Research Progress and Prospect in Cellular Processes and Molecular Mechanisms of Regeneration in Ascidians

Regenerative ability broadly but unevenly exists across metazoan phyla. Invertebrates such as the hydra or planarian regenerate the whole body from a piece of tissue or cell population, while only part of limited organs can be regenerated in vertebrates such as the limb of salamander. The ascidian, which is invertebrate chordates and the closest relatives of vertebrates evolutionarily, shows robust but variable regenerative ability among different species. Colonial ascidians such as Botrylloides leachii are capable of completing regeneration of entire new bodies with nothing but small fragments of extracorporeal vasculature, while solitary ascidians such as Ciona robusta can only regenerate distal structures upon injury, such as the siphons and their cerebral ganglion. The ascidian has gradually become an ideal animal model for regeneration study in view of its evolutionary position and coexisting of two kinds of regeneration. This review summarizes the cellular processes and molecular mechanisms of whole-body and partial-body regenerations that have been identified so far in different ascidian species. The difference of mechanisms underlying two kinds of regeneration types is discussed, and the prospect is put forward for future research that hope to be helpful for the understanding of the evolution and regulation of regeneration among metazoans.