Development of the Heart in a few lines.

“No flow, No grow.”

Background• The earliest steps

– Three germ layers—ectoderm, endoderm, and mesoderm• Cardiogenic field on two sides

– Mesenchymal cell at anterior part• Cardiogenic field meet in midline to form cardic • Cardiogenic fields can be subdivided into two groups

– First heart field[Anterior)-future RT ventricle– Second heart field[Posterior] –future left ventricle

• The two sides of the cardiac crescent fuse along the midline to form the primitive heart tube

• The primitive heart tube can itself be subdivided into regions along the caudal to rostral axis

– Sinus venosus– Primitive atria– Primitive ventricle– Bulbus cordis (conus)– Truncus arteriosus

• Heart beat at 5 weeks’ gestation

Earliest DevelopmentCardiovascular system makes its first appearance while the

embryo is still flat. Clusters of mesodermal cells specialise to form blood cells. Mesodermal cells around these flatten to form endothelium of blood vessels. These clusters are called blood islands of angiogenic (“blood vessel-forming”) cell clusters.

In the accompanying diagram note that these form a curve reaching well beyond the neural plate and the notochord. A mass of mesoderm, called cardiogenic area, near the head end (H) will give rise to the heart.

The sagittal section below illustrates the three germ layers, prochordal plate and the cardiogenic area.

H

Prochordal plate

Cardiogenic area

Head FoldWith the formation of the head fold (shown in the blue

circle), note how the cardiogenic area changes its position. Also observe that the endoderm (yellow) is beginning to form the gut tube. At this stage only the head and tail ends of the digestive tube are recognisable.

In the lowest picture, the gut tube is better seen and the heart is in fact in the form of a tube (red).

Heart Tube

HeartLiver

The Heart TubeIn the picture on the left the relationships of the

heart, the gut tube and the liver are clearer.In the magnified picture of the heart tube, the tail

end is the venous end and the cranial end is the arterial end. The changing shape of the tube also makes it possible to recognise the primitive chambers of the tube.

Remember that the tube is not partitioned at this stage.

Hereafter, for descriptive convenience, we shall view this tube in the vertical position, with the caudal (venous) end below and the cranial (arterial) end at the top as shown below.

The Tube Bends

This picture shows three successive stages in the growth of the tube. The tube, as it grows, cannot be accommodated within the pericardial cavity and undergoes bending.

The primitive chambers of the heart are recognisable, and are labelled in the last picture.SV – sinus venosus (receives veins from the body), A – atrium, V – ventricle. The ventricle

continues into the ‘bulbus cordis’which in turn leads to the arterial end.Two terms are used somewhat confusingly for the parts at the arterial end. These are

conus arteriosus and truncus arteriosus. In our discussion we shall simply say ‘arterial end’ of the heart.

SVA

V

BV D

Cardiac Looping

• Folds on itself and twists –looping

• Mechanism– Differential ballooning out of the chambers

• D Loop- the looping occurs to the right

• The first visible sign of left-right asymmetry

• Looping sets up the relationship between the inflow tract, the outflow tract, and the ventricular septum of the right ventricle

The Chambers

Recognise the chambers in these two views. In the view from the left side, the sinus venosus is partly hidden. Note that with the bending of the tube the atrium is now dorsal and the loop formed by the ventricle and the bulbus cordis (bulbo-ventricular loop) is ventral.

In the next slide we shall examine the interior of the unpartitioned heart.

A

A A

VB-V Loop

Left view Front view

The InteriorA portion of the ventral wall of the bulbo-

ventricular loop is removed to show the interior.

Since there is no partition, there is a single passage from the atrium to the ventricle. This passage is the atrioventricular canal. Note the direction of blood flow through the bulboventricular loop.

Also note that the single vessel leading out of the heart has given rise to what are called aortic arches.

RA LA

AVC

A-Ar

Left – Right Partitioning

• Interatrial septum• Interventricular septum• Spiral (aortico-pulomonary) septum• Endocardial cushions (A-V cushions)

• Functional requirements• There must always be a right to left passage!

Interatrial septum

• Partitioning

• Right to left passage

• Mechanism for closing the passage

A

V

Septum Primum

• This is a sagittal section seen from the right.

V

AVC

Foramen Primum

• Foramen primum :

Between

the septum and

the AV Cushions

Passage is a Must!

• Foramen secundum

• Foramen primum about to disappear

Septum Secundum

• To the right of primum

• Foramen primum has disappeared

Foramen ovale

• F. Ovale –• In septum secundum

• Further…

The ‘Valve’

• Two septa

• Two foramina

Sinus Venosus

• Originally a symmetrical structure

• Venous return more to the right

• Left horn becomes smaller

• Opening shifts to the right

• Later – part of right atrium

Left Atrium

• Four pulmonary veins

• Common opening

• “Absorption” of veins into atrium

• Rough part - auricle

The Ventricular Septum

Three Parts– Interventricular septum– AV Cushions– Spiral Septum

Ventricular septum• The primitive right ventricle is more anterior• The flow of blood comes into the left ventricle, then goes across the

bulboventricular foramen to the right ventricle and out the as-yet-undivided outflow tract. As development progresses, inflow becomes more directed toward both ventricles

• Failure of this process can result in a double-inflow left ventricle [DILV]—a situation much more common than double-inflow right ventricle

• The ventricular septum begins to grow toward the AV canal and outflow tract from the apical and inferior portion of the junction between the primitive right and left ventricle- muscular part of the interventricular septum

• Septation of the ventricle is complete when the muscular septum meets the canal septum between the AV valves and the conal septum just below the now separate outflow tracts

• The area at which these structures meet, there is the thinner membranous septum.

Ventricular Septum

R

Membranous

Muscular Spiral(Aorticopulmonary)

VEINS

Arch

Foetal Circulation

• Very little pulmonary flow

• Placental Circulation

• Right to Left Passages

Postnatal vs Foetal Circulation

• PostnatalBody RA RV Lungs LA LV Body

• FoetalBody RA RV Lungs LA LV Body

The basic difference between postnatal and foetal circulation is that foetal lungs are nonfunctional. Effectively, blood from the right side of the heart has nowhere to go and needs to be ‘shunted’ to the left. Such a shunting passage exists between the right and the left atria. However, if no blood flows through the right ventricle, that chamber will fail to develop. Thus some blood does pass to the RV. As it is pumped into the pulmonary artery, it needs to be shunted again, this time to the aorta. This illustrated below.

But we are jumping too far ahead! This was mentioned as one of the basic principles of the development of the heart…let us begin at the beginning.

• IVC : Blood from placenta– Ductus

venosus

• F. ovale

• Ductus arteriosus

Changes At Birth

• Closure of interatrial septum

• Closure of ductus arteriosus

• Closure of ductus venosus

Congenital Heart Disease

• Septal Defects – Atrial and Ventricular

• Endocardial cushion defects

• Aorticopulmonary defects

• PDA

• Others

Thank You