Synonym: development of the heart
German: Embryonale Herzentwicklung
During the development the cardiovascular system is the first operational system of the embryo which begins as early as the third development week with its activity picking up. Embryonic cardiac activity can be seen during pregnancy as early as the sixth week of gestation (post menstruationem) using ultrasound.
The starting point for cardiac development is the neck region before the prechordal plate in the so-called cardiogenic zone. This is located at the bottom of the intraembryonic body cavity (coelomic cavity) and is horseshoe-shaped in front of the neural tube. The paired heart tubes are initially produced in this cardiogenic zone. These merge into each other in the course of the lateral folding of the embryo, making them the unpaired primitive cardiac tube.
All internal spaces of the heart develop during organ development by the so-called loop and septation of the unpaired cardiac tube. They develop in the surrounding coelomic cavity, which later includes the pericardial cavity. The mesoderm surrounding the cardiac tube later forms the myocardium.
The entire heart development can be divided into two sections:
During the fourth development week, the cardiac tube extends and bends to form the cardiac loop. Through locally bonded extensions, the caudal is formed (cranially) starting with the following sections:
During the heart's development, only a atrium and a chamber exist. Separation takes place through a subsequent complicated septation action.
Because of the particular rotation of the cardiac loop, the atrium and also the sinus venosus with the inflowing veins (in their entirety, the porta venosa hereinafter) are presented dorsally. During the heart's development, the cardiac sinus venosus is incorporated primarily into the right atrium, where it forms the smooth-walled portion ("right sinus horn"). The other parts of the sinus venosus ("left sinus horn") are responsible for the formation of the sinus coronarius.
The ventral includes the ventricles, bulbus cordis and truncus arteriosus. From this come later emerge the aorta and the truncus pulmonalis (porta arteriosa). The U-shaped deflection in the sagittal plane in addition to a lateral displacement of the various portions of the heart means the cardiac loop is S-shaped.
Thus, the ventral system of the left ventricle goes to the left side and the bulbus cordis and truncus arteriosus go to the right. The bulbus cordis is located in the proximal section and later forms the portion of the right ventricle. The distal section of the bulbus cordis, also known as the conus cordis, forms the common smooth-walled outflow tract from the left and right ventricles. Finally, the truncus arteriosus forms the ascending part of the arcus aortae and the truncus pulmonalis.
The various portions of the heart are divided by septa.
The entire development of the inside of the heart is structured into three separate sections.
Between the two walls, the dorsal and the ventral wall of the atrioventricular canal, which forms the narrowed transition between the atrial and ventricular area, causes swellings (endocardial cushions). These then merge and divide the AV channel into a left and a right portion (canalis atrioventricularis dexter and sinister). For the development of the atrial septum, the septum primum is later connected to the merged endocardial cushion and is anchored by it. Later, from these fused endocardial cushions, the AV valves are formed, each separating the atria from the ventricles.
The ventricles are divided at the end of the fourth development week with the formation of a muscle strip. This grows from the caudal according to the cranial and thus forms the pars muscularis of the septum interventriculare.
For the time being on the cranial side a gap remains between the left and right ventricle (interventricular foramen). This foramen is closed at a later date primarily by the so-called bulge (part of the aortopulmonary septum and material from the endocardial cushion the atrioventricular canal) with connective tissue (pars membranacea des septum interventriculare).
In a complicated process, the conus cordis and truncus arteriosus are divided by a spirally extending aortopulmonary septum (also referred to as cone-trunk septum), structured into an outflow from the ventricles into the pulmonary trunk and the ascending aorta (from the left ventricle).
Proximally, the so-called bulge (endocardial cushion in the area of the conus cordis) grows and the distal trunk bulge (endocardial cushion in the area of the truncus arteriosos, trunk septum). The spiral twist is probably caused by the blood flow. The resulting spiral shape here is largely responsible for the later tortuous course of the pulmonary trunk near the aorta.
Towards the end of the fourth development week, the septum primum is growing from the roof (of the still undivided atria). The division of the atria through this septum is initially incomplete: At the bottom of the atrium, above the limit for the chamber, there is an open connection, the foramen primum. Immediately prior to the complete closure of the foramen primum, the septum primum tears on the cranial side so ultimately the foramen secundum is formed.
At the end of the 5th development week, the septum primum grows gradually from the floor and from the roof of the atrium towards the centre in the septum secundum, which is why the foramen secundum is ultimately covered. In the centre of the septum secundum, the foramen ovale cordis remains, which is covered by the septum primum.
Postnatally, the septum primum and secundum are usually firmly together, resulting in a physical closure of the foramen ovale. However, where incomplete fusion takes place, a gap may remain in the interatrial septum. Functionally there should be no impairments (in most cases). This is due to the slightly higher pressure in the left atrium, which creates a seal.
Disturbances in the various stages of cardiac development can lead to different clinically relevant diseases.
Due to the heart's complicated development, numerous malformations can occur. In addition to isolated ventricular septal defects so-called combined defects can also occur. This includes the clinical picture of Tetralogy of Fallot: in this heart malformation, there is a high ventricular septal defect, one above the ventricular septum "riding" the aorta and a pulmonary stenosis combined with right heart hypertrophy. Children are clinically affected by a pronounced cyanosis because deoxygenated blood arrives in the systemic circulation while too little blood arrives in the pulmonary circulation.
If there is no spiral subdivision through the aorticopulmonary septum, transposition of the large vessels occurs. Characteristic is the emanation of the aorta from the right and the execution of the pulmonary trunk from the left ventricle. This aberration is only compatible with life, as long as other malformations of the heart are present, through which a blood exchange between the large and small circulatory system is possible.
If the opening of the foramen ovale take place postnatally, this can cause small clots (thrombi) from the body's periphery to form over the foramen directly into the left atrium and thus into the systemic circulation. If the thrombus travels directly into the carotid artery, this can lead to a cerebral infarction. This defect often triggers a stroke in young people.
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