Fig. 1. First and second heart lineages. (A) The second lineage of cardiac progenitor cells is highlighted in a cardiac crescent-stage mouse embryo (E7.5) by the expression of a ß-galactosidase (lacZ) transgene under the transcriptional control of the Fgf10 locus (arrows). (B) Expression of -cardiac actin mRNA in differentiating cardiomyocytes of the cardiac crescent (first lineage) in a similarly staged mouse embryo. Comparison with staining in A demonstrates that the second lineage progenitors lie medial and caudal to the first lineage progenitors of the crescent. (C) Transverse section through an E8.5 mouse embryo, showing expression of the Fgf10-lacZ transgene in dorsal pericardial mesoderm (DPM) in continuity with myocardium of the heart tube (HT), and in pharyngeal arch mesoderm (PAM). (D) Illustration of the position of the first (red) and second (green) heart precursor lineages at E7.5 (small inset), and during deployment to the heart tube at E8.5 (main figure) in the mouse embryo. Arrows indicate deployment at both poles of the heart. Large inset shows the contribution of the first and second heart lineages to the E11.5 heart. Figures reprinted, with permission, from Kelly and Harvey (Kelly and Harvey, 2004), and Parmacek and Epstein (Parmacek and Epstein, 2005), Copyright 2005 with permission from Elsevier. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.