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

Heart development in the chick embryo proceeds from bilateral mesodermal pr imordia established during gastrulation. These primordia migrate to the mid line and fuse into a single heart trough. During their migration as a cohes ive sheet, the cells of the paired heart fields become epithelial and under go cardiac differentiation, exhibiting organized myofibrils and rhythmic co ntractions near the time of their fusion. Between the stages of cardiomyobl ast commitment and overt differentiation of cardiomyocytes, a significant t ime interval exists. Using a new riboprobe (usmaar) for whole-mount in situ hybridization in chick embryos, me report the earliest phases of smooth mu scle alpha-actin (smaa) mRNA distribution during the precontractile develop mental window. We show that ingressed heart-forming regions express smaa by the head-process stage (Hamburger and Hamilton stage 5). In addition, me u sed usmaar to study the formation and early morphogenesis of the heart. Con sistent with fate mapping studies (Garcia-Martinez and Schoenwolf [1993] De v. 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 f rom separated and paired heart rudiments, rather than from a single crescen t-shaped rudiment (that is, prior to fusion of the paired heart rudiments t o establish the straight-heart tube, the rostral midline of the cardiac cre scent 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 des cribe formation of the pericardium. Finally, we provide evidence of a post- transcriptional level of control of smaa gene expression in the heart field s. Our results suggest that the expression of smaa may mark a primitive mes odermal state from which definitive cell types can be derived through induc tive events. Dev Dyn 2000; 218:316-330. (C) 2000 Wiley-Liss, Inc.