CHAMBER FORMATION
During the time of cardiac looping, at approximately 3 weeks of
development, the arterial and venous poles of the heart decrease or cease cell
division. At the same time, cardiomyocytes at two distinct locations within the
intervening tissue reinitiate cell proliferation. This localized expansion of
cardiomyocytes gives rise anteriorly to the atria and posteriorly to the left
ventricle, with the area separating the two regions giving rise to the
atrioventricular canal. Studies in chickens and mice demonstrated that the
atria grow not only through proliferation but also by the recruitment of cells
to the venous pole of the heart. The left ventricle and the atria are largely
derived from a common pool of progenitors termed the first heart field (Fig.
1.4). In contrast, the second heart field gives rise to the dorsal mesenchymal
protrusion and primary atrial septum, which are tissues that are critically
important for atrioventricular septation, the outflow tract, and the right
ventricle. A con- served role for the second heart field is supported by the
observations that abnormalities that affect the expansion of the second heart
field are associated with congenital heart disease in mouse models and humans,
including atrial and atrioventricular defects, as well as outflow tract abnormalities.
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| FIG 1.4 Summary of heart tube derivatives. |
Contribution of cells from the second heart field to the heart is complete by the fifth week of human
development. At this stage, chamber identity can be established by inspecting
anatomic features and/or by the expression of left or right ventricular
chamber-specific genes. As the cardiovascular system develops to support
postnatal systemic and pulmonary circulations, the heart goes through a series
of complex remodeling events. Critical steps in this process are the formation
of the septa between individual components of the heart, with the purpose of
separating the respective blood flows within the heart, and the formation of
valves facilitating unidirectional flow among the respective components.
Together, these two events are commonly referred to as valvuloseptal morphogenesis.
