Even before gastrulation has completed, the internalized bilateral cardiac precursor pools continue to migrate in response to signaling cues from neighboring tissues. Remaining as cohesive epithelia, the heart fields move anteriorly and ventrally between 15 and 20 days of development, fusing at the embryonic midline to form the transient cardiac crescent (Fig. 1.1). Proper midline fusion of the bilateral cardiac primordia is essential for development of the heart. Several cardiac transcription factors are required for this process, and loss of function of any one of them causes extensive defects in further morphogenesis, including cardia bifida in severe cases.
Newly united as the cardiac crescent, the multipotent cardiac progenitors coalesce further to form a linear tube by 3 weeks of development, segregating into the future endocardial lining and myocardial walls (Fig. 1.2). The linear heart tube consists exclusively of differentiated first heart field cells; the second heart field persists as a mesenchymal population, which is a loose association of rapidly dividing precursor cells adjacent to the heart tube. Although no specialized electrical conduction system has yet arisen, the myocardium of the linear heart tube already exhibits autonomous contractions. Compared with those of a mature heart, these contractions are slow and weak, driven only by the intrinsic depolarizing activity and conductivity of the still-maturing cardiomyocytes. Once the conduction system develops and connects to the mature working myocardium, it will serve as an extrinsic regulator of the electrical impulses within the myocardium. Sufficient contractile force will, in turn, allow the heart to beat at the strength required to circulate blood throughout the body.