Late development of heart

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					Late Development of Heart
By Dr. Muhammad Rafique Assistant Professor Anatomy Department 25/01/2008

Objectives
• • • • • • • Different division of Heart Tube Rotation of Heart Tube Dorsal Mesocardium Partition of the Atrioventricular Canal Atrioventricular Canal Partition of the Atrium Sinus Venosus and Formation of the Definitive Atrium • Development of Right atrium • Semilunar Valves • Atrioventricular Valves

Different division of Heart Tube
The heart elongates as the embryo grows, and it acquires dilatations and constrictions. When fusion is completed during the fourth week, the heart has five distinct regions. These regional divisions, in the order followed by circulating blood, are the sinus venosus, atrium, ventricle, bulbus cordis, and truncus arteriosus. (The great veins of the heart, the superior vena cava and inferior vena cava, develop from the venous end of the primitive heart tube; the trunks arteriosus carries blood to the general circulation.)

Different division of Heart Tube
The bulbus cordis and ventricle grow more rapidly than the other regions, and the heart grows more rapidly than its superior and inferior attachments. Thus, the originally straight endocardial heart tube forms a U-shaped bulboventricular loop. The heart tube later curves back upon itself, forming an S-curve that gradually becomes more pronounced. The flexures of the heart reorient the regions so that the atrium and sinus venosus eventually lie superior to the bulbus cordis, ventricle, and truncus arteriosus.

Rotation of Heart Tube
The head fold causes the pericardial cavity and the endocardial tube to rotate approximately 180º so that they lie ventral to the foregut and caudal to the oropharyngeal membrane. This brings the septum transversum to a position between the pericardial cavity and yolk stalk. The heart tube then bulges dorsally into the pericardial cavity.

Dorsal Mesocardium
The heart tube sinks further into the pericardial cavity and becomes dorsomedially suspended by the dorsal mesocardium, formed from the right and left epimyocardial layers. The dorsal mesocardium later disappears, except at its cephalic and caudal ends. Circulation of blood starts by the beginning of the third week when the heart begins to contract. Peristaltic waves of contraction begin in the sinus venosus and force blood through the tubular heart.

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.

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.

Sinus Venosus and Formation of the Definitive Atrium
The sinus venosus consists of a small transverse portion and right and left sinus horns. The sinus venosus develops from the common cardinal, umbilical, and vitelline veins. The valves of the inferior vena cava (right and left sinal valves) guard the orifice between the sinus venosus and the right atrium.

Development of Right atrium
The sinus venosus incorporates into the dorsal heart wall. The right sinus horn and veins enlarge greatly and become the only communication between the original sinus venosus and the atrium. The right auricle and the rough portion of the right atrium that contain pectinate muscles represent illustrate the transformation of the bulbus cordis. Ao. Truncus arteriosus. Au. Atrium. B. remnants of the original Bulbus cordis. RV. Right ventricle. LV. Left embryonic right atrium. ventricle. P. Pulmonary artery.

Development of Right atrium
The original embryonic left atrium becomes the trabeculated left auricle. The smooth left atrium develops from the primitive pulmonary vein as its main branches incorporate into the wall. The left horn of the sinus venosus and the proximal part of the left common cardinal vein become the coronary sinus and oblique vein of the left atrium.

Semilunar Valves
Two very short pairs of tubercles alternate with the main truncus swellings in the portion of the bulbus cordis that contributes to the truncus arteriosus. The pulmonary and aortic channels each have one pair. A third swelling appears in both channels opposite the fused truncus swellings. These three guard the orifices of both the aorta and pulmonary artery. Gradually, they canalize to form the three cusps of the semilunar valves.

Atrioventricular Valves
The tricuspid and bicuspid valves arise by proliferation of subendocardial connective tissue around the atrioventricular openings and from the fused endocardial cushions. The proliferations canalize on their ventricular sides, but the newly formed valves remain connected to the trabeculae carnae of the ventricular wall by chordae tendineae attached to papillary muscles.


				
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Description: this presentation includes the further develoment of heart with formation rotation of heart tube and development of atria and ventricles