Download Embryology_Objectives heart 2008

Embryology Objectives – Heart 2008
Lecture 8: Heart
 Describe the formation of the cardiac tube:
 Cardiac precursor cells arise from the epiblast and associate with splanchnic mesoderm
 Underlying endoderm induces these cells to form cardiac myoblasts
 Cardiac cells surround blood islands which are forming nearby making a horseshoe shaped tube
known as the cardiogenic field. Two laterally situated tubes are present by day 19
 Initially the heart tube is suspended by dorsal mesentery but this starts to break down thus forming a
transverse pericardial sinus
 The heart consists of endothelium and splanchnic mesoderm
 Splanchnic mesoderm develops into two layers: the myocardium and cardiac jelly
 Cells from the splanchnic mesoderm also migrate and give rise to epicardium thus forming a three
layered tube consisting of epicardium, myocardium, and endocardium
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Describe formation of the cardiac loop by day 21
 The bulbus cordis moves anteriorly, inferiorly and to the right
 It can be divided into thirds:
 Truncus arteriosus: forms big arteries of the heart (aorta and pulmonary trunk)
 Conus cordis: forms area where the muscles get smooth
 Primitive right ventricle: full of cardiac muscle
 The ventricle moves anteriorly, inferiorly and to the left
 The atrium shifts posteriorly and superiorly
 Sinus venosus slides up posteriorly thus forming a kink- the cardiac loop
 Receives blood from three veins, bilaterally
 Cardinal veins: principal veins that drain the extremities and head
 Umbilical veins: drain the placenta
 Vitelline veins: drain the guts
 During the fifth week, there is a great venous shift
 The right sinus horn enlarges and becomes incorporated into the right atrium (sinus venarum)
 This forms two valves: the valve of the IVC (directs flow from IVC to left side of the heart)
and the valve of the coronary sinus (prevents backflow to coronary sinus)
 The crista terminalis is dividing between the trabeculated part of the right atrium and the smooth
walled part (sinus venarum) which originates from the right sinus horn
Describe septum formation of the atria
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 Two actively growing masses of tissue approach each other until they fuse, dividing the lumen into
two separate canals
 Between the bulbous cordis and Truncus
 Neural crest serves to partition the Truncus
 Common site for defects
 Between the atria and ventricle
 Splanchnic mesoderm serves to partition the atria, ventricles, and AV canals
 Septum Primum enters from the roof of the atrium and moves toward the ventricle
 Ostium Primum forms an opening from the right to left atrium which then closes off
 Ostium secundum stays intact
 Septum secundum grows towards the ventricles from the atrium
 This helps form an opening with the septum primum for blood flow from the right atrium to the
left atrium
 Ovale foramen: a connection between the atria which helps blood skip the lungs in the
embryo
Septum formation of the AV canal
 Towards the end of the fourth week, two mesenchymal cushions, the atrioventricular endocardial
cushions, appear at the superior and inferior borders of the AV canal
 Initially the AV canal gives access only to the primitive left ventricle and is separated from bulbus
cordis by bulbo ventricular flange which terminates midway
 The AV canal then enlarges to the right and blood passing through the AV orifice now has direct
access to the left and right ventricles
 Two lateral AV cushions appear on the right and left borders of the canals
 The superior and inferior cushions project into the lumen and then fuse thus competing the division
 Each AV orifice is now surrounded by mesenchymal tissue.
 Valves form and remain attached to the ventricle wall by muscular cords
 The muscle tissue in the cords degenerates and is replaced by dens connective tissue
 Valves are connective tissue covered by endocardium
 They are connected to a thick trabeculae in the wall, papillary muscles, by chordate tendineae
 This forms the bicuspid (mitral) valve in the left AV canal and the tricuspid on the right side
Septum formation in the ventricles
 By the end of the fourth week, two primitive ventricles expand
 There is continuous growth of the myocardium on the outside, continuous diverticulation and
trabecula formation on the inside
 Medial walls of the expanding ventricles become opposed and gradually merge, forming muscular
interventricular septum (sometimes the two walls do not merge completely thus allowing
communication between the two ventricles)
 The interventricular foramen located above the muscular portion of the interventricular septum
shrinks on completion of conus septum. As development continues, outgrowth of tissue from the
inferior endocardial cushion along the top of the muscular interventricular septum closes the foramen.
This forms the membranous part of the interventricular septum
 Often a site of defects
 When partitioning of truncus is almost complete, primordial of the semilunar valves become visible
as small tubercles on the main truncus swells. One of each is then assigned to pulmonary and aortic
channels. A third tubercle appears and the tubercles hollow out forming semilunar valves
Describe formation of the conducting system
 Initially the pacemaker is in the caudal part of the left cardiac tube
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 Later the sinus venosus assumes this function and as the sinus is incorporated into the right atrium,
pacemaker tissue lies near the opening of the superior vena cava  sinoatrial node
 Atrioventricular node and bundle of HIS
 Cells in the left wall of the sinus venosus
 Cells from the AV canal
 Once the sinus venosus is incorporated into the right atrium these cells terminate at base of interatrial
septum
Describe associated congenital anomalies related to development of the heart (159-180)
 Abnormalities of cardiac looping
 Dextrocardia: heart lies on right side of thorax instead of left; heart loops to the left instead of right
 Situs inversus: complete reversal of asymmetry in all organs
 Heterotaxy: random miss-sidedness (some organs are reversed, some are not)
 Left-sided bilaterality- polysplenia
 Right-sided bilaterality- asplenia or hypoplastic spleen
 Endocardial cushions and heart defects
 Atrial and ventricular septal defects, defects involving the great vessels (transposition of the great
vessels, tetralogy of Fallot)
 Teratogenic agents or genetic causes abnormalities in neural crest cells
 Often produce both heart and craniofacial defects in the same individual
 Heart defects
 Teratogens such as rubella virus, thalidomide, isotretinoin alcohol; diabetes, hypertension,
chromosomal abnormalities…all linked to heart defects
 Holt-Oram syndrome: limb abnormalities and atrial septal defects (a heart-hand syndrome)
 Atrial septal defect (ASD): congenital heart abnormality
 Ostium secundum defect causes large opening between left and right atrium or by inadequate
development of the septum secundum
 Cor triloculare biventriculare: complete absence of the atrial septum
 Premature closure of the oval foramen: leads to massive hypertrophy of the right atrium and
ventricle and underdevelopment of the left side of the heart
 Persistent atrioventricular canal: endocardial cushions fail to separate
 Ostium primum defect: interventricular septum is closed, defect in atrial septum
 Tricuspid atresia: obliteration of the right atrioventricular orifice, absence or fusion of the
tricuspid valves
 Ventricular septal defect (VSD)
 Most common congenital cardiac malformation: abnormalities in the membranous portion of
the septum
 Tetralogy of Fallot: conotruncal region abnormality; unequal division of the conus resulting
from anterior displacement of the conotruncal septum
 Persistent truncus arteriosus: conotruncal ridges fail to fuse and to descend toward ventricles
 Transposition of the great vessels: conotruncal septum fails to follow its normal spiral course
and runs straight down- aorta originates from right ventricle, pulmonary artery originates
from the left ventricle; trucus arteriosus
 DiGeorge sequence: pattern of malformations due to abnormal neural crest development
 Valvular stenosis: semilunar valves are fused for a variable distance
 Aortic valvular atresia: open ductus arteriosus
 Ectopia cordis: heart lies on the surface of the chest; failure of the embryo to close the ventral
body wall
Describe the development of the vasculature (180-194)
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 Blood vessel development occurs by two mechanisms
 Vasculogenesis: vessels arise by coalescence of angioblasts
 The major vessels, including the dorsal aorta and cardinal veins are formed this way
 Angiogenesis: vessels sprout from existing vessels
Development of the arteries
 All arteries, veins and lymphatic channels form from mesoderm and all vessels begin as groups of
mesodermal cell clusters
 Extraembryonic vessels form initially from the yolk sac. Embryonic vessels form shortly after the
extraembryonic vessels
 Blood cells form from:
 The yolk sac
 Later in the liver
 Later sources of blood cells include the spleen, thymus, and bone marrow
 During the 4th and 5th weeks of development, pharyngeal arches form at the cranial end of the embryo
and each of the arches receives a blood vessel
 Arterial Derivatives of the Aortic Arches
 Arch 1: Maxillary arteries
 Arch 2: Hyoid and stapedial arteries
 Arch 3: Common carotid and first part of the internal carotid arteries
 Arch 4:
 Left- Arch of the aorta from the left common carotid to the left subclavian arteries
 Right- Right subclavian artery (proximal portion)
 Arch 6:
 Left- left pulmonary artery and ductus arteriosus
 Right- right pulmonary artery
 Vitelline arteries: blood supply to abdomen
 Celiac Trunk: foregut
 Superior Mesenteric: midgut
 Inferior Mesenteric: hindgut
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 Other branches from the aorta
 Intercostal
 Middle suprarenal
 Renal
 Gonadal
Development of the Venous system
 During the fifth week of development, three systems of veins can be observed
 Vitelline (omphalomesenteric): carry blood from yolk sac to sinus venosus
 Form a plexus around duodenum and pass through septum transversum
 Umbilical: originate in chorionic villi and carry oxygenated blood to the embryo
 Ductus venosus- direct communication between left umbilical vein and right hepatocardiac
channel (after birth left umbilical vein and ductus venosus are obliterated and form the
ligamentum teres hepatis and ligamentum venosum, respectively
 Cardinal: drain the body of the embryo proper
 Anterior, posterior, and common cardinal veins form a symmetrical system during the fourth
week; additional veins form weeks 5-7
 Umbilical veins:
 The ductus venosus bypasses the sinusoids of the liver
 After birth the left umbilical vein obliterates to form the ligamentum teres hepatis
 After birth the ductus venosum obliterates to form the ligamentum venosum
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Describe circulation before and after birth (180-194)
 Before:
 Three shunts in the fetal circulation
 Ductus arteriosus
… protects lungs against circulatory overload
… allows the right ventricle to strengthen
… high pulmonary vascular resistance, low pulmonary blood flow
… carries mostly med oxygen saturated blood
 Ductus venosus
… fetal blood vessel connecting the umbilical vein to the IVC
… blood flow regulated via sphincter
… carries mostly high oxygenated blood
 Foramen ovale
… shunts highly oxygenated blood from right atrium to left atrium
 Blood from the placenta (about 80% saturated with oxygen) returns to the fetus by way of the
umbilical vein. On approaching the liver, most of this blood flows through the ductus venosus
directly into the IVC and bypassing the liver. A sphincter mechanism in the ductus venosus close
to the entrance of the umbilical vein regulates flow of umbilical blood through the liver sinusoids.
After a short course in the IVC where placental blood mixes with deoxygenated blood returning
from the lower limbs, it enters the right atrium. Here it is guided toward the oval foramen by the
valve of the IVC and most of the blood passes directly into the left atrium. A small amount is
prevented from doing so by the septum secundum, the crista dividens, and so this blood remains in
the right atrium. Here it mixes with desaturated blood returning from the head and arms by the
SVC. From the left atrium where it mixes with a small amount of desaturated blood returning
from the lungs, blood enters the left ventricle and ascending aorta. Heart and brain are thus
supplied with well oxygenated blood through the coronary and carotid arteries (first branches of
the ascending aorta). Desaturated blood flow from the SVC flows by way of the right ventricle
into the pulmonary trunk. During fetal life, resistance in the pulmonary vessels is high, so most of
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the blood passes directly through the ductus arteriosus into the descending aorta, where it mixes
with blood from the proximal aorta. After coursing through the descending aorta, blood flows
toward the placental by way of the two umbilical arteries. The oxygen saturation in the two
umbilical arteries is about 58%.
 After
 Closure of the umbilical arteries
 Distal parts of the umbilical arteries form the medial umbilical ligaments, and the proximal
portions remain open as the superior vesicle arteries
 Closure of the umbilical vein and ductus venosus
 Umbilical vein forms the ligamentum teres hepatis
 The ductus venosus forms the ligamentum venosum
 Closure of the ductus arteriosus
 Adult forms ligamentum arteriosum
 Closure of the oval foramen due to increased pressure in the left atrium (takes ~1 year to
completely close)
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