By Dr Robin Thomas

By Dr Robin Thomas

The vascular system appears in the middle of 3rd week , when embryo is no longer able to satisfy its nutritional requirement .

The Heart develops from splanchnopleuricmesoderm related - intraembryonic coelomthat forms pericardial cavity. This mesoderm called Cardiogenic area.

Cardiac development involves :

1.Establishment of cardiogenic field

2.Formation & position of heart tube

3.Cardiac looping

4.Cardiac septation

Cardiac progenitor cells lie in the epiblast , lateral to primitive streak.

Migrate towards primitive streak

Cells destined to form cranial segments of heart , outflow tract, migrate first & cells forming more caudal portions , right ventricle,

left ventricle & sinus venosus migrate in sequential order.

Cells procede towards cranium & position themselves rostral to buccopharyngealmembrane & neural folds- reside in splanchniclayer of lateral plate mesoderm.

Cardiac progenitor cells lies – epiblast ,lateral to primitive streak. Cells procede towards cranium –rostral to buccopharyngealmembrane & neural folds. Reside in splanchnic layer of lateral plate mesoderm.

Blood islands appear in this mesoderm –form bloodcells & vessels –vasculogenesis.

Blood islands unite & form horseshoe shaped endothelial lined tube surrounded by myoblasts- cardiogenic field – intraembryoniccavity over it develops into pericardial cavity.

Initially central portion of cardiogenic area is anterior to buccopharyngeal membrane & neural plate.

Closure of neural tube & formation of brain vesicles – central nervous system grows cephalad

Buccopharyngeal membrane is pulled forward –heart & pericardial cavity move first to cervical region & finally to thorax.

Embryo fold cephalocaudally & laterally- caudal region of paired cardiac primordia merge.

18 Days embryo 20 Days embryo

Central portion- cardiogenic area anterior to buccopharyngeal membrane & neural plate.

21Days embryo 22 Days embryo

growth of brain & buccopharyngeal memb

cephalic folding of pulled forward –heart &

embryo. pericardial cavity moves

first to cervical region-

thorax.

22 day embryo

Embryo folds cephalocaudally & laterally-caudal region of paired cardiac primordia merge-

Heart –continuous expanded tube with inner endothelial lining & outer myocardial layer. Pump blood –first aortic arch –dorsal aorta.

Heart - continuous expanded tube – inner endothelial lining & outer myocardial layer.

Receives venous drainage at caudal pole - pump blood - first aortic arch into dorsal aorta .

Heart tube - attached to pericardial cavity by dorsal mesocardium.

Dorsal mesocardium disappears-creates transverse pericardial sinus-connects both sides of pericardial cavity.

Myocardium thickens & secrete thick layer of extracellular matrix – rich in hyaluronic acid –separates from endothelium.

Mesothelial cells on surface of septum transversum form proepicardium-later form epicardium.

Heart tube consists of 3 layers

Endocardium-internal endothelial lining

Myocardium-muscular wall Epicardium-outside of heart tube

Right & left endothelial heart tubes.

Fusion of heart tubes

from cranial to caudal

ends.

Arterial end of heart tube Venous end of

heart tube

Single endothelial heart tube shows series of dilatations.

1.Bulbus cordis

2.ventricle

3.atrium

4.sinus venosus

Ventricle & atrium are connected by atrioventricular canal.

Sinus venosus has prolongations - right & left horns.

Bulbus cordis lies at arterial end of heart tube–divisible into proximal part - conus & distal part - truncus arteriosus.

Truncus arteriosus continuous distally with aortic sac-continuous with right & left pharyngeal arch arteries-continuous with right & left dorsal aorta.

Sinus venosus lies at venous end of heart-has right & left horns.

One vitelline vein (yolk sac), umbilical vein (placenta), common cardinal vein (bodywall) join each horn of sinus venosus.

Heart tube elongate & bend on day 21.

Cephalic portion bend ventrally,caudally & to right.

Atrial portion shift dorsocranially & to left.

Bending creates cardiac loop-complete by day 28.

Cephalic portion heart tube bend ventrally, caudally & to right.

Atrial portion shift dorso-cranially & to left. Creates Cardiac loop-complete by day 28.

Dextrocardia-heart loops to left instead of right.

Heart lies on right side of thorax instead of left.

May coincide with situs inversus-complete reversal of asymmetry in all organs.

Truncus arteriosus form ascending aorta & pulmonary trunk.

Conus absorbed into ventricle & form chamber –partitioned into right & left ventricles.

Primitive atrium partitioned into right & left atria.

Sinus venosus absorbed into right atrium.

Right common cardinal vein forms superior vena cava.

Umbilical vein disappears.

Right vitelline vein forms part of inferior vena cava.

Left horn of sinus venosus retrogresses-form part of coronary sinus.

Sinus venosus & primitive atrial chamber are connected by a wide opening

Opening becomes narrow-shifts to right-narrow slit-right & left venous valves.

Cranially 2 valves fuse to form septum spurium.

Atrioventricular canal divides into right & left halves.

2 thickenings-atrioventricular cushions appear on dorsal & ventral walls.

Atrio ventricular cushions grow towards each other & fuse –form septum intermedium.

Begins at day 26-ingrowth of large tissue masses.

Endocardial cushions at atrio ventricular & conotruncal junctions.

Endocardial cells de differentiate & form part of atrio- ventricular valves.

Complete septation of atrioventricular canal –fusion of endocardial cushions.

Most of atrioventricular valve tissue –derived from ventricular myocardium –undermining of ventricular walls.

Physical separation of 2 valves (mitral & tricuspid valve) produce atrioventricularseptum.

Absence causes-Atrio ventricular canal defects. (Endocardial cushion defects)

1.Complete endocardial cushion defects

Ostium primum ASD, VSD in interventricularseptum, clefts in anterior mitral valve & septalleaflet of tricuspid valve.

2.Partial endocardial cushion defects

Ostium primum ASD, clefts in mitral & tricuspid valve.

Septation of atria begins at day 30 day .

Atrial chamber undergoes division into right & left halves-formation of 2 septa.

Septum primum arises from roof of atrium , to right of septum spurium.

Septum primum grows downwards & fuses with atrioventricular cushions.

Upper part of septum primum degenerates-form foramen secundum.

Septum secundum formed to right of septum primum.

Septum secundum grows downwards from roof of atrial chamber-overlapps free edge of septum primum.

Blood now flows from left to right through oblique cleft b/w 2 septa-foramen ovale.

During fetal life oxygenated blood reaches right atrium from placenta.

Blood reaches left atrium tru foramen ovale.

After birth left atrium receives oxygenated blood from lungs- foramen ovale obliterated by fusion of septum primum & septum secundum.

Annulus ovalis –septum secundum.

Fossa ovalis-septum primum.

Inter atrial septal defects

1.Septum primum defect

2.Septum secundum defect

3.Patent foramen ovale

4.Premature closure of foramen ovale

Right atrium derived from :

1.Right half of primitive atrium-main part.

2.Sinus venosus.

3.Right half of Atrio ventricular canal.

SINUS VENOSUS

Left horn of sinus venosus –part of coronary sinus.

Right common cardinal vein becomes part of superior vena cava.

Right vitelline vein forms terminal part of inferior vena cava.

Right venous valve (right margin of sinu atrialorifice) forms crista terminalis, valve of IVC,

& valve of coronary sinus.

Left atrium is derived from :1.Primitive atrial chamber-left half2.Atrioventricular canal-left half3.Pulmonary veins -absorbed proximal parts

Single pulmonary vein opens into left half of atrium –when septum primum just beginning to form.

Pulmonary vein bifurcates-right & left branches-bifurcates-drain corresp. lung bud.

Parts of pulmonary vein nearest to left atrium –absorbed -4 pulmonary veins

Fate of Bulbus Cordis

Bulbus Cordis –divisible into conus (proximal part) & truncus arteriosus (distal part).

Spiral septum –union of right superior & left inferior truncus swelling or cushions.

Spiral septum appears within truncusarteriosus & subdivides into ascending aorta & pulmonary trunk.

Conus (bulbus cordis proper) merges with cavity of primitive ventricle .

Right & left ventricle formed –partition of this chamber.

Bulboventricular cavity consists of :

Dilated lower part –communicates with atria.

Conical upper part-communicates with truncus arteriosus.

Septation of ventricles begins at day 25.

Interventricular septum grows upwards from floor of bulboventricular cavity-divides lower dilated part into right & left halves.

Interventricular septum fuses with fused atrioventricular cushions.

Right & left bulbar ridges arise from conical part of bulboventricular cavity & fuse to form bulbar septum.

Bulbar septum grows towards interventricularseptum , doesnot reach it –gap left b/w two.

Gap b/w upper edge of interventricularseptum & lower edge of bulbar septum –filled by proliferation of tissue from atrioventricularcushions.

Inter ventricular septal defects

1.VSD

2.Tetralogy of Fallot

3.Persistant truncus arteriosus

4.Transposition of Great arteries

Mitral & tricuspid valves formed –proliferation of connective tissue under endocardium of right & left atrioventricular canals.

Pulmonary & aortic valves –endocardialcushions formed at junction of truncus -arteriosus & conus.

3rd week of life ( 15-21 days) 1.Splanchnopleuric mesoderm-18 days.

2.Cavitation of mesoderm-intraembryoniccoelom-pericardial, pleural, peritoneal cavities-18 days.

3.Endocardial heart tube-20 days.

4.Cardiac looping-21days.

4 th week of life ( 22-28 days )

1.Dvp. of left ventricle & right ventricle.

2.Circulation commences.

3.Cardiovascular septation.

4.Aortic arches begins.

5 th week of life ( 29-35 days )

1.LV,RV,ventricular septum continue to grow.

2.Approximation of aorta to interventricularforamen, mitral valve,LV.

3.Separation of ascending aorta,mainpulmonary artery.

4.Separation of mitral & tricuspid valve.

5.Tricuspid valve opens into right ventricle.

6.Ostium primum closed by endocardialcushions.

6th & 7th week of life ( 36-49 days )

1.Closure of conal septum

2.Closure of membraneous part of ventricular septum 38-45 days

Blood vessel development by 2 mechanisms.

1.Vasculogenesis-vessels arise by coalesenceof angioblasts.

2.Angiogenesis-vessels sprout from existing vessels.

Dorsal aorta & cardinal veins –vasculogenesis.

VEGF-vascular endothelial growth factor.

Pharyngeal arches develop during 4 th & 5th

weeks of development.

First arteries to appear in embryo-right & left primitive aortae.

Continuous with 2 endocardial heart tubes.

Each primitive aorta-ventral aorta (lying ventral to foregut), arched portion & dorsal aorta (lying dorsal to gut).

Fusion of 2 endocardial heart tubes-2 ventral aorta partially fuse to form aortic sac.

Unfused parts remain as right & left horns of sac.

Successive arterial arches appear in 2-6 pharyngeal arches, each connected ventrally to right or left horn of aortic sac & dorsally to dorsal aorta.

Greater part of 1st & 2nd arch arteries disappear ( 29 day embryo )

Small portion of 1st arch artery persists to form maxillary artery.

Second arch artery form hyoid & stapedialartery.

Fifth arch artery also disappears.

Aortic sac now connected with arteries of 3,4 & 6th arches.

3rd & 4th arteries open into ventral part & 6th

arch arteries into dorsal part of aortic sac.

Spiral septum formed in truncus arteriosus,extends into aortic sac –fuses with posterior wall-blood from pulmonary trunk passes into 6th arch artery –blood from ascending aorta passes into 3rd & 4th arch arteries.

Dorsal aorta b/w 3rd & 4th arches disappear on both sides.

Each 6th arch artery gives off artery to developing lung bud.

Right side-portion of 6th arch artery b/w lung bud & dorsal aorta disappears.

Left side-remains patent & forms ductusarteriosus.

Each 3rd arch artery gives off a bud –grows cranially & form external carotid artery.

Dorsal aorta gives off series of lateral intersegmental branches to body wall.

7th cervical intersegmental artery forms left subclavian artery.

Arch Arterial derivative

1 arch Maxillary artery

2 arch Hyoid & stapedial artery

3 arch Common carotid & 1st

part of internal carotid artery.

Left side of 4th arch Arch of aorta ( from left common carotid to left subclavian arteries ).

Right side of 4th arch Right subclavian artery (proximal portion)

Left side of 6th arch Left pulmonary artery & ductus arteriosus

Right side of 6th arch Right pulmonary artery

Ascending aorta & pulmonary trunk –formed from truncus arteriosus.

Arch of aorta –from aortic sac, its left horn, left 4th arch artery.

Descending aorta-from left dorsal aorta, partly from fused medial vessel.

Brachiocephalic artery from right horn of aortic sac.

Common carotid artery from part of 3rd arch artery.

Pulmonary artery from 6th arch artery.

Arteries to the gut-from ventral splanchnicbranches of dorsal aorta.

Renal,supra renal & gonadal arteries –from lateral splanchnic branches of dorsal aorta.

Left subclavian artery –part of 7th cervical intersegmental artery. Right side partly from 7th cervical intersegmental artery & right 4th

arch artery.

Portal vein from right & left vitelline veins & anastomosis b/w them.

Superior vena cava from part of right anterior cardinal vein & right common cardinal vein.

Inferior vena cava from right posterior cardinal vein, right subcardinal vein, right supra cardinal vein, right hepato cardiac channel.

Heart develops from splanchnopleuricmesoderm-cardiogenic area-3rd week.

1.Establishment of cardiogenic field2.Formation & position of heart tube3.Cardiac looping4.Cardiac septation5.Aortic arch dvp.

Heart begins to beat by day 22.

Cardiogenesis begins on 18th day of life & normally completed by 45th day of life with formation of membraneous part of interventricular septum.

1.Kliegman, Stanton, Geme ST, Schor, Behrman. Nelson Textbook of Pediatrics 19th edition 2012 :1527-1529.

2.Sadler WT. Langmans Medical Embryology 10th edition 2012 :159-189.

3.John FK, James EL, Donald CF Nadas Pediatric Cardiology 2nd edition :14-25

4.Myung K Park .Pediatric cardiology for Practitioners 2012 5th edition 120-180

5.Inderbir singh, Pal PK. Human Embryology 8th edition 2012 :191-231