Download Development of Cardiovascular System

Development of Cardiovascular System
As the embryo increase in size, a more efficient system for supplying nutrients & exchanging gases is
required instead of diffusion.
Development of Blood vessels
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At week 3, in the wall of yolk sac, groups of mesenchymal cells are induced by endoderm to form
angioblasts form blood islands. (fig 4.1)
As cavity develop in these island, primitive blood cells and vessels form. (fig 4.3)
Primitive blood vessels join to form a vascular network in the yolk sac wall and in mesenchyme
associated with connecting stalk & chorion (fig 5-6)
Blood vessels in the embryo soon join to those on the yolk sac & in the connecting stalk & chorion to
form a primitive CVS
Development of Heart
Formation
 At first, endothelial cell clusters (angiocyst) located on lateral side of embryo, but soon spread in
cephalic direction.
 Later, they unite to form a horseshoe-shaped plexus of small blood vessel.
 The anterior central portion of plexus is known as cardiogenic area (fig 8)
 The cardiogenic cords of cells become canalized to form 2 thin-walled endothelial tube called
endocardial heart tubes
 These tubes soon fuse to form a single heart tube
 While the developing heart tube bulges more, the splanchnic mesenchyme adjacent to the heart tube
form the future myocardium & epicardium (fig 10-11)
Position
 At first, the cardiogenic area is located anterior to the prechordal plate & the neural plate.
 With closure of neural tube and formation of brain vesicle, CNS grow rapidly in cephalic direction that
extend over the cardiogenic area.
 As a result of brain growth & cephalic folding of embryo, the prechordal plate is pulled forward, and
finally the heart & pericardial cavity is located in thorax.
 While embryo folds cephalocaudally, it also fold laterally. So the caudal region of 2 endothelial tubes
merge except he caudalmost ends.
 At the same time, the crescent part of the horseshoe-shaped area expands to form future outflow tract
and ventricular region.
 So the heart becomes a continuous expanded tube, receiving drainage from caudal pole and begin
pumping blood out.
Formation of cardiac loop
 The heart tube continues to elongate and bend on Day 23
 The cephalic (ventricular) portion of the tube bend in ventral & caudal direction and to the right (fig 12.13) while the atrial (caudal) part shift in dorsocranial direction and to the left (fig 12.1-3)
 So cardiac loop is created and completed on D28
Further Development of Heart
Formation of adult Right Atrium (fig 22)
 Sinus venosus is initially a separate chamber of primitive heart that opens into the right atrium.
 As development proceed, left horn of sinus venosus becomes coronary sinus, while right horn is
incorporated into the wall of right atrium to form the smooth portion of the adult right atrial wall.
 The right primitive atrium persists as the right auricle
Formation of adult Left Atrium (fig 22)
 Most of the adult left atrium is form by incorporation of the primitive pulmonary vein
 As the atrium enlarge, parts of the vein and its branch is absorbed, with the result that 4 pul veins enter
the adult left atrium.
 The smooth-walled pt of the left atrium is derived from absorbed pul vein tissue while the left auricle is
derived from the primitive atrium.
Formation of Four-Chambered Heart (week 4-5)
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Division of Atrioventricular canal (fig 14)
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2 localized proliferation of mesenchyme, called endocardial cushions develop in the superior &
inferior border of AV canal of the heart.
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These cushions grow toward each other (fig 14, R top) & fuse, so dividing the AV canal into R and L
AV canals (fig 14, R bottom)
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Development of Inter-Atrial Septum (fig 15-19)
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A sickle-shaped membranous partition called Septum Primum grows from the dorsal wall of
primitive atrium.
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A communication exist between R & L halves of primitive atrium through ostium primum. (fig 15)
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As the septum primum will eventually fuse with the endocardial cushion & obliterate the ostium
primum, the superior part of septum primum breaks down (fig 17.2), creating another opening
called ostium secundum (fig 18.1).
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As this hole develop, another sickle-shaped membranous fold called Septum Secundum grows into
atrium to the right of septum primum (fig 18.2). Septum Secundum overlaps the ostium secundum.
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There's also an opening between the free edge of septum secundum & the dorsal wall of atrium,
the foramen ovale (fig 19.1).
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The remain of septum primum form the flap-like valve of foramen ovale (fig 19.2).
Atrial Septal Defects (ASD) (fig 20-21)
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results from abnormal development of interatrial septum
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common defects is characterized by large opening in septum between R & L atria (persistent foramen
ovale) that results from
i) excessive cell death & resorption of septum primum (fig 21.1)
ii) underdevelopment of septum secundum (fig 21.2)
ostium secundum defect
iii) combination of i & ii
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Depends on the size, considerable intracardiac shunting may occur from L to R
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Formation of Ventricle
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Primitive ventricle give rise to most of the L ventricle while the bulbus cordis form most of the R
ventricle
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Development of ventricle is done by continuous growth of myocardium on the outside & continuous
diverticulation and trabecula formation on the inside
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Interventricular Septum formed by 2 parts: Muscular pt & Membranous pt
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Muscular pt begins as a ridge in the floor of the primitive ventricle & slowly grow towards the
endocardial cushions (fig 23)
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Membranous pt of inter-V septum form at the end of week 7, closing the interventricular foramen. It
is formed by fusion of AV endocardial cushion & L, R bulbar ridges
Cardiovascular Changes at birth
Fetal Circulation
 Most of oxygenated blood returns to the fetus via umbilical vein.
 A main portion bypass the liver by ductus venosus into IVC, small portion enters the liver sinusoids &
mix with blood fm portal circulation
 After a short course of IVC that placental blood mix with de-O2 blood from lower limb, it enter the R
atrium.
 Blood is guided toward the foramen ovale by valve of IVC, most bloodstream pass directly into L atrium.
Small portion of blood remains in RA and mix with de-O2 blood returning fm head & arms fm SVC.
 In L atrium, where RA blood mix with de-O2 blood fm lung, blood enter the LV and ascending aorta. In
this way, heart and head are supplied with well-oxygenated blood since the coronary & carotid arteries
are first branches of asc aorta.
 De-O2 blood fm SVC flows by way of RV into pul trunk. Due to high resistance in pul vessels, main
portion of blood pass through ductus arteriosus into descending aorta. Blood then flows towards the
placenta through two umbilical arteries
Changes at Birth
 The changes are caused by cessation of placental blood flow and the beginning of respiration
 The following changes occur in the vascular system after birth:
A) Closure of umbilical arteries
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done by contraction of smooth muscle in the wall that MAY cause by thermal & mechanical stimuli
and change in O2 tension.
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closed a few min after birth, actual obliteration may take 2-3 mth
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distal parts of umbilical art form medial umbilical ligament, while prox pt remains open as superior
visical arteries
B) Closure of umbilical Vein & Ductus Venosus
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occurs shortly after that of umb art.
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after obliteration, umb vein forms ligamentum teres of liver; while ductus venosus that course fm lig
teres to IVC, is obliterated & forms the ligamentum venosum.
C) Closure of ductus arteriosus
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done by contraction of muscular wall
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occurs immediately after birth and mediated by bradykinin release fm lungs during initial inflation
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complete obliteration takes 1-3 mth, forms ligamentum arterosum
D) Closure of foramen ovale
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caused by increased pressure in the LA, combine with decrease pressure in RA (due to interruption
of placental blood flow)
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septum primum is pressed against the septum secundum
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as the closure is reversible, crying creates a shunt fm R to L in the first day, causing cyanotic periods
in newborn
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2 septa gradually fuse in abt 1 yr, but 20% individual never get perfect closure.