Development%20of%20the%20Heart

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Development of the Heart
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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
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• 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 pericardium,
  (E,F) schematic T.S of 28
  days.

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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 endocardium.
• 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 epicardium.

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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 pericardium.

• 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.

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• 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 .

• Each horn receives umbilical, vitelline , common
  cardinal veins from the chorion, yolk sac
  embryo, respectively.

Ventral veiw ,By the end of 4th week

• Heart tube bends upon itself,giving rise an
  s-shaped heart,then u-shaped.

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• 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.

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• 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 embryo, yolk sac
  , and placenta. A

A,sagittal section of primordial 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.
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• C,ventral view of heart (35 days),Note the
  aortic arches arising from aortic sac and
  terminate in the dorsal aortae.

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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.

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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. septa.

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• 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 septum primium
  coalesce to form Foramen Secundum (C1-D1).

• A1 to D1 coronal sections
• A to D views of interatrial septum from right
  side.

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• 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
  ovale.

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• 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.

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Left side embryonic cardiovascular system (26
days) 4 week embryo
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• 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
  placenta, 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)

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• Changes on left side (B,C)
  1- left horn
  body of sinus venosus form the coronary sinus.

2-left common cardinal vein becomes small to form
oblique vein of left atrium.

3- left vitelline left umbilical veins,
degenerat.
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• Changes on right side

1- The right horn becomes absorbed into right
atrium to form its smooth part ,sinus venarum.
2- Right common cardinal vein enlarges to form
SVC.
3- Right vitelline vein becomes IVC. 4- Right
umbilical vein disapears.
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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
atrium.
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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.
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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.

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Embryological origin of the definitive atrium
Right Atrium
Left Atrium
1-Its rough part auricle from Right ½ of
primitive atrium. 2-right ½ of A-V
canal. 3- Its smooth part from Absorbed right
horn of sinus venosus.
1- Its rough part auricle from left ½ of
primitive atrium. 2- left ½ of A-V canal. 3- Its
smooth part from Absorbed part of pulmonary veins.
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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 together through IV foramen.
• This foramen closes by the end of 7th week as the
  2 bulbar ridges fuse with the endocadial cushion.

• A-sagittal section 5th week.
• Coronal section.6th week.

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Incorporation of the proximal part of bulus
cordis into the ventricles

• A sagittal s.at 5th w., showing the bulbus cordis
  in the primitive heart.
• B coronal s.at 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 valve.
  

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• 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 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 close I V foramen , so memb.
  IV septum is formed

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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.
• B-6weeks.
• C-7weeks.
• D-20 weeks.

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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.

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Partitioning of distal part of the Bulbus Cordis
Truncus Arteriosus

• G,diagram illustrating the spiral form of
  aortico-pulmonary septum.
• H,drawing showing aorta pulmonary trunk
  twisting around each other as they leave the
  heart.

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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

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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.

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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 septum(muscularmemb.pa
  rt)

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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 cusps.

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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.

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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 coronary sinus.

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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).

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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.

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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 atrium.
• B and B1, note incomplete adhesion of septum
  primum TO septum secundum and development of a
  probe patent oval foramen.

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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
  defect.

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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.

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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 common.
• 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.

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Tetralogy of Fallot

• It contains 4 cardiac defects
• 1- Pulmonary stenosis (obstruction of right
  ventricular outflow).
• 2- Ventricular Septal Defect (VSD).
• 3- Dextroposition of aorta (overriding aorta).
• 4- Right ventricular hypertrophy.
• cyanosis is one of the obvious signs of
  tetralogy .

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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.

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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 pulmonary trunk.
• Complication heart failure and death.
• This diagram showing transposition of great
  arteries (TGA) which leads to cyanosis. VSDASD
  allow mixing arterial venous blood.
• Transposition results from that the
  aortico-pulmonary septum descends straight
  (instead of spiral).

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The Aortic Arches Derivatives

• During the 4th week, as the pharyngeal arches
  develop, they are supplied by the aortic
  arches.
• 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 arteries.

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Aortic Arch Arteries
48
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.

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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.

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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.

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Derivatives of 1st 2nd pairs of aortic arches

• The 1st 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).

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Aortic Arches Arteries

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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.
• The distal part of Rt.subclavian artery
  develops from the right dorsal aorta right 7th
  intersegmental artery.
• The left subclavian artery . is not derived
  from aortic arch but from the left 7th
  intersegmental artery.

• Proximal part of the arch of aorta develops from
  the aortic sac , and the distal part
  from left dorsal aorta.

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Aortic arches arteries

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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
  develop.
• 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
  disappears.

• 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
  aorta.

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Aortic Arches Arteries

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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 artery).
  
• Proximal part of aortic sac forms the pulmonary
  trunk. 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.

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Final development of the arteries from the
aortic arches arteries
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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
  aorta.
• Child, showing the left R.L.N. hooked around
  ligamentum arteriosum arch of aorta.

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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.

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Coarctation of the aorta

• A, postductal coarctation of aorta.
• B, development of collateral circulation.
• 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.

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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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