Regeneration of Ascidians
Regeneration of amputated or injured tissues and organs is often thought to be a rare phenomenon, however if one surveys all metazoans, it becomes abundantly clear that regeneration is present in many phyla and may only be rare in specific clades. Why is it that so many diverse animals are capable of extraordinary regeneration when mammals, and more specifically humans, display only modest regeneration? Was efficient regeneration a common characteristic of our evolutionary ancestors that has since been lost in many animal lineages or did regeneration arise independently many times from ancestors that did not regenerate? The wide spread of good regenerators across the metazoan tree of life and the logic that a complex trait is much more easily lost that created, suggests that regeneration was a common characteristic in early metazoans. By that reasoning the rare vertebrates that are good regenerators, for example salamanders that can completely regenerate amputated limbs, may have simply retained what other vertebrates failed to retain. On the other hand, the existence of diverse cellular and molecular mechanisms utilized by different animals for regeneration may support independent origins, as natural selection may have acted upon different biological building blocks to produce similar regenerative powers.
These questions of the origins of vertebrate regeneration and the relationships between different cellular and molecular regeneration strategies drive our research. We have chosen to investigate this topic in ascidians because of their unique evolutionary position as the closest invertebrate relatives of the vertebrates and their position within the chordate phylum (our phylum). Moreover, ascidians efficiently regenerate a number of organ systems and some species are even capable of asexual reproduction, which is akin to whole body regeneration. Finally, ascidians, specifically Ciona intestinalis and Ciona savignyi, also offer a host of technical advantages, making sophisticated genetics and molecular biology routine in the laboratory.
We have chosen to focus our research on the regeneration of the oral siphon because of its accessibility for amputation, its rapid regeneration in juvenile Ciona, and the ease with which this transparent organ can be imaged. Along with these advantages the oral siphon is also sufficiently complex to allow us to observe the production of various missing cell types and their movements during organ morphogenesis within the newly regenerating siphon. Careful characterization of oral siphon regeneration promises to reveal important previously unknown information, such as the source of progenitor cells used to rebuild the siphon and how those newly made progenitors interact with existing tissues. Furthermore, the ability to do traditional genetics in these organisms will undoubtedly identify new molecules or new roles for already known molecules that are necessary for successful regeneration and are directly relevant to regenerative medicine in humans.