Like other organs, vertebrate appendages are composed of complex tissues that originate from multiple germ layers. The limb, for example, consists of epidermis and a peripheral nervous system, both derived from ectoderm, and other internal tissues such as muscle, bone, dermis, and blood vessels, which have a mesodermal origin. In a regeneration-competent vertebrate, damage or complete loss of an appendage initiates a regenerative response that typically involves the early formation of a growth zone of undifferentiated cells, the blastema, at the distal end of the stump. The origin of the newly formed blastemal cells and their fate during the regeneration process have been on-going topics of debate over the past century. Early studies using the regenerating salamander limb and tail indicated that injured multinucleated myofibers can dedifferentiate, give rise to mononucleate progeny, and contribute to the regenerating blastema. Tracing individually labeled myotubes after transplantation documented their capacity to redifferentiate into different lineages within the regenerate, indicating the multipotent nature of derived progenitors cells [1
]. Recent advances in generating green fluorescent protein (GFP)-expressing transgenic frogs, salamanders, and fish, combined with molecular marker analyses, have enabled in vivo
tracking of cells with high precision. Revisiting the open questions concerning the overall contribution and transdifferentiation of lineages, Kragl et al
] demonstrated that the salamander limb blastema primarily contains lineage-restricted progenitors that remain within their original lineage as they rebuild the lost tissue.
The first demonstration in a vertebrate that different tissues, such as muscle and nerve, are regenerated from distinct progenitor pools came from work on Xenopus
tadpole tail regeneration [3
]. These studies indicated that the activation of muscle-specific stem cells (that is, Pax7+ satellite cells localized adjacent to mature fibers, rather than dedifferentiation, drive muscle regeneration in premetamorphic frogs. In addition, the new study by Rodrigues and colleagues [4
] with amputated zebrafish larvae tails produced no evidence of dedifferentiation of the myofibers. Ultrastructural and gene expression data, however, revealed signs of incomplete dedifferentiation in regenerating tadpole tail muscle fibers. This unexpected phenotype might indicate that partial cellular dedifferentiation is sufficient to condition the muscle into a regeneration program, which might not just comprise the myofiber but also could include the activation of satellite cells. A lineage restriction for bone has also been documented in regenerating zebrafish fins, although a cycle of osteoblast dedifferentiation and redifferentiation was demonstrated during blastema formation [5
]. In mammals, appendage regeneration is limited to the digit tip, permitting the study of cartilage, bone, epidermal, and nervous tissues but not of muscle tissue as this lineage is not present in this distally amputated tissue. Using the adult limb [6
] or neonatal limb model [7
], in combination with tissue-specific and inducible mouse cre-reporter lines, these two conceptually similar lineage analyses reached the same conclusion: during mammalian digit-tip regeneration, tissue-resident stem or progenitor cells are fate restricted.
Thus, the recent data from frog, salamander, fish, and mouse models support the hypothesis that lineage restriction during regeneration is the norm. Apparently, each tissue provides a distinct progenitor cell pool to the regeneration blastema, indicating that the vertebrate blastema is a heterogeneous population of cells that have different tissue origins and restricted potentials, which together coordinately regenerate the complex appendage. These studies did not, however, address or conclusively answer the question of whether dedifferentiation occurs within a specific lineage. By contrast, in salamanders, abundant data exist for skeletal muscle dedifferentiation. This finding is supported by recent studies in salamander and zebrafish cardiac muscle regeneration, where dedifferentiation of heart muscle cells results in expansion and redifferentiation to the original cell type [8
]. Cre/loxP-based lineage tracing to compare the fates of skeletal muscle fibers and satellite cells will be crucial in finally determining the significance of skeletal muscle dedifferentiation versus stem cell activation in this lineage.