Cardiovascular System

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General

• 1st functional organ system in the embryo
• Cardiac pathology during development is either genetic, enviornmental or combination of both
• Cardiac structures arise initially from mesoderm which proliferates into angiogenic cell clusters. These intraembryonic clusters multiply and coalese to form a plexus with a lumen (= the embryonic cardiac tube)
• The extraembryonic cell clusters form the vitelline & umbilical vesels which by budding contact the heart tube

Development of the Heart

(Fig. 1)

• Mechamisms that initiate formation of the heart are not well understood; however the heart tube position is initially determined by brain growth
• Formation of the neural tube by folding causes fusion of the heart tube in the midline, to become a single endocardial tube
• The primitive heart tube buldges into the pericardial cavity where it is surrounded by splanchnopluric meso-derm. Resulting heart tube consists of 4 layers: the endocardium, the cardiac jelly (= endocardial cushions) the myocardium, and the epicardium
• At this stage embryo is 23 days old and the heart begins to beat from a focus in the sinus venosus

Formation of the Heart Loop

(Fig. 2)

1. Intrapericardial = bulboventricular portion of heart
2. Extrapericardial = atrium/sinus venous + Ao roots

• During the next stage of development, the bulboventricular portion of the heart forms a loop to the RIGHT, forming a flange (= the bulboventircular flange)
• Region proximal to flange expands to become the embryonic ventricles, while the distal portion becomes the bulbus cordis
• Mechanism of cardiac looping not well understood

Formation of the Embryonic Ventricles

(Fig. 3)

truncus arteriosus

• After looping, the cardiac tube develops local expansions which are precursors to the cardiac chambers

conus cordis

• The bulbus cardis develops three portions:

Proximal portion = trabeculated RV Midportion (conus cordis) = outflow tract for RV/LV Distal portion (truncus arteriosus) = Ao root + PA primitive right atrium primitive right ventricle

• The primitive ventricle becomes trabeculated and forms the major portion of the LV. Both ventricular chambers dialate as CO increases

bulboventricular fange

Development of the Sinus Venosis

(Fig. 4)

• The sinus venous consists of three parts:

Central unpaired portion proximal to the atrium Transverse portion R/L sinus horns: receive blood from the vitelline, umbilical and common cardinal veins bulbus cordis F1G 4. left sinus harn L sinus horn becomes the coronary sinus R sinus horn w/ associated vessels enlarges and is incorporated into the RA common cardinal vein sinus horns

Formation of the Cardiac Septa

umbilical vein vilelline vein left ventricle vena cava right ventricle

• Major septa in the heart are formed between the 27th & 37th day; these chambers

involve either active fusion of cushions or passive expansion of the cardiac ATRIAL SEPTATION & GROWTH (Fig. 5)

• Septation begins as a crest of tissue (septum primum growing downward from the roof of the common atrium towards the endocardial cushions at the AV canal
• Multiple perforations appear in the septum primum which coalese to form the ostium secundum
• The septum secundum forms to the R of the septum primum where the R sinus horn incorporates into the RA and also grows downward towards the cushions

• RA expands by incorporation of the sinus venous into the atrial body.

Embryonic RA = R atrial appendage

• LA expands as the common pulmonary vein is incorporated into the posterior wall. The common PV branches and connects to the splanchnic plexus of the (FIct o. lung buds.

common atrio- Embryonic LA = L atrial appendage (aka the RGW appendage) ventricular canal cost. endocardial. Cushion

THE ATRIOVENTRICULAR CANAL

(Fig. 6)

• AV canal septation occurs by growth of the endocardial cushions. The anterior (superior) and posterior (inferior) AV cushion extend into the

ant endocardial cushion lumen of the canal, while the lateral cushions form the R/L borders of the canal. Fusion in the midline divides the canal into the R/L AV orifices

• AV cushions also arch and fuse w/ the septum primum to close the foramen primum w/ the following result:

Cushion L of septum primum = Ao leaflet of MV Cushion R of septum primum = septal leaflet of TV + crest of IVS

MUSCULAR INTRAVENTRICULAR SEPTUM FORMATION

(Fig. 7)

• Muscular IVS formed by two processes:

Medial walls of the expanding RV/LV oppose and fuse together Condensation of trabeculae in ventricular walls

• Primary IV foramen between RV/LV never closes and forms LV outflow tract

CONOTRUNCAL SEPTATION

(Fig. 8)

• Truncus arteriosus of the embryonic heart is divided by conotruncal cushions

sentum primum anta

• Concurrently, similiar cushions occur in the conus cordis and fuse both w/ each other & w/ the truncal cushions.

With fusion the septum divides the conus into an anterio-lateral portion which w/ primitive RV form the definitive RV. The posteriomedial portion becomes continuous w/ the primitive LV to become the definitive LV

• Spiriling of the outflow tracts follows cushion growth & fusion. The superior cushion grows L-ward/inferior cushion grows R-ward as well as both growing downward causing the twisting of the Ao/PA ounflow tracts, by a mechanism that is not well understood
• The IV foramen becomes reduced in size w/ completion of the conus septum by growth of tissue inferiorly from the septum which fuses w/ the inferior endocardial cushion to the top of the muscular septum
• The membranous IVS forms after completion of septation. Part of the EC cushion located between the septum primum & ventricular septum thins and forms the membranous septum

FORMATION OF THE AV VALVES

• AV valves are derived from EC cushion, endothelium, and ventricular tissue.

A skirt of ventricular tissue covered by atrial mesenchyme is found at each AV orifice w/ attachments to the ventricular wall (=cordae tendineae)

• Mitral valve initially has 4 cusps; 2 of the cusps grow larger and their papillary muscles fuse in pairs to form anterior/posterior papillary muscles. The 2 smaller cusps become commissural cusps
• Tricuspid valve forms from EC cushion, conus septum (= the conal papillary muscle + medial portion of the anterior cusp) and ventricular muscle

FORMATION OF THE SEMILUNAR VALVES

• Semilunar valve form at the interface of the truncal cushions and the Ao/PA septum. Initially seen as paired tubercles in the septated truncus, a 3rd tubercle appears & by hollowing of the upper surface the Ao/PA valves form

FORMATION OF THE ARTERIAL SYSTEM

• Initial arterial system = blood islands that coalese to form vessels. Ist

major embryonic vessels = dorsal aortae which form along the axis of the embryo and are contigeous w/ the heart tube

• Cranial portion of dorsal Ao = mandibular Ao arches of which the dialated

junction is the Ao sac. This sac contributes to the formation of the 6 paired Ao arches

SELECTION OF AORTIC ARCHES

(Fig. 9)

• 1st Arch = Regresses except small portion - maxillary artery
• 2nd Arch = Regresses
• 3rd Arch = forms the common carotid arteries + lst part of

internal carotid artery;? gives portion to the external carotid

• 4th Arch = L side becomes Ao arch between L common carotid

and L subclavian R side becomes proximal R subclavian

• 5th Arch = Regresses
• 6th Arch = Proximal part forms bilateral branches which form

proximal RPA/LPA L distal portion persists as the ductus arteriosus

• R dorsal Ao regresses leaving the L dorsal Ao as the descending aorta

Formation of the Venous System

(Fig. 10)

• 3 pair of veins form the major veins in the embryo:

Cardinal veins = drain the body of the embryo w/ the anterior/posterior veins entering the sinus horns Vitelline vein = drain yolk sac and enter the sinus venosus. Forms a plexus w/ the liver and around the duodenum which ultimately forms the portal vessel Unbilical veins = initially paired structures on both both sides of the liver.Both veins regresses except for L umbilical vein which becomes the ductus venosus

VENA CAVAE

• Formed from the cardinal & subcardinal veins which together enlarge to form the hepatocardinal channel. After anastimosis w/ renal veins and regression of multiple branches, remaining

portion = IVC

• SVC is formed by the junction of the R common cardinal vein & the proximal portion of the R anterio cardinal vein

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