The development of the human heart is like a carefully planned event. Many cells must act at the right time and in the right place. Gene actions are also essential to turn a small group of cells into a fully working heart with four chambers.
Learning how the heart forms helps us understand how it works normally.
- It also helps us understand how problems in this process can cause congenital heart defects. These are the most common type of birth defect.
This outline explains how the heart forms: - It all begins with early heart cells, which are the first to help form the heart.
- It ends with a fully formed heart with chambers, valves, and big blood vessels.
- It also talks about the important genes and signals that control this development.
- It explains how problems in these steps can lead to heart defects.
Heart formation can be explained under these main points:
- Heart formation can be explained under these main points:
- Formation of the early Heart tube :
- Heart tube morphogenesis and looping :
- Chamber Septation :
- Development of the cardiac valve :
- Formation of the Great Vessels
- SUMMARY
Formation of the early Heart tube :
Cardiac development begins at the end of the second week of human development. This occurs during gastrulation, which is a critical phase. This process begins as mesodermal cells migrate from the primitive streak. The primitive streak is a transient structure in the developing embryo. The cells move towards the anterolateral border of the trilaminar embryonic disc.
Around day 21–22, the growing embryo starts to fold. This folding helps the two heart tubes come together. They then join into one simple heart tube. This early heart has three parts. It includes an inner endocardium and a middle cardiac jelly. There is also an outer muscle layer, which is the myocardium. The presence of this acellular matrix suggests it plays a role in providing structural support. It may also mediate signaling interactions between the endocardium and myocardium during these early stages. Soon after its formation, the heart tube exhibits slow, peristaltic-like contractions, which start at the venous pole.
Heart tube morphogenesis and looping :
Around the fourth week of development, the straight heart tube begins to bend and curve in a process called looping. By this process, the heart starts forming its final shape. This process involves the rupture of the dorsal mesocardium. This tissue bridge attaches the heart tube to the dorsal body wall along its midline. As the heart tube grows, it bends to the right and forms a C-shape. Later, the bending becomes more complex as the heart keeps developing. Eventually forming an S-shape. The looping process is critical. It brings the various segments of the developing heart into their approximate definitive spatial relationships.
This morphogenetic event sets up the basic configuration needed for the development of the four-chambered heart. It marks the first visible sign of left-right asymmetry in the embryo. The looping of the heart tube appears to be a simple bending. However, it is influenced by a complex interplay of factors. These include differential growth within the tube, biomechanical forces exerted by the surrounding pericardial cavity, and underlying genetic programming. The initially linear heart tube is divided into five primary segments. These segments are organized from caudal to cranial. They include the sinus venosus, the primitive atrium, the primitive ventricle, the bulbus cordis, and the truncus arteriosus. These segments will eventually give rise to specific structures in the mature heart.
Chamber Septation :
The transformation of the single-chambered heart tube into a four-chambered heart is complex. It involves the critical process of septation. Septation primarily occurs between the fourth and seventh weeks of development. This consists of the formation of septa that divide the atria and ventricles.
Atrial septation :
Atrial septation starts around day 26 of development. At this time, a thin, curved sheet of tissue called the septum primum begins to grow. It extends from the upper back (posterosuperior) part of the common atrium. It moves toward the endocardial cushions, which are soft tissue structures located at the center of the heart. These cushions are where the upper and lower chambers connect.
Ventricular septation :
Ventricular septation follows a slightly different developmental path. Around the fourth week, the walls of the primitive ventricle begin to expand. The medial ventricular walls gradually appose and merge. This process forms a muscular interventricular septum. Initially, this muscular septum does not fully extend to divide the ventricles. It leaves an opening superiorly known as the interventricular foramen. The ventricles finish closing when a thin wall called the interventricular septum forms between them. Tissue grows and fuses, derived from the endocardial cushions of the atrioventricular canal. The aorticopulmonary septum, developing in the outflow tract, contributes as well.
Development of the cardiac valve :
The formation of the cardiac valves is essential for ensuring unidirectional blood flow. It occurs between the fifth and ninth weeks of development. This process involves the development of both the atrioventricular valves. These valves regulate blood flow between the atrium and ventricles. It also involves the semilunar valves, which control the flow from the ventricles into the great vessels.
Atrioventricular valve :
The atrioventricular valves include the mitral valve on the left and the tricuspid valve on the right. They develop from mesenchymal tissue surrounding the atrioventricular canals. The endocardial cushions play a crucial role in dividing the common atrioventricular canal into distinct right and left orifices. Subsequently, the mesenchymal tissue surrounding these orifices undergoes thinning. It remodels to form the leaflets of the mitral valve. The mitral valve has two leaflets. It also forms the leaflets of the tricuspid valve. The tricuspid valve has three leaflets. The chordae tendineae support these valve leaflets. These are thin fibrous cords. They connect the valve leaflets to the papillary muscles. The papillary muscles develop from the ventricular walls. The AV valves and the semilunar valves have different origins. This reflects their distinct functional requirements. It also reflects the hemodynamic environments in which they operate.
Semilunar Valves:
The semilunar valves, having a half-moon shape include the aortic valve and pulmonary valve. They begin to form from small bumps called tubercles. These bumps appear on swellings inside the truncus arteriosus. This is a part of the early heart tube. It will later split into the major arteries. As development continues, these bumps shape into thin flaps. These flaps make up the valves. They help control blood flow out of the heart. These truncal swellings originate from the bulbar ridges and subendocardial valve tissue. These tubercles undergo excavation and thinning. This process sculpts them into the three characteristic cusps of the aortic and pulmonary valves. The aortic valve is located at the junction between the left ventricle and the aorta. The pulmonary valve is situated at the junction between the right ventricle and the pulmonary trunk. While the general principles of semilunar valve development are understood, there is less specific knowledge about the process in humans. We know less compared to the AV valves. This highlights an area where further research is needed.
Formation of the Great Vessels
The development of the great vessels is closely linked to the formation of the heart itself. This includes the aorta and pulmonary artery. This linkage is particularly evident in the outflow tract. The ascending aorta originates from the truncus arteriosus. The pulmonary trunk also originates from this most cranial segment of the primitive heart tube.
The truncus arteriosus is an early heart tube. It splits into the aorta and pulmonary artery. This process occurs with the help of a wall called the aorticopulmonary septum.
This septum forms from two opposing ridges. The truncal and conal swellings come from endocardial cushions. They grow toward each other and fuse. This process is strongly guided by neural crest cells moving into the truncal and conal ridges. Importantly, the aorticopulmonary septum goes through a spiraling process. This process is key to setting the correct position of the aorta and pulmonary trunk. The pulmonary trunk lies in front and to the left of the aorta. Problems in this spiraling can cause heart defects like transposition of the great arteries.
- The aortic arches are blood vessels. They originate from the aortic sac. This sac is located at the front end of the early heart tube called the truncus arteriosus.
- The arch of the aorta develops from the left fourth aortic arch. It also develops from parts of the aortic sac and dorsal aorta. These parts combine to form the main curved section of the aorta.
- The right horn of the aortic sac forms the brachiocephalic artery, a key vessel in the heart’s arterial supply.
- The pulmonary arteries develop from the forward sections of the sixth aortic arches on both the right and left sides. These sections give rise to the vessels that carry blood from the heart to the lungs.
SUMMARY
| Developmental stage | Timeline(days) | Key events |
| Gastrulation and cardiac crescent formation | 14-21 | Migration of mesodermal cells, specification of cardiac progenitors, and formation of cardiac crescent. |
| Heart tube formation | 21-22 | Fusion of bilateral endocardial tubes into a single linear heart tube. |
| Heart tube looping | 22-28 | Bending and twisting of the heart tube into a C- shape and then an S-shape. |
| Atrial and ventricular septation | 26-49 | Formation of the atrial and ventricular septa, division of the atrioventricular canal. |
| Valve development | 34-63 | Formation of the atrioventricular (mitral and tricuspid) and semilunar (aortic and pulmonary) valves. |
| Great vessel formation | 35 onwards | Development of the aorta, pulmonary artery, and other major vessels. |
MBBSWALAH 
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