During limb regeneration adult tissue is converted into a zone of undifferentiated progenitors called the blastema that reforms the diverse tissues of the limb. Previous experiments have led to wide acceptance that limb tissues dedifferentiate to form pluripotent cells. Here we have reexamined this question using an integrated GFP transgene to track the major limb tissues during limb regeneration in the salamander Ambystoma mexicanum (the axolotl). Surprisingly, we find that each tissue produces progenitor cells with restricted potential. Therefore, the blastema is a heterogeneous collection of restricted progenitor cells. On the basis of these findings, we further demonstrate that positional identity is a cell-type-specific property of blastema cells, in which cartilage-derived blastema cells harbour positional identity but Schwann-derived cells do not. Our results show that the complex phenomenon of limb regeneration can be achieved without complete dedifferentiation to a pluripotent state, a conclusion with important implications for regenerative medicine.

Vertebrate body axis extension involves progressive generation and subsequent differentiation of new cells derived from a caudal stem zone; however, molecular mechanisms that preserve caudal progenitors and coordinate differentiation are poorly understood. FGF maintains caudal progenitors and its attenuation is required for neuronal and mesodermal differentiation and to position segment boundaries. Furthermore, somitic mesoderm promotes neuronal differentiation in part by downregulating Fgf8. Here we identify retinoic acid (RA) as this somitic signal and show that retinoid and FGF pathways have opposing actions. FGF is a general repressor of differentiation, including ventral neural patterning, while RA attenuates Fgf8 in neuroepithelium and paraxial mesoderm, where it controls somite boundary position. RA is further required for neuronal differentiation and expression of key ventral neural patterning genes. Our data demonstrate that FGF and RA pathways are mutually inhibitory and suggest that their opposing actions provide a global mechanism that controls differentiation during axis extension.

Retinoic acid (RA) is involved in the induction of neural differentiation, motor axon outgrowth and neural patterning. Like other developmental molecules, RA continues to play a role after development has been completed. Elevated RA signalling in the adult triggers axon outgrowth and, consequently, nerve regeneration. RA is also involved in the maintenance of the differentiated state of adult neurons, and disruption of RA signalling in the adult leads to the degeneration of motor neurons (motor neuron disease), the development of Alzheimer's disease and, possibly, the development of Parkinson's disease. The data described here strongly suggest that RA could be used as a therapeutic molecule for the induction of axon regeneration and the treatment of neurodegeneration.

SUMMARYEvolutionary modification has produced a spectrum of animal defense traits to escape predation, including the ability to autotomize body parts to elude capture1,2. Following autotomy, the missing part is either replaced through regeneration (e.g. urodeles, lizards, arthropods, crustaceans) or is permanently lost (mammals). While most autotomy involves the loss of appendages (e.g. leg, cheliped, antennae, tail), skin autotomy can occur in certain taxa of scincid and gekkonid lizards3. Here we report the first demonstration of skin autotomy in Mammalia (African spiny mice, Acomys). Mechanical testing revealed a propensity for skin to tear under very low tension and the absence of a fracture plane. Following skin loss, rapid wound contraction was followed by hair follicle regeneration in dorsal skin wounds. Surprisingly, we found regenerative capacity in Acomys extended to ear holes where they exhibited complete regeneration of hair follicles, sebaceous glands, dermis, and cartilage. Salamanders capable of limb regeneration form a blastema (a mass of lineage-restricted progenitor cells4) following limb loss, and our findings suggest that ear tissue regeneration in Acomys may proceed through assembly of a similar structure. This study underscores the importance of investigating regenerative phenomena outside of traditional model organisms and suggests that mammals may retain a higher capacity for regeneration than previously believed. As re-emergent interest in regenerative medicine seeks to isolate molecular pathways controlling tissue regeneration in mammals, Acomys may prove useful in identifying mechanisms to promote regeneration in lieu of fibrosis and scarring.

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