Development and evolution of the ascidian cardiogenic mesoderm

The heart and other blood pumping organs are close to being universally essential in the animal kingdom. These organs present a large anatomical, morphological, and cellular diversity, which is thought to have arisen by building developmental modules on a conserved core of ancestral heart regulatory units. In this context, studies using the ascidian model system Ciona intestinalis offer a distinctive set of theoretical and experimental advantages, which we herein discuss in details. Development of the heart and related muscles in Ciona has been analyzed with a cellular to subcellular resolution unprecedented in Chordate model systems. Unique derived developmental characters of the cardiogenic mesoderm appear to be shared between Ciona and vertebrates. Notably, accumulating evidence point to an early Chordate origin of the cardiopharyngeal population of mesoderm cells that may have provided the foundation for the emergence of the second heart field in higher vertebrates.