Development of the Heart and Cardiovascular System

II Septation of the Heart

 

 

Professor Alfred Cuschieri, Department of Anatomy, University of Malta

 

 

 

Septation of the atrioventricular canal - Septum Intermedium.

26 days :

Superior and inferior endocardial cushions arise from the superior and inferior walls of the atrio-ventricular canal. They are formed of sub-endothelial thickenings of cardiac jelly.

35 days: The AV cushions fuse separating right and left AV openings. The fused AV cushions constitute the septum intermedium

 

 

Development of the atrioventricular valves

 

The mitral and tricuspid atrioventricular valves form between the 5th and 8th weeks. They valve cusps and their chordae tendinae are formed by undermining of the ventricular myocardium.

 

Development of the Atrial Septum

 

Two atrial septa are formed, both of which contribute to the definitive atrial septum. They are associated with two inter-atrial communications (ostia).

 

1.      The septum primum begins to develop at 28 days. It is a thin, crescentic fold of endocardium that arises craniodorsally and grows down to the AV cushions, leaving an ostium primum below its free edge. It fuses with the AV cushions at approx. 35 days, obliterating the ostium primum.

 

 

 

2.      The ostium secundum is an opening in the upper part of the septum primum. It forms at about 33 days i.e. before the ostium primum closes. It forms by apoptosis (programmed cell death) as a number of small perforations that coalesce.

 

 

 

3.      The septum secundum begins to develop at about 33 days. It is a thick muscular septum that arises to the right of the septum primum in the intersepto-valvular space (between the septum primum and the left venous valve of the SA opening). It grows from the roof of the atrium but never reaches the AV cushion forming the fossa ovalis.

 

 

The final atrial septum is formed from both septum primum and septum secundum:

 

 

 

 

 

The final atrial septum is formed from both septum primum and septum secundum:

 

 

 

1.      The muscular part of the atrial septum is derived from the septum secundum fused with the septum primum

2.      The ostium secundum is covered by the septum secundum

3.      The limbus fossae ovalis is the free border of the septum secundum

4.      The floor of the fossa ovalis is formed of septum primum - it is thin and membranous and forms the flap valve mechanism

 

 

 

 

 

 

 

 

 

 

The Definitive Atria

 


The original sinu-atrial opening communicates entirely with the right atrium. The SA opening dilates greatly, and the right horn of the sinus venosus is absorbed into the right atrium.

 


The definitive right atrium is formed from two parts:

a)     the muscular part derived from the embryonic atrium - this part has musculi pectinati;

b)     the smooth part derived from the sinus venosus (also called the sinus venarum part).

c)      The smooth part of the right atrium receives three openings (I) the superior vena cava; (ii) the inferior vena cava and (iii)the opening of the coronary sinus.

d)     The crista terminalis, separating the smooth and muscular parts, and the valves of the inferior vena cava and coronary sinus are derived from the right venous valve. They form one continuous curved line.

 

 

The definitive left atrium receives no contribution from the sinus venosus. Right and left pulmonary veins establish communication with the left atrium. The left atrium also consists of two parts:

a)     The smooth part is derived from the pulmonary veins that have been resorbed into the the left atrium till the level of their division. Thus this part receives the openings of the four pulmonary veins.

b)     The muscular part of the left atrium is derived from the left half of the embryonic atrium

 

Formation of the ventricular septum

 


Initially, the ventricle communicates with the atrium and the bulbus cordis communicates with the truncus arteriosus.

After folding of the heart at 28 days, the bulbus cordis and truncus arteriosus are situated to the right of the ventricle. The part of the bulbus cordis that tapers to merge with the truncus arteriosus is the conus cordis. The conus cordis and truncus arteriosus together form the outflow tract or cono-truncus.

 

The ventricle and the bulbus cordis merge into one big chamber. This has the AV openings (inflow) to the left and the conotruncus (outlfow ) to the right. Two important rearrangement that occurs at this stage are the realignment of the atrioventricular openings and the cono-truncus to the middle of the common bulbo-ventricular cavity. This is essential for correct septation of the ventricle.

 

The muscular ventricular septum grows from the bulbo-ventricular sulcus and is directed dorsally and to the right towards the atrioventricular cushions but does not fuse with them. Growth of the ventricular septum is arrested at the seventh week, leaving a communication between the right and left ventricles. This gap is closed during the eighth week by growth from endocardial tissue, which forms the membranous part of the ventricular septum.

 

 

 

The right ventricle is derived mainly from the bulbus cordis whereas the left ventricle is derived mainly from the embryonic ventricle.

 

 

Septation of the cono-truncus

A pair of bulbar ridges (also called conotruncal ridges) arises from opposite sides of the cono-truncus. They approach one another and fuse in the midline to form the spiral aortico-pulmonary septum, separating the aorta and pulmonary trunk. The ridges are spirally oriented, and the relative positions of the aorta and pulmonary trunk are also spirally arranged.

The development of the bulbar ridges begins at the lower end of the truncus arteriosus (level 3 in Figure) and extends cranially into the truncus and caudally into the conus. The uppermost part of the septum fuses with the dorsal wall of the truncus just beyond the origin of the 6th aortic arch.

The spiral extension of the bulbar ridges downwards into the conus forms the membranous part of the the ventricular septum, together with a contribution from the AV cushions. This downward extension continues the spiral and brings the aortico-pulmonary septum in line with the ventricular septum.

 

 

Development of Aortic and pulmonary valves

 

These develop at the lower end of the truncus arteriosus. At this level there are four swellings of sub-endocardial tissue - the right and left bulbar swellings and two acessory dorsal and ventral swellings. Separation of the fused bulbar ridges forms the aortic and pulmonary vessels each containing three swellings. Growth and excavation of the swellings results in the formation of the semilunar valves. Formation of the semilunar valves is complete by the end of the 9th week.

Note the positions of the valves as in adult anatomy.

The aorta has one posterior valve and two anterior valves, above which the right and left coronary arteries arise. The pulmonary trunk has one anterior and two posterior valves.

 

 

Development of the conducting system of the heart

 

Contraction of the heart by myogenic activity begins at about 28 days. The conducting system of the heart (SA node, AV node, bundle of His and Purkinje fibres) consist of specialized cardiac muscle cells. The SA node is thought to be derived from neural crest cells and is initially situated in the wall of the sinus venous. The rest of the conduction system is thought to be derived from cardiogenic mesoderm.