A primer on regeneration

Centuries of observation have uncovered a diverse range of organisms capable of overcoming loss of tissue. The act of restoring lost anatomy and function is known as regeneration, and it is broadly represented in both plant and animal kingdoms. Cumulative studies have identified a series of events that take place during regeneration of complex animal structures. First, the organism recognizes damage and undergoes wound healing. Then, programmed cell death in the vicinity of the damaged tissue precedes proliferation and migration of cells that foster the development of replacement tissue. Finally, rearrangement of pre-existing tissue and integration with newly differentiated cells take place to restore the function and proportionality displayed previous to damage . Although the ability to regenerate is believed to be ancestrally common and lost throughout evolution, there is significant heterogeneity of some basic mechanisms displayed during regeneration in different animal species. Perhaps one of the most noticeable differences is the cellular source contributing to formation of the new tissue during regeneration. Organisms such as planarians and Hydra rely on active reservoirs of somatic pluripotent stem cells abundantly distributed throughout their bodies and maintained throughout their life. On the other hand, vertebrates rely mostly on progenitor cell activation and dedifferentiation, to regenerate cells with limited potential to regenerate specific structures. However, not all regenerative events rely on cellular replacement. Leading edge research has begun to uncover mechanisms involved in autonomous repair and functional regeneration of single cells – be it neurons or ciliated protozoa. The fact that organisms can achieve regeneration through diverse cellular sources is remarkable, but just as remarkable is the possibility that conserved molecular pathways could be activated to achieve regeneration in different species. Analysis of these pathways will contribute to understanding human development and potential avenues for regenerative medicine.