dorsal to truncus
arteriosus, bulbus cordis
& ventricle.
                                        Blood Flow through
                                        the Primitive Heart :
                                        By the end of 4th week,
                                        unidirectional blood flow begins
                                        at sinus venosus by peristalsis-
                                        like waves. A
                                        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
A,sagittal section of primordial
                                        embryo, yolk sac , and placenta. A
heart(24 days),showing blood flow.
B,dorsal view of heart (26 days)
,illustrating hornes of sinus venosus
– Note also dorsal location of
primordial atrium & sinus venosus.
C,ventral view of
heart (35 days),Note
the aortic arches
arising from aortic sac
and terminate in the
dorsal aortae.
Partitioning of the primitive Heart

A, sagittal section of primordial heart (24days),showing blood flow.

   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.
   Partitioning of
   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 AV-
valves + membranous septa of
interventricular septum.
Note in D,cronal section ,begining of
development of interatrial & intervent.
                                          Partitioning of
                                          primordial Atrium :
                                It begins at the end of 4th week by
                                development of 2 septa.

                                1-Septum primum : a thin crescent-
                                shaped 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
                                the endocard.cushions,(A1-C1).
                                Before closure of foramen primum ,
                                perforations appear in central part of
A1 to D1… coronal sections     septum primium… coalesce to form
                                Foramen Secundum (C1-D1).
A to D… views of interatrial
septum from right side.
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
‘’foramen ovale’’.
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 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 ,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 : (A) It consists of
body and 2 hornes,right &
left.each horn receives 3 veins
1- Vitelline vein from yolk sac.
2- Umbilical vein from
3-Common cardinal vein from
body of embryo.
(B) Later , due to shuting of
blood from left side to right
side in the connection by
anastomosis between the 2
anterior cardinal veins. this
shunt becomes left
brachiocephalic vein… (C)
Changes on left side :
1- left horn & body of sinus
venosus form the coronary
2-left common cardinal vein
becomes small to form
oblique vein of left atrium.
3- left vitelline & left
umbilical veins, degenerat.
 Changes on right side:
1- The right horn becomes
absorbed into right atrium to
form its smooth part ,sinus
2- Right common cardinal
vein enlarges to form SVC.
3- Right vitelline vein
becomes IVC.
4- Right umbilical vein
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, 8-weeks
3- The remainder of the wall
of right atrium + conical
muscular pouch (auricle)…..
have rough trabeculated area
and derived from primordial
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
S.secundum and
incorporated with it into
interatrial septum.
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 2-
                  pulm.veins at 6th week , (C).
                  lastly , aborption of the 4-
                  pulm.veins into left atrium , with
                  separate orifices at 8th week. (D).
                  Left auricle is derived from
                  primordial left atrium.
Embryological origin of the definitive atrium:
    Right Atrium                 Left Atrium
1-Its rough part + auricle   1- Its rough part +auricle
from Right ½ of primitive    from left ½ of primitive
atrium.                      atrium.
2-right ½ of A-V canal.      2- left ½ of A-V canal.
3- Its smooth part from      3- Its smooth part from
Absorbed right horn of       Absorbed part of
sinus venosus.               pulmonary veins.
    Development of muscular part of
    interventricular septum :
                                Primordial muscular
                                interventricular( IV )septum
                                arises in the floor of ventricle
                                , as thick crescentic fold with
                                concave free edge.
                                This septum subdivides the
                                original ventricular cavity
                                incompletely into right & left
                                ventricles that communicate
A-sagittal section 5th week.   together through IV foramen.
Coronal section.6th week.      This foramen closes by the
                                end of 7th week as the 2 bulbar
                                ridges fuse with the endocadial
Incorporation of the
proximal part of bulus
cordis into the ventricles
A sagittal 5th w., showing the
bulbus cordis in the primitive heart.
B coronal 6th w. after
incorporation of the proximal part of
bulbus cordis into the ventricles to
forms :
In right ventricle …Conus arteriosus
(infundibulum), which gives origin of
pulmonary trunk.
In left ventricle…. Aortic vestibule part
of ventricular cavity just inferior to aortic
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.
3-fused endocardial cushions.
A,sagittal 5th w.
B, coronal 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
close I V foramen , so memb. IV
septum is formed
Cavitation of Ventricular Walls
                  Leads to formation of
                  spongy muscular bundles
                  (trabeculae carneae).
                  These bundles become
                  the papillary muscles &
                  tendinous cords
                  (attached to the cusps of
                  tricuspid & mitral valves).
                  A-5 weeks.
                  D-20 weeks.
Partitioning of distal part of the Bulbus
Cordis & Truncus Arteriosus :
                    A, 5th w. ventral v.of heart.
                    B,5th w. transverse sections of truncus
                    arteriosus & bulbus cordis,illustrating
                    truncal & bulbar ridges.
                    C,5th w. truncal & bulbar ridges , after
                    removal of ventral wall of heart &
                    truncus arteriosus.
                    D,heart after partitioning of truncus
                    arteriosus into aorta & pulmonary trunk.
                    E, transverse sections through newly
                    formed aorta & pulm.trunk showing
                    aortico-pulmonary septum.
                    F,6th w.removal of ventral wall to show
                    aotico-pulmonary septum.
Partitioning of distal part of the Bulbus
Cordis & Truncus Arteriosus :

                    G,diagram illustrating the spiral
                    form of aortico-pulmonary
                    H,drawing showing aorta &
                    pulmonary trunk twisting around
                    each other as they leave the
Partitioning of distal part of the Bulbus
Cordis & Truncus Arteriosus :
                     During 5th w. firstly , a right &
                    left bulbar ridges are developed
                    in the lower part.
                    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 primitive pharynx
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 & pulmonary trunk.
                    Because of spiraling of aortico-
                    pulmonary 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.
                     B,6th w.
                     C,7th w. complete
                     development of tricuspid
                     & mitral valves….. Note
                     also development of
                     compelete interventricular
Development of aortic & pulmonary valves :
                       Results after
                       development of bulbar &
                       truncal ridges and formation of
                       aorticopulmonary septum.
                       3 Semilunar valves
                       begin to develop from 3
                       swellings of
                       subendocardial tissue
                       around aortic &
                       pulmonary orifices.
                       These swellings are
                       hollowed out to form the
                       thin walled semilunar
Development of aortic & pulmonary valves
                      A, long. Section showing
                      bulbar & truncal ridges.
                      B, transverse section of
                      bulbus cordis.
                      C,fusion of bulbar ridges.
                      D,formation of walls & valves
                      of aorta & pulmonary trunk.
                      E, rotation of the vessels &
                      the valves.
                      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
           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 &
                    coronary sinus.
Development of conducting system :
                    Left sinuatrial valve is
                    incorporated into the
                    interatrial septum forming AV-
                    node & bundle ,which are
                    located superior to
                    endocardial cushions.
                    Right & left bundle
                    branches arising from AV-
                    bundle , pass from atrium into
                    the ventricular myocardium.
                    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 heart).
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
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
                                septum secondum.
                                A1, interatrial septum,
                                illustrating development
                                of oval fossa in right
                                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 :

The most common form of ASD is patent oval foramen:

                           A, patent oval foramen due
                           to abnormal resorption or
                           perforations of septum
                           primum, (in abnormal
                           locations), during formation
                           of foramen secondum.
                           B, patent oval foramen due
                           to excessive resorption of
                           septum primum ‘’short flap
Various Types of Atrial Septal Defect (ASD) in the
right aspect of interatrial septum :

Patent oval foramen :
                           C, patent oval foramen
                           ,resulting from an abnormally
                           large oval foramen because
                           of defective development of
                           septum secundum ,so a
                           normal septum primum will
                           not close the abnormal oval
                           foramen at birth.
                           D, patent oval foramen
                           resulting from a combination
                           of an abnormally large oval
                           foramen + excessive
                           resorption of septum primum.
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…. Less
                           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
                           septum secundum.
Tetralogy of Fallot :   It contains 4 cardiac
                        defects :
                        1- Pulmonary stenosis
                        (obstruction of right
                        ventricular outflow).
                        2- Ventricular Septal Defect
                        3- Dextroposition of aorta
                        (overriding 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) +
                      heart failure.
                 Due to excessive cavitation of the
Muscular VSD :   muscular part of the interventricular
                 septum….. Producing multiple small
                 defects (Swiss Cheese VSD).
                 Or absence of the IV septum--Single
                 ventricle + Transposition of aorta &
                 pulmonary trunk.
                 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
               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
Aortic Arch Arteries :
Left side embryonic cardiovascular
system (26 days) 4 –week embryo :
             The paired dorsal aortae fuse to form a
             single dorsal aorta, just caudal to the
             pharyngeal arches.

             Branches of the dorsal aorta :
             1- Cervical dorsal intersegmental arteies join
             to form vertebral artery on each side
             (7th cervical intersegmental artery forms the
             subclavian artery).
             2- Thoracic dorsal intersegmental arteries
             persist as intercostal arteries.
             3- in the lumbar region, they persist forming
             lumbar arteries, but 5th lumbar enlarge and
             forms common iliac artery.
             4- in the sacral region, they form lateral
             sacral arteries , but the caudal end of dorsal
             aorta becomes the median sacral artery.
The aortic Arches :
                A, left sided-embryo (26-
                days) showing the
                pharyngeal arches.
                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.
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.
Derivatives of 1st & 2nd pairs of aortic arches
                        The 1 aortic arches

                        largely disappear. small
                        parts persist to form the
                        maxillary artereis.
                        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).
Aortic Arches Arteries
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
                                The distal part of Rt.subclavian artery
aorta develops from the aortic develops from the right dorsal aorta &
sac ,      and the distal part right 7th intersegmental artery.
from left dorsal aorta.
                                   The left subclavian artery …. is not
                                   derived from aortic arch but from the
                                   left 7th intersegmental artery.
Aortic arches arteries
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 6th arch artery :
                                  a- proximal part on both sides …
The dorsal aorta on the          forms the pulmonary artery.
right side caudal to 4th arch
disappears down to the single     b- distal part of left artery : forms
dorsal aorta, while persists on   ductus arteriosus which connects left
left side to form descending      pulmonary artery with arch of aorta.
aorta.                            C- distal part of right artery :
Aortic Arches Arteries
Development of the arch of aorta
                    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
                    aortic arch.
                    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 :
The relation of recurrent laryngeal
nerves to the aortic arches :
                   A, 6-weeks, showing R.L.Ns.
                   hooked around the distal part of
                   6th pair of aortic arches.
                   B, 8-weeks, showing the
                   Rt.R.L.N. hooked around the
                   Rt. Subclavian artery, and the
                   left R.L.N. hooked around the
                   ductus arteriosus & arch of
                   Child, showing the left R.L.N.
                   hooked around ligamentum
                   arteriosum & arch of aorta.
Development of Recurrent Laryngeal Nerves :
                        Firstly, these nerves supply the 6th
                        pharyngeal arch so, they hook around
                        the 6th pair of aortic arches.
                        On the right , the distal part of right
                        6th aortic arch degenerates ,so right
                        R.L.N. hooks around the right
                        subclavian artery.
                        On the left , the left R.L.N. hooks
                        around the ductus arteriosus formed
                        by the distal part of 6th aortic arch.
                         when DA is transformed after birth
                        into ligamentum arteriosum ,left
                        R.L.N. hooks around lig.arteriosum &
                        arch of aorta.
Coarctation of the aorta :
                       A, postductal coarctation of aorta.
                       B, development of collateral
                       C and D, preductal coarctation.
                       E, 7-week embryo, showing normal
                       area of involution in the distal segment of
                       right dorsal aorta as the right
                       subclavian artery develops.
                       F, abnormal area of involution in the
                       distal segment of the left dorsal aorta.
                       G, later stage, showing the abnormally
                       involuted segment appearing as a
                       coarctation of aorta which moves with
                       the left subclavian artery to the region of
                       ductus arteriosus….. E to G illustrate
                       one hypothesis about the embryological
                       basis of coarctation of aorta.
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.

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