Figure 1.
Schematic representation of heart tube formation in mouse (a, b, c) and chick (d, e, f). First heart progenitors are shown in blue and second heart progenitors in red. (a,d) Fate mapping studies have defined the locations of chick and mouse cardiac progenitors in the heart fields. (a) Lateral view of an E6.5 mouse embryo. (d) Dorsal view of an HH5 chick embryo. (b,e) Fusion and differentiation of the heart fields is more rapid in the mouse leading to a cardiac crescent that is not seen in the chick. Second heart progenitors are located medially in both chick and mouse heart fields but quickly change positions to cranial as the heart fields converge on the midline. (b) Ventral view of an E8.5 mouse embryo showing the cardiac crescent and its relationship with the anterior intestinal portal (curved black line). (e) Ventral view of an HH8 chick embryo showing convergence of the heart fields in the ventral midline and how the first and second heart field progenitors have rotated their position from medial-lateral to craniocaudal. (c,f) Second heart progenitors are gradually added to the elongating cardiac tube. (c) Ventral view of an E9.5 mouse embryo. (f) Ventral view of an HH12 chick embryo. The caudal second heart progenitors are shifted by formation of the foregut pocket and anterior intestinal portal (curved black line) to cranial, thus putting them in place to contribute to the outflow pole. Some of the second heart field progenitors are also added to the venous pole: parts of the atrium and atrial septum but these are incorporated later than the stages shown, hence no red cells are seen at these stages in the venous pole. The proximal and distal outflow myocardium is added over an extended period of time (reprinted from 222 with permission).