The limb blastemal cells of an adult salamander regenerate the structures distal to the level of amputation, and the surface protein Prod 1 is a critical determinant of their proximodistal identity. The anterior gradient protein family member nAG is a secreted ligand for Prod 1 and a growth factor for cultured newt blastemal cells. nAG is sequentially expressed after amputation in the regenerating nerve and the wound epidermis-the key tissues of the stem cell niche-and its expression in both locations is abrogated by denervation. The local expression of nAG after electroporation is sufficient to rescue a denervated blastema and regenerate the distal structures. Our analysis brings together the positional identity of the blastema and the classical nerve dependence of limb regeneration.

Most but not all phyla include examples of species that are able to regenerate large sections of the body plan. The mechanisms underlying regeneration on this scale are currently being studied in a variety of contexts in both vertebrates and invertebrates. Regeneration generally involves the formation of a wound epithelium after transection or injury, followed by the generation of regenerative progenitor cells and morphogenesis to give the regenerate. Common mechanisms may exist in relation to each of these aspects. For example, the initial proliferation of progenitor cells often depends on the nerve supply, whereas morphogenesis reflects the generation of positional disparity between adjacent cells-the principle of intercalation. These mechanisms are reviewed here across a range of contexts. We also consider the evolutionary origins of regeneration and how regeneration may relate to both agametic reproduction and to ontogeny.

The regeneration of complex structures in adult salamanders depends on mechanisms that offer pointers for regenerative medicine. These include the plasticity of differentiated cells and the retention in regenerative cells of local cues such as positional identity. Limb regeneration proceeds by the local formation of a blastema, a growth zone of mesenchymal stem cells on the stump. The blastema can regenerate autonomously as a self-organizing system over variable linear dimensions. Here we consider the prospects for limb regeneration in mammals from this viewpoint.T he goal of regenerative medicine is to restore cells, tissues, and structures that are lost or damaged after disease, injury, or aging. The current approaches are influenced by our understanding of embryonic development, of tissue turnover and replacement in adult animals (1-3), and by tissue engineering and stem cell biology (4). The regeneration of organs and appendages after injury occurs in diverse animal groups and provides another important viewpoint, in addition to the demonstration that complex adult tissues can be rebuilt. The lessons of biological regeneration have not been extensively assimilated, in part because this attribute appears remote and exceptional from a mammalian perspective. This Review is concerned principally with lessons from regeneration in salamanders, the species of adult vertebrates that possesses the most extensive abilities (5, 6). We identify three properties of regeneration in salamandersautonomy, scaling, and plasticity-and discuss some of the cellular and molecular mechanisms underlying them. It may be desirable to implement these properties in the context of mammalian regeneration.Regenerative medicine currently uses three approaches ( Fig. 1) (4): the implantation of stem cells to build new structures, the implantation of cells pre-primed to develop in a given direction, and the stimulation of endogenous cells to replace missing structures. Each of the different aspects identified in the first two examples-the generation of an appropriate cohort of regenerative cells, their regulated division and differentiation, and the restoration of the appropriate part of the structure-must be evoked from endogenous cells in the third approach. These processes operate in adult animals that regenerate, and in addition, the regenerative response must be initiated by signals responsive to tissue injury or removal. One candidate signal in salamanders is the local activation of thrombin, a regulator of hemostasis and other aspects of the response to injury, as well as an activator of S phase (the phase of chromosome replication) reentry in differentiated cells (7-9).A salamander can regenerate its limbs and tail, upper and lower jaws, ocular tissues such as the lens and retina, the intestine, and small sections of the heart (10-13). The various contexts for regeneration do not present an equivalent degree of difficulty. To restore the intricate and discontinuous pattern of the vertebrate limb is a different proposition from replac...

Many regeneration contexts require the presence of regenerating nerves as a transient component of the progenitor cell niche. Here we review nerve involvement in regeneration of various structures in vertebrates and invertebrates. Nerves are also implicated as persistent determinants in the niche of certain stem cells in mammals, as well as in Drosophila. We consider our present understanding of the cellular and molecular mechanisms underlying nerve dependence, including evidence of critical interactions with glia and non-neural cell types. The example of the salamander aneurogenic limb illustrates that developmental interactions between the limb bud and its innervation can be determinative for adult regeneration. These phenomena provide a different perspective on nerve cells to that based on chemical and electrical excitability.