The replacement of damaged tissues and organs with tissue and organ transplants or bionic implants has serious drawbacks. Regenerative biology seeks to understand the cellular and molecular differences between regenerating and non-regenerating tissues. Regeneration is accomplished by three mechanisms, each of which uses or produces a different kind of regeneration competent cell. Compensatory hyperplasia is regeneration by the proliferation of cells which maintain all or most of their differentiated functions (e.g., liver). The urodele amphibians regenerate a variety of tissues by the dedifferentiation of mature cells to produce progenitor cells capable of division. All regeneration-competent cells have two features in common. First, they are not terminally differentiated and can re-enter the cell cycle in response to signals in the injury environment. Second, their activation is invariably accompanied by the dissolution of the extracellular matrix (ECM) surrounding the cells, suggesting that the ECM is an important regulator of their state of differentiation.
Regeneration of complex structures after injury requires dramatic changes in cellular behaviour. Regenerating tissues initiate a program that includes diverse processes such as wound healing, cell death, dedifferentiation, and stem (or progenitor) cell proliferation; furthermore, newly regenerated tissues must integrate polarity and positional identity cues with pre-existing body structures. Gene knockdown approaches and transgenesis-based lineage and functional analyses have been instrumental in deciphering various aspects of regenerative processes in diverse animal models for studying regeneration.