Development of heart tube and fetal circulation

Dr. Mohamed El fikyProfessor of anatomy and embryology

Embryonic disc

q Appears in the middle of third week.

q Mesenchymal cells in the splanchnic mesoderm proliferate and form isolated cell clusters known as angiogenic clusters.

q Angiogenic clusters at first located in the lateral end but rapidly spread to cephalic end.

Angiogenic clusters.

q The angiogenic clusters acquire lumen.

q They unite to form a horseshoe-shaped plexus of small blood vessels.

q The anterior portion of the plexus is called cardiogenic area.

q The intraembryonic coleomic cavity located over the plexus later form pericardial cavity.

Formation of endocardial heart tubes

q After formation of neural tube and brain vesicles CNS grows rapidly in cephalic direction.

q It finally extends over the cardiogenic area and future pericardial cavity.

q Finally,the procordal plate and the cardiogenic plate are pulled forward.

q The cardiogenic plate and pericardial cavity become located ventrally and caudally.

1. Epimyocardial cells2. Heart tube3. Endoderm4. Endocardial cells5. Notochord6. Dorsal aorta7. Neural crest8. Neural fold

1. Foregut2. Intraembryonic coelom3. Heart tubes4. Dorsal mesocardium5. Epimyocardium6. Neural groove7. Neural crest8. Notochord9. Dorsal aorta

Heart tubes

Fusion of the heart tubes

Embryo after foldingHead swelling

Cardiac swelling

Umbilical cord

Y.S

GUT

q The embryo folds in cephalocaudal and transversely bringing the two heart tubes closer.

q The two endocardial heart tube fuse in cephalo-caudal direction.

q The tube is attached to the dorsal side of the pericardial cavity by dorsal mesocardium.

q The mesoderm adjacent to the endocardial tube form epimyocardial mantle.

q The epimyocardial mantle seperated from endocardial tube by cardiac jelly.

q Tube consists of endocardium,myocardium and epicardium.

Development of primitive heart tube

Ø It develops early in the middle of 3rd week , from aggregation of

splanchnic mesodermal cells, in cardiogenic area.

Ø They form 2 angioblastic cords that canalize to form

2 endocardial heart tubes.

Ø The embryo folds in cephalocaudal and

transversely bringing the two heart tubes closer.

Ø The two endocardial heart tube fuse in cephalo-caudal direction.

Ø The tube is attached to the dorsal side of the pericardial cavity by dorsal mesocardium.

Ø After lateral folding of embryo, 2 endocard.tubes fuse to form….

Single heart tube

Ø This heart tube lies inside the pericardial cavity , its dorsal wall is

connected to foregut by dorsal mesocardium

Ø The central part of dorsal mesocardium degenerates ,forming

transverse passage dorsal to heart ,called transverse sinus of

pericardium,

ØThe primitive heart tube elongates and develops alternate dilatations and constrictions :

Ø1-truncus arteriosus.

Ø2-bulbus cordis.

Ø3-primitive ventricle.

Ø4-primitive atrium.

Ø5-sinus venosus.

ØTruncus arteriosus is continous cranially with aortic sac ,from which aortic arches develop.

ØSinus venosus has right & left hornes .

Ø Each horn receives umbilical, vitelline ,& common cardinal veins .

Main subdivisions of heart tube

Ø Bulbus cordis & ventricle grow faster than other regions, so the heart

bends upon itself,forming U-shaped bulboventricular loop.

Ø The atrium & sinus venosus also come to lie dorsal to truncus arteriosus, bulbus cordis & ventricle .(S-shaped heart tube).

Formation of cardiac loopq Heart tube elongates and bends.

q The cehpalic portion: bends in ventral and caudal direction to the right.

q The caudal portion: shifts in a dorsocranial direction and to the left.Thebendings creates a cardiac loop.

Primitive heart tube Twists

q Local expansion become visible after cardiac loop is formed.

q The atrial portion lie outside the pericardial cavity,

q later incorporated inside the cavity.

q The atrioventricular junction remains narrow and form atrioventricular canal.

q The bulbus cordis is narrow except its proximal third which later forms trabeculated part of right ventricle.

q The distal part of bulbus called the truncus arteriosus.

q The conus cordis forms the outflow tract of both ventricles.

q The atrial portion of bulbus remain temporarily smooth walled.

q The proximal portion of the bulbus form the primitive right ventricle.

q The primitive ventricle becomes trabeculated and form the primitive left ventricle.

Development of sinus venosusq In 4th week,it consists of a

transverse portion and right and left sinus horn.

q Each horn receives blood from three important veins: - .

a. Vitelline or omphalomesenteric veins.

b. The umbilical vein.c. Common cardinal vein.

q At 5th and 7th week the left umbilical vein and Left vitelline vein disappear.

q The left common cardinal vein disappear at 10th week.

q The remaining part in the left horn of sinus venosus isa.The oblique vein of left atrium.b.The coronary sinus.

q Due to left to right shunt the right sinus horn and veins enlarge.q The right horn is the only communication between sinus venosus and the

atrium.q The right horn is incorporated into right atrium to form the smooth part of right

atrium

q The entrance the sinoatrial orifice is flanked On each side by right and left venous valves.

q Dorsocranially , the valves fuse , forming a ridge known as septum spurium

q The superior portion of right venous valve disappear.q The inferior part form two parts:

a.The valve of inferior vena cava.b. The valve of coronary sinus.

q The crista terminalis originates from right sinus horn.

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.Endocardial cushions: these are masses of cells and extracellular matrices develop in the atrioventricular and conotruncal regions . • in the atrioventricularregion they are :

– Dorsal & ventral swellings– Fuse, dividing the single AV canal into paired canals– Involved in formation of interatrial & interventricular septa– Derived from neural crest

Partitioning

•Attheendofthefourthweek,asickle-

shapedcrestgrowsfromtheroofofthe

commonatriumintothelumen.Thiscrest

isthefirstportionoftheseptumprimum.

•Thetwolimbsofthisseptumextend

towardtheendocardialcushionsinthe

atrioventricularcanal.

•Theopeningbetweenthelowerrimof

theseptumprimumandtheendocardial

cushionsistheostiumprimum

Septum formation in the common atrium

With further development,

extensions of the superior and

inferior endocardial cushions

grow along the edge of the

septum primum, closing the

ostium primum Before closure

is complete, however, cell

death produces perforations in

the upper portion of the

septum primum. Coalescence

of these per- forations forms

the ostium secundum,

ensuring free blood flow from

the right to the left primitive

atrium,D).

When the lumen of the right atrium expands as a result of incorporation of the sinus horn, a

new crescent-shaped fold appears. This new fold, the septum secundum never forms a

complete partition in the atrial cavity . Its anterior limb extends downward to the septum in

the atrioventricu- lar canal. When the left venous valve and the septum spurium fuse with the

right side of the septum secundum, the free concave edge of the septum secundum begins to

overlap the ostium secundum.The opening left by the septum secundum is called the oval

foramen (foramen ovale).

When the upper part of the septum primum gradually disappears, the remaining

part becomes the valve of the oval foramen. The passage between the two atrial

cavities consists of an obliquely elongated cleft through which blood from the

right atrium flows to the left side.

After birth, when lung circulation begins and pressure in the left atrium

increases, the valve of the oval foramen is pressed against the septum secundum,

obliterating the oval foramen and separating the right and left atria. In about

20% of cases, fusion of the septum primum and septum secundum is incomplete,

and a narrow oblique cleft remains between the two atria.This condition is called

probe patency of the oval foramen; it does not allow intracardiac shunting of

blood.

Fetus• right side high pressure (high pulmonary resistance, etc.)• well oxygenated blood streams through foramen ovale.• valve of foramen ovale closes with left atrial contraction.

After birth• right side low pressure (low pulmonary resistance).• valve remains closed (physiological closure).• valve eventually fuses (anatomical closure): fossa ovalis.

•Bytheendofthefourthweek,thetwoprimitiveventriclesbegintoexpand.Thisis

accomplishedbycontinuousgrowthofthemyocardiumontheoutsideandcontinuous

diverticulationandtrabeculaformationontheinside.

•Themedialwallsoftheexpandingventriclesbecomeapposedandgraduallymerge,

formingthemuscularinterventricularseptum.Sometimes,thetwowallsdonotmerge

completely,andamoreorlessdeepapicalcleftbetweenthetwoventriclesappears.

Thespacebetweenthefreerimofthemuscularventricularseptumandthefused

endocardialcushionspermitscommunicationbetweenthetwoventricles.

•Theinterventricularforamen,abovethemuscularportionoftheinterventricular

septum,shrinksoncompletionoftheconusseptum.Duringfurtherdevelopment,out-

growthoftissuefromtheanterior(inferior)endocardialcushionalongthetopofthe

muscularinterventricularseptumclosestheforamen.Thistissuefuseswiththeabut-

tingpartsoftheconusseptum.Completeclosureoftheinterventricularforamen

formsthemembranouspartoftheinterventricularseptum

SeptumFormationintheVentricles

• Continuous set of ridges in bulbus cordis(bulbar ridges) and truncus arteriosus (truncal ridges).

• Grow toward each other, spiraling 180º.

Partitioning of Truncus Arteriosus

• Fuse to form spiraling aorticopulmonaryseptum, dividing aorta & pulmonary trunk

• Bulbar ridges involved in formation of IV septum

• Bulbar & truncal ridges derived from neural crest cells—clinical implications

Partitioning of Truncus Arteriosus

1. Placenta plays the role of lungs; lungs are not functional:

• Like pulmonary veins, left umbilical vein carries highly oxygenated

blood from placenta to heart.

• Like pulmonary artery, right and left umbilical arteries braing

deoxygenated blood to placenta.

2. Mixing potentially occurs at 4 sites:

• Left umbilical vein (Oxygen saturation ~80%) – Portal vein

(Much bypassed by ductus venosus to Inferior Venacava)

• Left umbilical vein – Inferior venacava – Portal vein (Oxygen

saturation of IVC blood is ~65%)

• Superior venacava – Right atrium (Much bypassed by Foramen

ovale)

• Pulmonary veins – Left atrium (Much bypassed by Foramen ovale

and ductus arteriosus)

Following are the important features of fetal circulation

Placenta

Umbilical vein

Portal vein

Inferior vena cava

Right atrium

Left Atrium

Left ventricle

Aorta

Whole body tissue

Fetal circulation(oxygenated)

Superior & inferior vena cava

Right atrium

Right ventricle

Pulmonary trunk

Arch of Aorta

Descending aorta

Common and internal iliac

2 Umbilical arteries

Placenta

Fetal circulation(Deoxygenated)

PATHWAY

Placenta

Umbilical Vein

Umbilical Arteries

Liver

Ductus VenosusInferior VenacavaRight AtriumForamen Ovale

Right LungArch of Aoarta

Ductus Arteriosus

Left Atrium

Left Ventricle

Right Ventricle

Portal Vein

Overview of Fetal Circulation1. Blood is oxygenated in the placenta.

2. Highly oxygenated and nutrient-enriched blood returns to the fetus from the

placenta via the left umbilical vein.

3. Some blood enters liver sinusoids; most of the blood bypasses the sinusoids by

passing through the ductus venosus and enters the inferior vena cava (IVC)

4. From the IVC, blood enters the right atrium, where most of the blood bypasses the

right ventricle through the foramen ovale to enter the left atrium.

5. From the left atrium, blood enters the left ventricle and is delivered to fetal tissues

via the aorta.

6. Poorly oxygenated and nutrient-poor fetal blood is sent back to the placenta via

right and left umbilical arteries.

7. Some blood in the right atrium enters the right ventricle; blood in the right

ventricle enters the pulmonary trunk, but most of the blood bypasses the lungs

through the ductus arteriosus.

Fetal circulation

Circulatory Changes at Birth

Changes in the vascular system at birth are

caused by cessation of placental blood flow

and the beginning of respiration. :

1- 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 vesical arteries

2-Closure of the umbilical vein and ductus

venosus , the umbilical vein forms the

ligamentum teres hepatis and The ductus

venosus forms the ligamentum venosum.

3- Closure of the ductus arteriosus : forms

the ligamentum arteriosum.

4-Closure of the oval foramen : forms fossa

ovale

Congenital anomalies of the heart1- Defects of the atrial septum:

a)Patent foramen ovale (ASD):

b)Is caused by incomplete anatomic fusion of septum primum and septum secudum.

Present in approximately 25% of people.

c)Foramen secundum defect : is caused by excessive resorption of septum primum

or septum secundum. This results in a large opening between the right and left atria.

d)Common atrium : caused by complete failure of septum primum and septum

secundum to develop.

e)Premature closure of foramen oval: closure of foramen ovale during pre-natal life.

This results in hypertrophy of the right side of the heart and under-development of the

left side of the heart.

Congenital anomalies of the heart

• 2- Defects of atrio-ventricular canal :

a) Persistent AV canal: caused by failure of AV cushions to fuse, accompanied

by abnormal tricuspid and bicuspid valves.

b) Tricuspid atresia: Obliteration of the right AV canal, characterized by absence

of the tricuspid valve and accompanied by the following :

– 1) Patent foramen ovale.

– 2) I.V. septal defect.

– 3) Over-developed left ventricle.

– 4) Under-developed right ventricle.

Congenital anomalies of the heart

3. Defects of I.V. septum (VSD):

a)Membranous VSD: caused by failure of the membranous I.V. septum to

develope.

b)Muscular VSD: caused by single or multiple perforations in the muscular I.V.

septum.

c)Common ventricle: caused by failure of the membranous and muscular I.V.

septa to develope.

Congenital anomalies of the heart4, Defects of the aortico- pulmonary septum:a)Persistent truncus arteriosus: This is due to failure of development of the spiral aortico-pulmonary septum. The truncus overrides the inter-ventricular septum and receives blood from both ventricles. b)Tetralogy of Fallot : characterized by four classic malformations :c)Pulmonary stenosis, overriding aorta, inter-ventricular septal defect and right ventricular hypertrophy. d)Congenital aortic valve stenosis : this is due to fusion of the cusps of the aortic valve leading to a very narrow aortic orifice. The left ventricle is markedly hypertrophied. e)Aortic valve atresia : The aortic orifice is completely closed. The left ventricle is under-developed and the ascending aorta is narrow. The ductus arteriosus is patent to carry blood into the aorta. f)Pulmonary valve stenosis and atresia: The pulmonary trunk is narrow and the right ventricle is under-developed. The ductus arteriosus remains patent and carries blood in an opposite direction from the arch of aorta to the pulmonary arteries. The foramen ovale remains patent. g)Transposition of ascending aorta and pulmonary trunk: This is due to a reversal development of the spiral aortico-pulmonary septum.

Congenital anomalies of the heart

5. Abnormal positions of the heart:

a)Isolated dextro-cardia : the heart is abnormally positioned on the right side of

the thorax. It is usually associated with other severe cardiac anomalies.

b) Dextro-cardia with situs inversus: is dextro-cardia with inversion of the

viscera.

Thank You

Dr. Fiky Thorax