Evidence that translation of smooth muscle alpha-actin mRNA is delayed in the chick promyocardium until fusion of the bilateral heart-forming regions

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Abstract
Heart development in the chick embryo proceeds from bilateral mesodermal primordia established during gastrulation. These primordia migrate to the midline and fuse into a single heart trough. During their migration as a cohesive sheet, the cells of the paired heart fields become epithelial and undergo cardiac differentiation, exhibiting organized myofibrils and rhythmic contractions near the time of their fusion. Between the stages of cardiomyoblast commitment and overt differentiation of cardiomyocytes, a significant time interval exists. Using a new riboprobe (usmaar) for whole‐mount in situ hybridization in chick embryos, we report the earliest phases of smooth muscle alpha‐actin (smaa) mRNA distribution during the precontractile developmental window. We show that ingressed heart‐forming regions express smaa by the head‐process stage (Hamburger and Hamilton stage 5). In addition, we used usmaar to study the formation and early morphogenesis of the heart. Consistent with fate mapping studies (Garcia‐Martinez and Schoenwolf [ 1993 ] Dev. Biol. 159:706–719; Schoenwolf and Garcia‐Martinez [ 1995 ] Cell Mol. Biol. Res. 41:233–240; Garcia‐Martinez et al., in preparation), our results with this probe, combined with detailed histological and SEM analyses of the so‐called cardiac crescent, demonstrate unequivocally that the heart arises from separated and paired heart rudiments, rather than from a single crescent‐shaped rudiment (that is, prior to fusion of the paired heart rudiments to establish the straight‐heart tube, the rostral midline of the cardiac crescent lacks mesodermal cells and consequently fails to label with usmaar). Smaa is also expressed in the splanchnic and somatic mesoderm, marking the earliest step in coelom formation. Consequently, we also used usmaar to describe formation of the pericardium. Finally, we provide evidence of a post‐transcriptional level of control of smaa gene expression in the heart fields. Our results suggest that the expression of smaa may mark a primitive mesodermal state from which definitive cell types can be derived through inductive events. Dev Dyn;218:316–330.