Fig. 1. Cardiac development. Mouse heart development at embryonic day (E) (A) 7.75-8.0, (B) E8.0-8.5, (C) E9.5 and (D) E12.5, and at (E) late embryonic/postnatal stages. Anterior is towards the top. (A-C) The myocardium of the heart develops from two populations of cells called the first heart field (FHF) (red) and a more medial region called the second heart field (SHF) (blue) that lie adjacent to each other at the cardiac crescent stage of development (E7.75-8.0, A). Broken line indicates the midline. (B) Lateral regions of the FHF migrate towards the ventral midline to fuse and form the primitive heart tube, while the SHF remains concentrated in the dorsal pharyngeal mesoderm. (C) Later, SHF cells (blue) migrate into the heart from both anterior and posterior regions, as noted by Isl1 immunostaining and fate mapping with Isl1-cre mice (arrows). (D) By E12.5, the four chambers of the heart are well delineated and septation of the outflow tract is observed so that (E) by late postnatal stages, the outflow tract (OFT) is completely septated and both ventricular and atrial septation are complete in preparation for postnatal life. In D, broken line indicates the OFT septum/cardiac neural crest cells. Ao, aorta; LA, left atrium; LV, left ventricle; PT, pulmonary tract; RA, right atrium; RV, right ventricle.