Wherefore heart thou? Embryonic origins of cardiogenic mesoderm

The developing heart in avian embryos has been examined extensively over the past several decades using classic embryologic and, more recently, molecular and genetic approaches. Still, conflicting reports arise as to the location and regulation of early heart progenitors in the embryo. In addition, a new source of cardiomyocytes has been identified recently that contributes to the outflow tract after the heart initially forms. The focus of this review is the examination of the tissue interactions, signaling molecules, and gene regulatory mechanisms that, together, control heart formation from primary and secondary heart forming fields of the embryo. Early studies of the induction and regulation of the secondary heart field indicate that at least some of the events of primary cardiomyogenesis are recapitulated when the conotruncal myocardium is recruited into the heart. The consideration of classic embryologic studies of the heart forming fields in terms of modern molecular and genetic tools provides reinforcing evidence for the location of cardiac progenitors in the embryo. The accurate definition of early cardiac regulatory events provides a necessary foundation for the generation of new therapeutic sources of cardiomyocytes.