Altering the spatial relationships of embryonic or postembryonic tissues is one of the oldest and most valuable techniques for investigating mechanisms of development.Urodele amphibians(salamanders and newts) are unique among vertebrates in their ability to regenerate several complex structures, including limbs, via the dedifferentia-tion of mature cells to embryonic-like progenitor cells, which proliferate to form ablastemathat self-organizes into the missing structures. Thespatial relationships of tissuesin the regenerating limb can be altered surgically either prior to amputation or after blastema formation or chemically by the administration ofretinoic acid(RA). These alterations evoke regenerative abnormalities that can be correlated with cell interactions, leading to the expression of specific genes in organized spatiotemporal patterns (1 -3 ). Such experiments are thus valuable as a first level of functional genomic analysis, as well as providing a wealth of information aboutcell interactionsduring regeneration.
