The roles of endogenous retinoid signaling in organ and appendage regeneration.

The ability to regenerate injured or lost body parts has been an age-old ambition of medical science. In contrast to humans, teleost fish and urodele amphibians can regrow almost any part of the body with seeming effortlessness. Retinoic acid is a molecule that has long been associated with these impressive regenerative capacities. The discovery 30 years ago that addition of retinoic acid to regenerating amphibian limbs causes "super-regeneration" initiated investigations into the presumptive roles of retinoic acid in regeneration of appendages and other organs. However, the evidence favoring or dismissing a role for endogenous retinoids in regeneration processes remained sparse and ambiguous. Now, the availability of genetic tools to manipulate and visualize the retinoic acid signaling pathway has opened up new routes to dissect its roles in regeneration. Here, we review the current understanding on endogenous functions of retinoic acid in regeneration and discuss key questions to be addressed in future research.