The embryology of the heart is the interaction among a set of cells, called cardiac cells, that results in the formation of the heart. The cardiovascular system originates from the mesodermal germ layer to form the cardiogenic field. At the beginning, there are a couple of heart tubes, which at the day 22 of development begin to merge to form a single tube that is slightly curved. This tube is known as the cardiac tube, consisting of a myocardial mantle, which will wrap and protect it. High concentrations of retinoic acid induce the formation of atrial structures. Between the fourth and seventh week of development the heart begins to develop a curve handle and form the walls that separate the heart cavities. The formation of such partitions gives rise to endocardial tissue pads in the conduit in the atrioventricular conotruncal region.
- 1 Establishment of cardiogenic field
- 2 Training and heart tube position
- 3 Formation of the heart handle
- 4 Venous sinus 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 Creation of the pacemaker and conduction system
- 12 References
Establishment of cardiogenic field
The cardiogenic field appears halfway through the third week of embryo development, with progenitor cells appearing in the epiblast. From here the cells migrate to form the cranial and caudal segments of the heart and continue towards the skull and to be placed in front of the oropharyngeal membrane and the neural folds. Here the cells lie on the visceral layer of the lateral plate of the mesoderm. Then, the underlying pharyngeal endoderm induces the cells to form cardiac myoblasts. Blood islands also appear which will be the forerunners of blood cells and blood vessels. These islets merge and form a horseshoe-shaped pipe later known as cardiogenic field, which will form the pericardial cavity.
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 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 on day 23 it curves. The cephalic portion of the tube is curved in a ventral direction, moving in a 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.
Venous sinus 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 spurium septum. 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 venae cavae 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 tissues mass 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 cushion in the atrioventricular canal. The opening between the bottom edge of the spetum primum and endocardial cushions are 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 secundum (second opening), which allows blood to flow freely from the right atrium to the left. When the light of the atrium expands due to the incorporation of the pole of the sinus, a new fold appears, called 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 form the trabecular atrial appendage, and the right atrium becomes the embryonic right atrial appendage.
Septum formation of the atrioventricular canal
At the end of the fourth week, two atrio-ventricular 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.
Creation 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 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. Myocytes initiate rhythmic electrical activity, before the fusion of the endocardia side tubes. The heartbeat begins in the region of the pacemaker which has a spontaneous depolarization time faster than the rest of myocardium.
The primitive heart tube 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 called sinoatrial node, on the left venous valve. After the development of the NA, the superior endocardial cushions begin to form a pacemaker as known as atrial-ventricular node (AV). With the development of the NA, a beam made 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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