Outline of the formation of the heart
Heart development begins from the formation of two endocardial tubes which merge to form the tubular heart, also called the primitive heart tube, that loops and septates into the four chambers and paired arterial trunks that form the adult, human heart. The heart is the first functional organ in vertebrate embryos, and in the human, beats spontaneously by week 4 of development.
The tubular heart quickly differentiates into the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus. The truncus arteriosus splits into the ascending aorta and pulmonary artery. The bulbus cordis forms part of the ventricles. The sinus venosus connects to the fetal circulation.
The heart tube elongates on the right side, looping and becoming the first visual sign of left-right asymmetry of the body. Septa form within the atria and ventricles to separate the left and right sides of the heart.
- 1 Endocardial tubes
- 2 Training and heart tube position
- 3 Formation of the heart handle
- 4 Sinus venosus development
- 5 Wall formation of the heart
- 6 Septum formation of the atrial in common
- 7 Development of the atria
- 8 Septum formation of the atrioventricular canal
- 9 Atrioventricular valves
- 10 Truncus septum formation and arterial cone
- 11 Development of the pacemaker and conduction system
- 12 References
In the splanchnopleuric mesenchyme on either side of the neural plate, a horseshoe-shaped area develops as the cardiogenic region. This has formed from cardiac myoblasts and blood islands as forerunners of blood cells and vessels.By day 19, an endocardial tube begins to develop in each side of this region. These two tubes grow and by the third week have converged towards each other to merge together, using programmed cell death to form a single tube, the tubular heart.
Training and heart tube position
The central part of cardiogenic area is in front of the oropharyngeal membrane and the neural plate. The growth of the brain and the cephalic folds push the oropharyngeal membrane forward, while the heart and the pericardial cavity move first to the cervical region and then into the chest. The curved portion of the horseshoe-shaped area expands to form the future ventricular infundibulum and the ventricular regions, as the heart tube continues to expand. The tube starts receiving venous drainage in its caudal pole and will pump blood out of the first aortic arch and into the dorsal aorta through its polar head. Initially the tube remains attached to the dorsal part of the pericardial cavity by a mesodermal tissue fold called the dorsal mesoderm. This mesoderm disappears to form the transverse pericardial sinus, which connects both sides of the pericardial cavity.
The myocardium thickens and secretes a thick layer of rich extracellular matrix containing hyaluronic acid which separates the endothelium. Then mesothelial cells form the pericardium and migrate to form most of the epicardium. Then the heart tube is formed by the endocardial, which is the inner endothelial lining of the heart, and the myocardial muscle wall which is the epicardium that covers the outside of the tube.
Formation of the heart handle
The heart tube continues stretching and by day 23, the cephalic portion curves in a frontal clockwise direction. The atrial portion starts moving in a cephalic ally and then moves to the left from its original position. This curved shape approaches the heart and finishes its growth on day 28. The conduit forms the atrial and ventricular junctions which connect the common atrium and the common ventricle in the early embryo. The arterial bulb forms the trabecular portion of the right ventricle. A cone will form the infundibula blood of both ventricles. The arterial trunk and the roots will form the proximal portion of the aorta and the pulmonary artery. The junction between the ventricle and the arterial bulb will be called the primary intra-ventricular hole. The tube is divided into cardiac regions along its craniocaudal axis: the primitive ventricle, called primitive left ventricle, and the trabecular proximal arterial bulb, called the primitive right ventricle.
Sinus venosus development
In the middle of the fourth week, the sinus receives venous blood from the poles of right and left sinus. Each pole receives blood from three major veins: the vitelline vein, the umbilical vein and the common cardinal vein. The sinus opening moves clockwise. This movement is caused mainly by the left to right shunt of blood, which occurs in the venous system during the fourth and fifth week of development.
When the left common cardinal vein disappears in the tenth week, with the only the oblique vein of the left atrium and the coronary sinus remaining. The right pole joins the right atrium to form the wall portion of the right atrium. The right and left venous valves fuse and form a peak known as the septum spurium. At the beginning, these valves are large but the left venous valve and the septum spurium fuse with the developing atrial septum. The upper right venous valve disappearing. The bottom evolves into: the inferior valve of the vena cava and the coronary sinus valve.
Wall formation of the heart
The main walls of the heart are formed between day 27 and 37 of the development of the early embryo. The growth consists of two tissue masses actively growing that approach one another until they merge and split light into two separate conduits. Tissue masses called endocardial cushions develop into atrioventricular and conotroncal regions. In these places, the cushions will help in the formation of auricular septum, ventricular conduits, atrio-ventricular valves and aortic and pulmonary channels.
Septum formation of the atrial in common
At the end of the fourth week, a crest grows that leaves the cephalic part. This crest is the first part of the septum primum. The two ends of the septum extend into the interior of the endocardial cushions in the atrioventricular canal. The opening between the bottom edge of the septum primum and endocardial cushions is the ostium primum (first opening). The extensions of the upper and lower endocardial pads grow along the margin of the septum primum and close the ostium primum. Coalescence of these perforations will form the ostium secundum (second opening), which allows blood to flow freely from the right atrium to the left. When the right of the atrium expands due to the incorporation of the pole of the sinus, a new fold appears, called the septum secundum. At its right side it is fused with the left venous valve and the septum spurium. A free opening will then appear, called the foramen ovale. The remains of the upper septum primum, will become the valves of the foramen ovale. The passage between the two atrial chambers consists of a long oblique slit through which blood flows from the right atrium to the left.
Development of the atria
Initially, a single pulmonary vein develops in the form of a bulge in the back wall of the left atrium. This vein will connect with the veins of the developing lung buds. As development proceeds the pulmonary vein and its branches are incorporated into the left atrium and they both form the smooth wall of the atrium. The embryonic left atrium remains as the trabecular left atrial appendage, and the embryonic right atrium remains as the right atrial appendage.
Septum formation of the atrioventricular canal
At the end of the fourth week, two atrioventricular endocardial cushions appear. Initially the atrioventricular canal gives access to the primitive left ventricle, and is separated from arterial bulb by the edge of the ventricular bulb. In the fifth week, the posterior end terminates in the center part of the upper endocardial cushion. Because of this, blood can access both the left primitive ventricle and the right primitive ventricle. As the anterior and posterior pads project inwardly, they merge to form a right and left atrioventricular orifice.
When forming intra-atrial septa, atrio-ventricular valves will begin to grow. An intra-muscular ventricular septum begins to grow from the common ventricle to the atrio-ventricular endocardial cushions. The division begins in the common ventricle where a furrow in the outer surface of the heart will appear the interventricular foramen eventually disappears. This closure is achieved by further growth muscular interventricular septum, a contribution of trunk crest-conal tissue and a membranous component.
Truncus septum formation and arterial cone
The arterial cone is closed by the infundibular cushions. The trunk cones are closed by the forming of an infundibulotroncal septum, which is made from a straight proximal portion and distal spiral portion. Then, the narrowest portion of the aorta is in the left and dorsal portion. The distal portion of the aorta is pushed forward to the right. The proximal pulmonary artery is right and ventral, and the distal portion of the pulmonary artery is in the left dorsal portion.
Development of the pacemaker and conduction system
The rhythmic electrical depolarization waves that trigger myocardial contraction is myogenic, which means that they begin in the heart muscle spontaneously and are then responsible for transmitting signals from cell to cell. Myocytes that were obtained in the primitive heart tube, start beating as they connect together by their walls in a syncytium. Myocytes initiate rhythmic electrical activity, before the fusion of the endocardial tubes. The heartbeat begins in the region of the pacemaker which has a spontaneous depolarization time faster than the rest of myocardium.
The primitive ventricle acts as initial pacemaker. But this pacemaker activity is actually made by a group of cells that derive from the sinoatrial right venous sinus. These cells form an ovoid sinoatrial node (SAN), on the left venous valve. After the development of the SAN, the superior endocardial cushions begin to form a pacemaker as known as the atrioventricular node. With the development of the SAN, a band of specialized conducting cells start to form creating the bundle of His that sends a branch to the right ventricle and one to the left ventricle. Most conduction pathways originate from the cardiogenic mesoderm but the sinus node may be derived from the neural crest.
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