LECTURE NOTES

Dr Nusrat Zareen

Associate professor Anatomy

MBBS; FCPS

Heart Development I, II, III

Learning Objectives:

1. Recall the source of heart. 2. Describe the development of the cardiogenic region

correlating the process of gastrulation with the changes that

occur to form a single heart tube in the thorax, and the role

that embryonic folding plays in this process. 3. Describe the looping and folding of the heart tube that gives

rise to the adult location of the developing heart chambers. 4. Describe the development of the inflow tract.

5. Describe the development of the atria and the interatrial septum.

6. Describe the development of the atrioventricular valves and the aortic and pulmonary valves.

7. Describe the development of the ventricles and the interventricular septum.

8. Describe the partitioning of the outflow tract and the contribution of neural crest cells to this process.

THE THREE LAYERS OF THE HEART

THE ENDOCARDIUM. (Mesodermal- splanchnic mesoderm)

THE MYOCARDIUM. (Mesodermal- splanchnic mesoderm)

THE EPICARDIUM. (Mesodermal)- it is derived from the mesothelial cells from the region of SINUS

VENOSUS migrating over the heart.

The 5 stages to heart development are:

1. Specification of cardiac precursor cells.

2. Migration of cardiac precursor cells and fusion of the primordia.

3. Heart looping .

4. Heart chamber formation.

5. Septation and valve formation

There are 02 types of cardiac precursors

� Splanchnic mesoderm from primitive streak. (Cardiogenic mesoderm at the

cranial end of embryonic disc.

� Out flow portions of the heart from NCC

The cardiogenic cells ingression from the primitive streak & migrate cranially rostral to the neural

plate to form a horse shoe shaped cardiogenic area.

Horse shoe shaped ---- CARDIOGENIC

FIELD

ROLE OF NEURAL CREST CELLS

The contribution of NCC: Migration and positioning

The cardiac neural crest migrating into the heart region is responsible for forming the entire

musculoconnective tissue wall of the large arteries emerging from the heart, the membranous portion

of the ventricular septum, and the septum between the aorta and pulmonary artery.

STAGES OF HEART TUBE FORMATION

The first sign of heart development is the formation of ENDOCARDIAL TUBES/cardiogenic cords.

These tubes fuse to form a single heart tube. This tube then loops and folds and remodel to

finalize the chambering of heart.

Notice the formation of the paired endocardial tubes, derived from the splanchnic mesoderm.

These tubes then fuse to form the single heart tube, suspended in the pericardial cavity. All

encircled.

THE THREE Histological LAYERS OF THE HEART, all mesodernal

THE ENDOCARDIUM. (Angioblastic Mesoderm derived from splanhnic mesoderm)

THE MYOCARDIUM.(Splanchnic Mesoderm)

THE EPICARDIUM. (Mesoderm derived from the mesothelial cells on the surface of septum trnsversum

migrating over the heart.)

What is cardiac jelly? How is it derive

As the heart develops, it

changes its relation to the

buccopharyngeal membrane.

What is this changing relation

and what is the cause of this?

The changed relation is that originally the heart is anterior / cranial to the membrane then it

gradually comes in____________________________ relation to the membrane, and sinks into

the thoracic cavity. (see the figure and fill in the blank.)

The causes of this changing relation are:

1. Closure of Neural tube.

2. Formation of brain vesicle

3. FOLDING OF THE EMBRYO

Buccopharyngeal / oropharyngeal membrane

SUMMARY OF EVENTS - DEVELOPMENT OF PRIMITIVE HEARTTHE

• The cardiogenic cells in EPIBAST (immediately lateral to Primitive streak) migrate

through the PRIMITIVE STREAK, proceed cranially and reside in SPLACHNIC layer of

Lateral Plate mesoderm.

• The Pharyngeal endoderm beneath the mesoderm induces them to become cardiac

myoblasts and vascoulognesis also starts.

• Paired Endothelial strands (THE ANGIOBLASTIC CORDS) form.

• These cords CANALIZE to form ENDOCARDIAL HEART TUBES (mid 3rd

wk)

• Paired ENDOCARDIAL HEART TUBES fuse to form TUBULAR HEART (late in the 3rd week)

• HEART BEGINS TO BEAT BY DAY 22- 23 (BEGINNING OF 4RTH WK)

Development time line of heart

Exercise: Summarize the events of heart formation

THE HEART TUBE

Looping of heart tube

The fused heart/ endocardial tube begin

to loop at day 23 and complete at day 28.

Observe:

• The cephalic portion moving ventrally, caudally and to the right.

And

• The caudal portion looping dorsally, cranially and to the left.

Cephalic end

Caudal end

WHAT FORMS FROM WHAT

EMBRYONIC DILATATION ADULT STRUCTURE

TRUNCUS ARTERIOSUS Aorta

Pulmonary trunk

BULBOUS CORDIS Smooth part of right ventricle

Smooth part of left ventricle

PRIMITIVE VENTRICLE Trabeculated part of right ventricle

Trabeculated part of left ventricle

PRIMITIVE ATRIUM Trabeculated part of right Atrium

Trabeculated part of left Atrium

SINUS VENOSUS Smooth part of right Atrium

Coronary vein

Oblique vein of left Atrium

Circulation route through the heart Tube.

Blood enters the “venous end” (sinus venosus) and leaves through the arterial

end (truncus arterious)

UNIDIRECTIIONAL BLOOD FLOW

Blood enters the Sinus venosus - common atrium -AV canal - Common Ventricle -

Bulbous cordis - Truncus arteriosus - Aortic sac - Aortic arches and into Dorsal

aorta.

Compare the primordia of looped heart and the

respective adult structures formed

After the looping of the heart tube, internal specifications start taking form. We

shall see the following:

� Fate of Sinus venosus.

� Septae formation / Partitioning of fused endocardial tube:

� Partitioning of common atrium.

� Partitioning of Atrioventricular canal.

� Partitioning of common ventricle.

� Partitioning of common out flow tract (truncus arteriosus)

The fate of sinus venosus- what forms from it? Observe

• Absorption of left

sided veins

• Formation of oblique

vein & coronary sinus

on left

• Incorporation of right

horn into the right

atrium.

Notice the sino artial junction and the veins of the right sinus horn opening into the right atrium

after incorporation of the right horn into it.

The atrioventricular canal septates (DIVIDES):

The common atrioventricular canal is wide communication between the undivided atrium and

ventricle. But due the formation of the endocardial cushions and their fusion (see figure above),

this common AV canal is divided into right and left AV canals. These canals will provide routes

of blood from right and left atria to right and left ventricles after their formations.

THE INTERATRIAL SEPTA - (Wall between the atria)

Following is the shape of the looped heart (tube). Notice the position of the atrium and the

ventricle. Both these chambers are undivided originally with an atrioventricular canal between

them. Later septa develop inside them dividing them into right and left chambers separated by

right and left AV Canals

Notice the position of the AV canal (ENDOCARDIAL CUSHIONS) and the position of septae in the

atrium and the ventricle respectively

The interatrial septum develops from the roof of the

atrium and grows towards the endocardial cushions in

the AV canal. The interventricular septum develops

from the floor of the ventricle and grows towards the

endocardial cushions in the AV canal.

Common atrium

Common ventricle

Position of the Av CANAL where

endocardial cushion form, dividing it into

right & left halves

The steps of interatrial septum formation:

1. During the 4th week, the septum primum grows from the roof of the primitive

atrium toward the endocardial cushions. The lower end does not completely descend to

the cushions leaving a space called osteum primum

2. Later the osteum primum closes off. Vacuoles develop In the septum primum which

coalesce to form the osteum secundum.

3. Another septum, the septum secundum grows from the roof of the primitive

atrium toward the endocardial cushions, curtaining off the osteum secundum. This

septum also leaves a gap – the foramen ovale.

4. The septum primum forms the valve for the foramen ovale which shunts blood from the right to

the left atrium.

5. This completes the formation of interatrial septa with a defect, the foramen ovale. At birth

pressure in the left atrium increases and the septum primum flaps close the foramen ovale. So

the development of interatrial septa completes at birth.

ASD: Atrial septum defect. (Patent foramen ovale)

Partition of the conotruncal region- Conus cordis and truncus arteriosus (the out

flow tract) & FORMATION OF AORTICO PULMONARY SEPTUM

• Neural crest cells migrate into the outflow tract populating the wall and

participating/inducing the formation of the septum.

• The first indication of a developing septum is the appearance of two ridges

projecting into the outflow tract from opposite sides. Curiously, these ridges

spiral in a counter-clockwise direction up the developing outflow tract.

1. Pair of ridges growing from the opposite walls of the outflow tract.

2. The two ridges grow toward each other spirally and eventually fuse resulting in the

formation conotruncal sptum (Aortico pulmonary septum) that divides

the tract into aorta and the pulmonary trunk.

The spiral pattern results in the apparent curve of the aorta up, over, and behind the

pulmonary trunk seen in the adult.

INTERVENTRICULAR SEPTUM FORMATION

As the interatrial septa are forming, the primitive ventricle is also partitioning.

There are 2 portion s of Intrerventricular septum;

� Membranous

� Muscular

1. The forming interventricular septum is initially a muscular partition defining an interventricular

foramen between its upper border and the fused endocardial cushions.

2. A portion of the muscular interventricular septum will eventually fuse to the endocardial

cushions contributing to the formation of a membranous portion of the interventricular

septum.

The muscular ridge- IV

septum

IV septum is contributed by:

• Endocardial cushions.

• Interventricular septum

• Aortico pulmonary septum

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FOR cardiac anomalies consult LANGMAN’S EMBRYOLOGY