Partitioning of Heart

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					Partitioning of atrium & Ventricle
By Dr. Muhammad Rafique Assistant Professor Anatomy Department 30-01-2008

Formation of Atrioventricular Canal Division of Atrioventricular Canal Partitioning of Atrium Partitioning of Venticle

Partition of the Atrioventricular Canal
Partitioning of the primitive heart, with its single atrium and ventricle into the typical four-chambered structure occurs between the fourth and seventh weeks by formation of interatrial and interventricular septa. Many congenital heart problems develop during this crucial time.

Atrioventricular Canal
The single atrial cavity connected to ventricular cavity. Both atrium and ventricle are connected by means of single tube like, the common atrioventricular canal. Toward the end of the fourth week, dorsal and ventral endocardial cushions are developed in the walls of the common atrioventricular canal at junction of atrium and ventricle,

Atrioventricular Canal
These grow toward each other and, during the sixth week, meet and fuse, dividing the common atrioventricular canal into right (tricuspid) and left (mitral, or bicuspid) atrioventricular canals. Now single atrium connected to single ventricle by two right and left atrioventricular canals. Partitioning of both atrium and ventricle begin

Partition of the Atrium
The septum primum first appears during the fourth week as a partition in the dorsocephalic wall of the primitive atrium. During the fifth and sixth weeks, the septum primum grows rapidly toward the endocardial cushions, partially dividing the atrium, but leaving the foramen primum.

Partition of the Atrium
The foramen primum obliterates when the septum primum meets the fused endocardial cushions. As this contact occurs, a second opening, the foramen secundum, appears cranially in the septum primum. The foramen secundum rapidly enlarges, permitting most blood in the right atrium to pass to the left.

Development of Foramen Ovale.
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 interseptovalvular 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 foramen ovale.

Final Development of inter-atrial Septum
The Septum Secundum fused with the upper part of septum primum above the foramen secundum. The septum secundum still grow downward towards atrioventricular cushion but never fused with endocardial with atrioventricular cushion. The opening between septum secundum and atrioventricular cushion is called as Foramen Ovale.

Development of Foramen Ovale and Valve
The foramen ovale is guarded by the lower remaining portion of septum primum which as flap like valve which guard the foramen ovale. This valve maintained unidirectional flow of blood from right to left side in fetus.

Development of Right Atrium
The sinus venosus has shifted to the right as the proximal portions of the left vitelline and umbilical veins are obliterated by the liver. The right sinus venosus becomes incorporated into the right atrium forming the smooth portion of the right atrium. The primitive right atrium is seen in the adult as the rough portion (auricle) of the right atrium.

Definitive Right Atrium
The remainder of the left sinus horn is the coronary sinus and the oblique vein (of Marshall) in the adult heart. The definitive right atrium is formed from two parts: The muscular part derived from the embryonic atrium this part has musculi pectinati;

Definitive Right Atrium
The smooth part derived from the sinus venosus (also called the sinus venarum part). 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

Definitive Left Atrium
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: The smooth part is derived from the pulmonary veins that have been reabsorbed into the left atrium till the level of their division.

Definitive Left Atrium
Thus this part receives the openings of the four pulmonary veins. The muscular part of the left atrium is derived from the left half of the embryonic atrium

Separation of Ventricle
The muscular interventricular septum grows as a ridge of tissue from the caudal heart wall toward the fused Atrioventricular cushions. The remaining opening is the interventricular foramen. The Muscular part of Interventricular Septum but does not fused with Atrioventricular cushions.

Separation of Ventricle
An opening persists between Atrioventricular cushions and muscular part of Interventricular septum. This opening is closed by the Membranous Part of Atrioventricular Septum, which grows downwards from the Atrioventricular cushions. This portion of the I.V. septum is called the membranous part of the interventricular 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 bulboventricular cavity. This is essential for correct separation of the ventricle. The right ventricle is derived mainly from the bulbus cordis whereas the left ventricle is derived mainly from the embryonic ventricle.

Separation 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 aorticopulmonary 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.

Separation of the Cono-truncus
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 aorticopulmonary septum in line with the ventricular septum.

Spiral division of Cono-truncus result in unequal division of Cono-truncus, so that larger Aorta lies on Right side and smaller Pulmonary trunk lies on left side

Aortic Seminular Valve


Pulmonary seminular Valve

Conotruncal crease

Pulmonary Trunk

Development of Aortic and pulmonary valves
• These develop at the lower end of the truncus arteriosus. At this level there are four swellings of subendocardial 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.

Development of Aortic and pulmonary valves
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. 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.

Atrioventricular Valves
The atrioventricular valves develop as subendocardial and endocardial tissues and project into the AV canal. These bulges are excavated from the ventricular side and invaded by muscle. Eventually, all the muscle, except that remaining as papillary muscle disappear and three cusps of the right AV (tricuspid) valve, and two cusps of the left AV (mitral) valve remain as fibrous structures.

Atrioventricular Valves
The atrioventricular valves form during the fifth to eighth week of development. Initially, endocardial cushion tissue forms bulges at the atrioventricular junction. These bulges have the appearance of valves, and although such tissue may play an important role in the eventual formation of the atrioventricular valves, endocardial cushion tissues are not the precursors of the mitral and tricuspid 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.

Description: different stages of partitioning of heart into four chambers along with development of each chambr of in human embryo. Also division of cous cordis into great vessels.