Development of the Heart
“If a single cell, under appropriate conditions, becomes a man in the space of a few years, there can surely be no difficulty in understanding how, under appropriate conditions, a cell may, in the course of untold millions of years, give origin to the human race.”
― Herbert Spencer, The Principles of Biology, Vol 1 -
In the process of development of heart, there are 4 main steps. They are,
- Cardiac tube formation
- Cardiac looping
- Septum formation
- Positioning of heart tube
Q. How does the endocardial tube is formed?
Cardiac progenitor cells which are in the splanchnic layer of the lateral plate mesoderm form cardiac myoblasts. Also blood islands appear in the mesoderm. These blood islands combine together & forms a horse shoe shaped endothelial lined tube which is known as endocardial tube. The collection of endocardial tube & surrounding myoblasts are known as cardiogenic field.
Q. How does the cardiac tube is formed?
Initially it formed as two parallel tubes on either sides of the embryo. As a result of growth of brain & cephalic folding of the embryo the cardiogenic field is brought ventrally to the thoracic region. Then the lateral folding of the tar tubes merge and form cardiac tube.
Q. What is cardiac looping?
As the outflow tract lengthens, cardiac tube begins to bend on day 23 I complete by day 28. Cephalic portion bends ventrally, caudally & to the right. Caudal portion bends dorsally, cranially & to the left.
Externally the junctions between bulbus cordis & primitive ventricle becomes bulboventricular junction, lately which becomes primitive interventricular foramen. The junction between primitive ventricle & primitive atrium becomes inter atrioventricular canal.
The abnormalities of cardiac looping is dextrocardia where the heart is in the right side of the chest.
Q. Describe the formation of interatrial septum.
Septum primum appears from the posterior wall of the roof of the atrium towards the endocardial cushions. Septum primum is a sickle shaped fold. When it reaches endocardial cushions it leaves a free margin in between the septum & the endocardinal cushion which is known as ostium primum. Ostium primum closes by fusion of septum primum & endocardial cushions. At that time the cell death Occurs in the upper part of septum primum. This opening is known as ostium secondum. Then another septum known as septum secondum appears from the common atria, right to the former septum primum. Septum secondrum grows towards the endocardial cushions & overlaps ostium secondum. The opening left by septum secondum is known as foramen ovale. Rest of the septum primum which is connected to the endocardial cushion acts as a valve of foramen ovale. At birth left atrial pressure increases as the lung circulation begins.
Q. Describe the formation of the ventricular septum.
There are 2 components of the ventricular septum. They are muscular portion & membranous portion. The muscular portion is formed by the end of forth week, two primitive chambers of ventricles begin to expand. The medial walls of the chambers merge each other to form the muscular portion of the interventricular septum. Two ventricles are connected by interventricular foramen. The interventricular foramen is closed by fusion of an outgrowth tissue of the inferior endocardial cushion with the muscular portion of the interventricular septum. Conus septum also helps.
There can be abnormalities of the endocardial cushion formation. They are Atrial Septal Defects (ASD), Ventricular Septal Detects (VSD), Tetralogy of Fallot, Transposition of great vessels, Common Truncus Arteriosus.
