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.
