In contrast to mammals, salamanders can regenerate complex structures after injury, including entire limbs. A central question is whether the generation of progenitor cells during limb regeneration and mammalian tissue repair occur via separate or overlapping mechanisms. Limb regeneration depends on the formation of a blastema, from which the new appendage develops. Dedifferentiation of stump tissues, such as skeletal muscle, precedes blastema formation, but it was not known whether dedifferentiation involves stem cell activation. We describe a multipotent Pax7+ satellite cell population located within the skeletal muscle of the salamander limb. We demonstrate that skeletal muscle dedifferentiation involves satellite cell activation and that these cells can contribute to new limb tissues. Activation of salamander satellite cells occurs in an analogous manner to how the mammalian myofiber mobilizes stem cells during skeletal muscle tissue repair. Thus, limb regeneration and mammalian tissue repair share common cellular and molecular programs. Our findings also identify satellite cells as potential targets in promoting mammalian blastema formation.

Urodele amphibians, like the newt, are the "champions of regeneration" as they are able to regenerate many body parts and tissues. Previous experiments, however, have suggested that the newt heart has only a limited regeneration capacity, similar to the human heart. Using a novel, reproducible ventricular resection model, we show for the first time that adult newt hearts can fully regenerate without any evidence of scarring. This process is governed by increased proliferation and the up-regulation of cardiac transcription factors normally expressed during developmental cardiogenesis. Furthermore, we are able to identify cells within the newly regenerated regions of the myocardium that express the LIM-homeodomain protein Islet1 and GATA4, transcription factors found in cardiac progenitors. Information acquired from using the newt as a model organism may help to shed light on the regeneration deficits demonstrated in damaged human hearts.

SUMMARY:In Duchenne muscular dystrophy patients, the pathological hallmark of the disease, namely, the chronic accumulation of sclerotic scar tissue in the interstitial space of skeletal muscle is attributed to manifestation of secondary pathological processes. Such anomalous generation of matrix protein is thought to be driven by the continuous degeneration and regeneration of muscle both in Duchenne Muscular Dystrophy and in the mdx mouse homolog. We examined mdx and the control strain C57bl/10 mice over a range of ages with respect to the amounts of collagen present in muscles and other organs, finding that the mdx have significantly higher collagen content at later time points in their kidney and lung as well as their muscles. Surprisingly, when we bred the mdx mice on the nu/nu background, the time course of fibrogenesis was modified depending on the tissue and the collagen content was significantly different in age-matched mice. Transplantation of normal thymic tissue into the mdx-nu/nu mice replenished their T-cells and concomitantly altered the collagen content in their tissues to levels comparable with those in immunocompetent mdx mice. This suggests that T-cells play a role in the onset of the fibrotic events that undermines the ability of dystrophic muscle to regenerate. (Lab Invest 2000, 80:881-891). D uchenne muscular dystrophy (DMD) was originally described as a myosclerosis (Duchenne, 1868), in recognition of the conspicuous deposition of collagenous scar tissue within the muscles. Fibrosis entails the excessive and inappropriate deposition of collagenous extracellular matrix. It is a central feature of many chronic diseases because it tends to disrupt and destroy the function of tissue, irrespective of the organ involved, be it kidney, heart, skin or skeletal muscle and is a major cause of suffering and death. It has also been suggested that this connective tissue proliferation may be important in the pathogenesis of the disease (Ionasecu and Ionasecu, 1982). Certainly, in primary myopathies such as Duchenne and congenital muscular dystrophies, striking increases in the levels of matrix proteins are a major histopathological feature (Stephens et al, 1982). In skeletal muscle, in addition to its obvious impact on the mechanical function of the tissue, progressive fibrosis may have widespread effects on a variety of mechanisms, which are crucial to the proper function of this tissue. Thus, the mechanisms of vascular and extravascular perfusion, which are critical for the supply of nutrients and removal of waste products in this metabolically active tissue, are likely to be compromised by the accumulation of dense, sclerotic scar tissue in the interfibre spaces. It has also been suggested that this scar tissue may itself assume a pathogenic role and contribute to the disease progression by interfering with effective muscle regeneration and re-innervation (Lipton, 1979).Experimental investigation of dystrophinopathies has been rendered possible by the advent of authentic animal models of DMD, of whic...

Duchenne muscular dystrophy was initially described as a myosclerosis because of the conspicuous progression of interstitial fibrosis. Using the mdx mouse homologue, we have shown previously that the accumulation of intramuscular collagen is profoundly influenced by the presence or absence of T lymphocytes. Here we have used thymectomy and antibody depletion to examine the effect of ablating CD4 or CD8 or both subsets of T lymphocytes on skeletal muscle fibrosis in mdx and C57BL10 (wild-type) mice. Depletion of either or both subsets at 4 weeks of age did not influence fibrosis in mdx mice, as determined by measuring hydroxyproline levels and collagen deposition in diaphragm. Additionally, expression of transforming growth factor-beta1, which is implicated in collagen deposition, either decreased (mdx mice) or increased (C57BL/10 mice) after double CD4/8 depletion. Our data suggest that depletion of lymphoid cells may affect the tight regulatory control of transforming growth factor-beta1, with possible pleiotropic effects, and more importantly, that the fibrotic process is self-sustaining from a very early stage.

Salamander limb regeneration depends on local progenitors whose progeny are recruited to the new limb. We previously identified a Pax7(+) cell population in skeletal muscle whose progeny have the potential to contribute to the regenerating limb. However, the plasticity of individual Pax7(+) cells, as well as their recovery within the new limb, was unclear. Here, we show that Pax7(+) cells remain present after multiple rounds of limb amputation/regeneration. Pax7(+) cells are found exclusively within skeletal muscle in the regenerating limb and proliferate where the myofibers are growing. Pax7 is rapidly down-regulated in the blastema, and analyses of clonal derivatives show that Pax7(+) cell progeny are not restricted to skeletal muscle during limb regeneration. Our data suggest that the newt regeneration blastema is not entirely a composite of lineage-restricted progenitors. The results demonstrate that except for a transient and subsequently blunted increase, skeletal muscle satellite cells constitute a stable pool of reserve cells for multiple limb regeneration events.-Morrison, J. I., Borg, P., Simon, A. Plasticity and recovery of skeletal muscle satellite cells during limb regeneration.