Regeneration and stem cells in ascidians

Abstract

Understanding and utilizing the ability of stem cells to expand and differentiate into tissues and organs is a major goal of biomedical science. Ascidians are basal chordates (Tunicata) which offer unique opportunities to investigate the biology of stem cells. These marine organisms begin their life as a larva with a typical chordate body plan, including a notochord, dorsal hollow nerve tube and a striated musculature. After a swimming phase, the larvae settles and undergoes an extensive metamorphosis during which most of the chordate characteristics are resorbed, leaving a filter feeding sessile invertebrate adult. Due to its small size (in many species a larva consists of ca. 2,500 cells) and rapid development (a fertilized egg can complete development in less that 24 h in Ciona species), the study of ascidian larvae has a long history and continues to be an outstanding model for studying specification and differentiation events which occur during chordate embryogenesis. In comparison, the adult body plan is relatively unstudied at a molecular level, but several examples of extensive regeneration following surgical ablation of different tissues have been described. In addition, within this chordate subphylum two distinct adult body plans exist: solitary and colonial. Following larval metamorphosis, solitary species grow into an adult that can range from several millimeters to tens of centimeters in length. In addition, colonial species grow not by increasing in size, but by asexually propagating, eventually creating a colony of genetically identical individuals that can cover areas of several square meters. Thus colonial ascidians have two independent developmental pathways to create an identical adult body plan, and are the only chordates with this regenerative ability. Taken together, the ascidians represent an excellent system to study the biology of both embryonic and adult stem cells. © 2008 Springer Netherlands.