Early Cardiac Growth and the Ballooning Model of Cardiac Chamber Formation

This chapter outlines the mechanisms of formation of the building plan of the vertebrate heart, placing emphasis on cardiac growth and chamber development. It attempts to set the scene for understanding the relationships between the building plan and mechanisms of cardiac patterning in the various fields of cardiac precursor cells. The pulmonary circulation is the essential difference between the mammalian heart and that of its vertebrate ancestors, with their purely systemic circulatory function. It also discusses the significance of the development of the cardiac components required for the pulmonary circulation. The local reinitiation of proliferation in the primary myocardial heart tube indicates that the formation of the myocardium of the chambers, in contrast to the initial formation of the primary myocardium, represents a localized process. As the working myocardium of the chambers is formed, the nondifferentiating and nonexpanding flanking components become visible. The subsequent temporal repression by Tbx2, Tbx3, and other factors of the formation of working myocardium in these flanking regions initially permits these regions to function as peristaltic valves prior to the formation of the definitive valvar leaflets. It will be crucially important to unravel the regional function of cardiac transcription factors such as Nkx2-5, which are essential but nonetheless play divergent roles in the first and second heart fields and in distinct regions of the heart.