Unlike humans, some vertebrate animals are able to completely regenerate damaged appendages and other organs. For example, adult zebrafish will regenerate the complex structure of an amputated caudal fin to a degree that the original and replacement fins are indistinguishable. The blastema, a mass of cells that uniquely forms following appendage amputation in regenerating animals, is the major source of regenerated tissue. However, the cell lineage(s) that contribute to the blastema and their ultimate contribution(s) to the regenerated fin have not been definitively characterized. It has been suggested that cells near the amputation site dedifferentiate forming multipotent progenitors that populate the blastema and then give rise to multiple cell types of the regenerated fin. Other studies propose that blastema cells are non-uniform populations that remain restricted in their potential to contribute to different cell lineages. We tested these models by using inducible Cre-lox technology to generate adult zebrafish with distinct, isolated groups of genetically labeled cells within the caudal fin. We then tracked populations of several cell types over the entire course of fin regeneration in individual animals. We found no evidence for the existence of multipotent progenitors. Instead, multiple cell types, including epidermal cells, intra-ray fibroblasts, and osteoblasts, contribute to the newly regenerated tissue while remaining highly restricted with respect to their developmental identity. Our studies further demonstrate that the regenerating fin consists of many repeating blastema "units" dedicated to each fin ray. These blastemas each have an organized structure of lineage restricted, dedifferentiated cells that cooperate to regenerate the caudal fin.
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