Heart malformations are among the commonest human inborn defects. But,although some of the genes involved in vertebrate heart morphogenesis have been identified, little is known about the specification of heart progenitors. On , Satou and co-workers report that in the ascidian Ciona savignyi, a primitive chordate, Cs-Mesp specifies heart precursor cells. Cs-Mesp is the sole Ciona orthologue of the vertebrate Mesp genes, which have been implicated in heart development. The researchers show that Cs-Mesp is specifically and transiently expressed in embryonic heart progenitor cells in C. savignyi, and that knockdown of its expression blocks heart development. In addition, Cs-Mesp drives the expression of genes, including Nkx and HAND, that are essential for vertebrate heart development. Although the ascidian heart is simpler than the vertebrate heart, these results suggest that the mechanism for heart specification is conserved among chordates.
