Circulatory System: Heart Tube
Formation Of The Heart Tube
During the third week of development blood islands appear in the lateral plate mesoderm (Figure 27.1) from angioblasts that accumulate as a syncytium (rather like the formation of the syncytiotrophoblast that we saw form during the development of the placenta in Chapter 13). From these cells new blood cells and blood vessels form through vasculogenesis. Blood islands at the cranial end of the embryo merge and assemble a horseshoe‐shaped tube lined with endothelial cells which curves around the embryo in the plane of the mesoderm.
Progenitor cells that migrated from the epiblast differentiate in response to signals from the nearby endoderm to become myoblasts and surround the horseshoe‐shaped tube (Figure 27.2). This developing cardiovascular tissue is called the cardiogenic field.
The early heart tube expands into the newly forming pericardial cavity (Figure 27.3) as it begins to link with the paired dorsal aortae cranially and veins caudally. The developing central nervous system and folding of the embryo (see Chapter 20) pushes it into the thorax and brings the developing parts of the cardi towards one another (Figures 27.1–27.3).
The early, simple heart tube (Figure 27.4) undergoes a ser i foldings to bring it from a straight tube to a folded shape ready to become four chambers. The heart tube begins to bend at 23 days (stops at 28 days) and develops two bulges. The cranial bulge is called the bulbus cordis and the caudal one is the primitive ventricle (Figure 27.5). These continue to bend and create the cardiac (or bulboventricular) loop during the fourth week of development.
When the heart tube loops, the top bends towards the right so that the bulboventricular part of the heart becomes U‐shaped. This looping changes the anterior–posterior polarity of the heart into the left–right that we see in the adult. The bulbus cordis forms the right part of the ‘U’ and the primitive ventricle the left part. You can see the junction between the bulbus cordis and ventricle by the presence of the bulboventricular sulcus. The looping causes the atrium and sinus venosus to move dorsal to the heart loop.
The atrium is now dorsal to the other parts of the heart and the common atrium is connected to the primitive ventricle by the atrioventricular canal. The primitive ventricle will develop into most of the left ventricle and the proximal section of the bulbus cordis will form much of the right ventricle. The conus cordis will form parts of the ventricles and their outflow tracts, and the truncus arteriosus will form the roots of both great vessels.
The sinus venosus comprises the inflow to the primitive heart tube and is formed by the major embryonic veins (common cardinal, umbilical and vitelline) as they converge at the right and left sinus horns (see Chapter 30). The sinus venosus passes blood from the veins to the primitive atrium.
With time, venous drainage becomes prioritised to the right side of the embryo and the left sinus horn becomes smaller and less significant, eventually forming the coronary sinus and draining the coronary veins into the right atrium. The right sinus horn persists, enlarges and becomes part of the inferior vena cava entering the heart and incorporated into the right atrium, forming much of its wall.
Similarly, a single pulmonary vein is initially connected to the left side of the primitive atrium and divides twice during the fourth week to form four pulmonary veins. These become incorporated into the wall of the future left atrium and extend towards the developing lungs.
Many congenital heart defects occur later in development during the division of the heart into its four chambers.
Dextrocardia is a condition in which the heart lies on the right, with the apex of the left ventricle pointing to the right, instead of the left. This is often associated with situs inversus, a condition in ymmetrical. Other congenital heart defects an occur with dextrocardia but it is often asymptomatic.