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Objectives
After studying this chapter, you should be able to:
Describe the origins, formation, and folding of the heart tube.
Explain how the heart tube becomes partitioned into the left and right atria and ventricles with separate outflow vessels.
Summarize the development of the conducting system of the heart.
Trace blood flow through the prenatal circulatory system and describe the circulatory changes that occur at birth.
Describe significant embryonic arteries and veins, and name the mature structures that they become.
Relate the processes of cardiovascular development to their corresponding defects.
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Formation of the Heart Tube
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The cardiovascular system begins to form in the middle of the third week.
The heart is derived from the primary and secondary heart fields: collections of cells in the splanchnic mesoderm at the cranial end of the germ disc (Figure 4-1).
The primary heart field arcs around the cranial end of the neural folds.
The secondary heart field is located ventral to the pharynx.
These progenitor cells differentiate into cardiac myoblasts and blood islands and fuse into a crescent-shaped tube called the cardiogenic region.
Lateral folding of the embryonic disc brings the caudal portions of the cardiogenic region together to fuse into the midline cardiac tube (Figure 4-2).
Craniocaudal folding of the embryonic disc—combined with elongation of the neural tube dorsally—carries the developing heart tube through the cervical region to the thoracic region (Figure 4-3).
The cardiac tube develops dilations along its length (Figure 4-4; Table 4-1):
Blood enters the heart tube through the most caudal dilation: the sinus venosus, which will give rise to the smooth-walled portion of the right atrium and the coronary sinus. (The smooth portion of the left atrium forms by incorporating the pulmonary vein into its wall as this chamber expands.)
The next dilation is the primitive atrium, which will give rise to the trabeculated portions of both atria.
The primitive ventricle will give rise to the trabeculated portions of both ventricles.
The bulbus cordis will form the smooth portions of both ventricles.
The conus cordis and truncus arteriosus will form the ventricular outflow tracts and proximal portions of the aorta and pulmonary trunk. Collectively, the conus cordis and truncus arteriosus are called the conotruncus.
The primary heart field forms more caudal components of the cardiac tube (primitive atrium, primitive ventricle), and the secondary heart field forms more cranial components (bulbus cordis, conus cordis, truncus arteriosus).
The cardiac tube elongates while remaining tethered by blood vessels at its cranial and caudal poles. As a result, the tube folds in a process ...