Dorsoventral compartmentalization of mesoderm in heart-forming area of chick embryo

In early chick development (stages 5-8) the seemingly homogeneous mesoderm in the heart-forming area splits to somatic and splanchnic cardiogenic laye rs. Little is known about dorsoventral compartmentalization before splittin g. Electron microscopic analysis shows the early dorsoventral polarization of precardiomyocytes. The dorsal compartment has epithelial and the Ventral compartment mesenchymal features with numerous protrusions. At stage 5(+)- 6 staining for wheat germ agglutinine (WGA) transiently demarcates the vent ral part of mesoderm. The glycosomes (beta-glycogen) show a dorsoventral gr adient in the mesoderm of the cardiogenic field during the initial step of the compaction. The differential expression of glycosomes depends on the ac tivity of glycogen synthase kinase 3-beta, a component of the wnt-signaling pathway, and might in this spatiotemporal developmental window be involved in the commitment of presumptive cardiogenic and somatic cells. To verify this hypothesis simulation experiments with LiCl in vitro were carried out. The normal splitting of the mesoderm and the development of heart primordi a were disturbed. Blocking the receptors of WGA by WGA in vitro at stage 5- 5(+) perturbs the migration of mesoderm to anterio-medial direction. It app ears that early specification of dorsal and ventral compartments of the mes oderm in the heart-forming area correlates with the gradient of glycosomes. Our results suggest that the target of LiCl action (glycogen synthase kina se 3-beta) might be involved in the specification of heart primordia and th at WGA receptors mediate the migration of mesoderm to the anteriomedial dir ection.