Fig. 3. Two phases of the relationship between RALDH2 and the cardiac field. (A) HH7⁺. GATA4 in situ hybridization (left) was converted to grayscale image (right) and submitted to morphometry. Anterior and posterior limits of the cardiac field (chick Gata4 expression) were measured relative to a horizontal bar transecting the anterior tip of Hensen's node. (B) HH7⁺. RALDH2 in situ hybridization (left) was converted to grayscale image (right) and submitted to morphometry. The anterior limit of RALDH2 expression in the lateral mesoderm was measured relative to the anterior tip of Hensen's node. Chick GATA4 and chick RALDH2 parameters above or below Hensen's node were attributed positive or negative values, respectively. (C) Two phases of cardiac anteroposterior (AP) signalling by RA. Phase 1 is characterized by increasing proximity between RALDH2 and prospective sino-atrial precursors in the posterior extremity of the cardiogenic plate. Phase 2 is characterized by a progressive encircling of sino-atrial precursors in the posterior half of the cardiogenic plate by a field of RALDH2. Data are presented as means±s.e.m.