Cellular differences in the regeneration of murine skeletal muscle: a quantitative histological study in SJL/J and BALB/c mice

Myoid cell density in the thymus is reduced during mdx dystrophy and after muscle crush

Thymic myoid cells share structural and behavioural features with cells of the skeletal muscle lineage: they express regulatory genes and contractile proteins, and they can form myofibers in culture. Historically, those features suggested that myoid cells could be precursors for muscle repair in addition to the satellite cells in muscle that are typically designated as the only muscle precursors. Muscles of the mutant mdx dystrophic mouse strain have a large demand for precursors, which is greatest at a young age. In the present study, immunostaining for troponin T was used to localize myoid cells. We tested the hypothesis that the myoid cell population changes when there is a demand for muscle precursors and that these changes would be anticipated if myoid cells have a role as myogenic precursors or stem cells in muscle. Chronic demands for muscle precursors in mdx dystrophic mice were accompanied by lower myoid cell density in comparison with density in two normal strains (C57BL10/ScSn and Swiss Webster). Acute demand for precursors was accompanied by a sharp decline in thymic myoid cell density within 2 days after a crush injury to one tibialis anterior muscle in normal but not dystrophic animals. To standardize the developmental age of the thymus, density was determined in all animals at 28 days of age. Given the current interest in nonmuscle sources of myogenic stem cells, these data suggest that changes in the density of thymic myoid cells may accompany acute and chronic demands for muscle precursors. Further experiments are required to determine whether thymic myoid cells are participants in distant muscle cell proliferation, new fiber formation, or the establishment of new stem cells in regenerated muscle.Key words: thymus, myoid cells, troponin T, MyoD, tissue repair, myoblasts, mdx dystrophy.

Regeneration of skeletal and cardiac muscle in mammals: do nonprimate models resemble human pathology?

Most of the available information regarding the regenerative potential and compensatory remodeling of mammalian tissues has been obtained from nonprimate animals, mainly rodent experimental models. The increasing use of transgenic mice for studies of the mechanisms controlling organogenesis and regeneration also requires a clear understanding of their applicability as experimental models for studies of similar processes in humans and other mammals. Application of modern cell biology methods to studies of regenerative processes has provided new insights into similarity and differences in cellular responses to injury in the tissues of different mammalian species. During more than 200-million years of progressive divergent evolution of mammals, cellular mechanisms of tissue regeneration and compensatory remodeling evolved together with increasingly adaptive functional specialization and structural complexity of mammalian tissues and organs. Rodents represent a phylogenetically ancient order of mammals that has conservatively retained a number of morphofunctional characteristics of early representatives of this class, which include enhanced regenerative capacity of tissues. A comparative analysis of regenerative processes in skeletal and cardiac muscle, as well as in several other mammalian tissues, shows that time courses and intensities of regeneration in response to the same type of injury vary even within taxonomically related species (e.g., rat, mouse, and hamster). The warm bloodedness of mammals facilitated the development of more complex mechanisms of metabolic, immune, and neurohumoral regulation, which resulted in a stronger dependence of regenerative processes on vascularization and innervation. For this reason, interspecies modifications of regenerative responses are limited by the capacity of the animal to resorb rapidly the foci of necrosis and to revascularize and reinnervate the volume of the regenerating tissue. These differences, among other factors, result in significantly lower rates of reparative regeneration in mammals possessing larger body sizes than rodents. A review of these data strongly indicates that the phylogenetic age and biological differences between different species should be taken into account before extrapolation of regenerative properties of nonprimate tissues on the regenerative responses in the primates.

Age-associated changes in the response of skeletal muscle cells to exercise and regeneration

This paper looks at the effects of aging on the response of skeletal muscle to exercise from the perspective of the behavior of muscle precursor cells (widely termed satellite cells or myoblasts) and regeneration. The paper starts by outlining the ways in which skeletal muscle can respond to damage resulting from exercise or other trauma. The age-related changes within skeletal muscle tissue and the host environment that may affect the proliferation and fusion of myoblasts in response to injury in old animals are explored. Finally, in vivo and in vitro data concerning the wide range of signaling molecules that stimulate satellite cells and other aspects of regeneration are discussed with respect to aging. Emphasis is placed on the important role of the host environment, inflammatory cells, growth factors and their receptors (particularly for FGF-2), and the extracellular matrix.

Heterogeneity among muscle precursor cells in adult skeletal muscles with differing regenerative capacities

Skeletal muscle has a remarkable capacity to regenerate after injury, although studies of muscle regeneration have heretofore been limited almost exclusively to limb musculature. Muscle precursor cells in skeletal muscle are responsible for the repair of damaged muscle. Heterogeneity exists in the growth and differentiation properties of muscle precursor cell (myoblast) populations throughout limb development but whether the muscle precursor cells differ among adult skeletal muscles is unknown. Such heterogeneity among myoblasts in the adult may give rise to skeletal muscles with different regenerative capacities. Here we compare the regenerative response of a masticatory muscle, the masseter, to that of limb muscles. After exogenous trauma (freeze or crush injuries), masseter muscle regenerated much less effectively than limb muscle. In limb muscle, normal architecture was restored 12 days after injury, whereas in masseter muscle, minimal regeneration occurred during the same time period. Indeed, at late time points, masseter muscles exhibited increased fibrous connective tissue in the region of damage, evidence of ineffective muscle regeneration. Similarly, in response to endogenous muscle injury due to a muscular dystrophy, widespread evidence of impaired regeneration was present in masseter muscle but not in limb muscle. To explore the cellular basis of these different regenerative capacities, we analyzed the myoblast populations of limb and masseter muscles both in vivo and in vitro. From in vivo analyses, the number of myoblasts in regenerating muscle was less in masseter compared with limb muscle. Assessment of population growth in vitro indicated that masseter myoblasts grow more slowly than limb myoblasts under identical conditions. We conclude that the impaired regeneration in masseter muscles is due to differences in the intrinsic myoblast populations compared to limb muscles.

Studies of the dynamics of skeletal muscle regeneration: the mouse came back!

Regeneration of skeletal muscle tissue includes sequential processes of muscle cell proliferation and commitment, cell fusion, muscle fiber differentiation, and communication between cells of various tissues of origin. Central to the process is the myosatellite cell, a quiescent precursor cell located between the mature muscle fiber and its sheath of external lamina. To form new fibers in a muscle damaged by disease or direct injury, satellite cells must be activated, proliferate, and subsequently fuse into an elongated multinucleated cell. Current investigations in the field concern modulation of the effectiveness of skeletal muscle regeneration, the regeneration-specific role of myogenic regulatory gene expression distinct from expression during development, the impact of growth and scatter factors and their respective receptors in amplifying precursor numbers, and promoting fusion and maturation of new fibers and the ultimate clinical therapeutic applications of such information to alleviate disease. One approach to muscle regeneration integrates observations of muscle gene expression, proliferation, myoblast fusion, and fiber growth in vivo with parallel studies of cell cycling behaviour, endocrine perturbation, and potential biochemical markers of steps in the disease-repair process detected by magnetic resonance spectroscopy techniques. Experiments on muscles from limb, diaphragm, and heart of the mdx dystrophic mouse, made to parallel clinical trials on human Duchenne muscular dystrophy, help to elucidate mechanisms underlying the positive treatment effects of the glucocorticoid drug deflazacort. This review illustrates an effective combination of in vivo and in vitro experiments to integrate the distinctive complexities of post-natal myogenesis in regeneration of skeletal muscle tissue.Key words: satellite cell, cell cycling, HGF/SF, c-met receptor, MyoD, myogenin, magnetic resonance spectroscopy, mdx dystrophic mouse, deflazacort.

Pax7 includes two polymorphic homeoboxes which contain rearrangements associated with differences in the ability to regenerate damaged skeletal muscle in adult mice

Pax7 is a paired-type homeobox gene which has previously been shown to play an important role in skeletal muscle formation. It is expressed in skeletal muscle of the limbs during embryogenesis and in adulthood. The aims of this study were firstly to determine the degree of polymorphism of Pax7 amongst inbred laboratory mice using Southern blotting and Pax7 regional specific sub-probes. Secondly, functional studies were performed on mice with each of the different structural forms of Pax7 to determine whether they were associated with differences in the ability to regenerate damaged skeletal muscle. Four different allelic forms of Pax7 have now been identified in laboratory mice indicating that the previously reported DNA sequence of Pax7 is not applicable to all laboratory mice. Hybridisation patterns of TaqI digested DNA representing each of the different Pax7 alleles with the Pax7 specific sub-probes suggested that in contrast to previous findings, Pax7 is associated with two highly polymorphic homeoboxes. The presence of two homeoboxes in BALB/c mice has been confirmed by DNA sequencing. Results of functional studies have also shown that the ability to regenerate damaged skeletal muscle in adult mice is strongly associated with the presence of a 0.15-kb TaqI fragment derived from one of the homeoboxes.

The identification of myogenic cells in skeletal muscle, with emphasis on the use of tritiated thymidine autoradiography and desmin antibodies

The identification of myogenic precursor cells (mpc) is a key factor in determining the early events in the myogenesis and regeneration of skeletal muscle. Although satellite cells have long been established as the providers of myoblastic cells, very little is really known (apart from their anatomical location in relation to muscle fibres and their ability to migrate) about the precise role of satellite cells in myogenesis. Numerous techniques for labelling mpc have been devised, but none of these has proven to be completely reliable in firmly establishing the origin of myogenic cells. The use of tritiated thymidine to label DNA in proliferating mpc (which are not specifically distinguishable at the time) and the subsequent location of their labelled progeny in myotube nuclei has revealed a great deal of data on the timing of myogenesis, but not about the nature of mpc themselves. DNA synthesis can also be detected by antibodies to the thymidine analogue, bromodeoxyuridine, and also by antibody staining for proliferating nuclear cell antigen. Like tritiated thymidine, these other markers are not specific for muscle but are general markers for DNA synthesis. In situ hybridisation of various muscle-specific genetic markers and their products has been informative, as has immunolabelling of myogenin, MyoD1 and desmin. Desmin labelling has been particularly instructive in identifying mpc because it is one of the first muscle-specific proteins to be produced in mpc. This review covers some of the techniques mentioned above and their usefulness in determining the early events in myogenesis.

Levels of MyoD protein expression following injury of mdx and normal limb muscle are modified by thyroid hormone

Thyroid hormone (T3) affects muscle development and muscle regeneration. It also interacts with the muscle regulatory gene MyoD in culture and affects myoblast proliferation. We studied the localization of MyoD protein using a well-characterized polyclonal antibody for immunohistochemistry. Relative numbers of myogenic precursor cells per field were identified by their MyoD expression during muscle regeneration in normal and mdx dystrophic mice, with particular reference to the expression in mononuclear cells and myotubes at various T3 levels. In regeneration by normal muscles, relatively few MyoD+ nuclei per field were present in mononuclear cells of euthyroid and hypothyroid mice. MyoD staining of mononuclear cell nuclei was approximately doubled in fields of regenerating muscles of normal hyperthyroid compared to euthyroid mice, and was observed in precursors that appeared to be aligned before fusion into myotubes. In mdx regenerating muscle, twofold more mononuclear cells positive for MyoD were present in all three treatment groups compared to normal muscles regenerating under the same conditions. Localization was similar to the pattern in normal euthyroid mice. However, in muscles regenerating in hyperthyroid mdx mice, both mononuclear cell nuclei and centrally located nuclei in a subpopulation (about 15%) of new myotubes formed after the crush injury were intensely stained for MyoD protein. The changes observed are consistent with reports on T3-induced alteration of muscle repair, and propose a link between MyoD regulation and the accelerated differentiation during regeneration under high T3 conditions. (J Histochem Cytochem 46:59-67, 1998)

The host environment determines strain-specific differences in the timing of skeletal muscle regeneration: cross-transplantation studies between SJL/J and BALB/c mice

The difference in the timing of the regeneration process of skeletal muscle between SJL/J and BALB/c mice was investigated using grafts of whole skeletal muscle (both autografts and allografts). Histological, autoradiographic and immunohistochemical techniques were used in the investigation. Infiltration of leucocytes into autografts, numbers of desmin-positive myogenic cells and myotube formation were all more advanced in the SJL/J compared with BALB/c mice. Furthermore, autoradiographic evidence showed that myoblasts in the SJL/J autografts were synthesising DNA 12 h earlier than myoblasts in BALB/c autografts. In allografts, where SJL/J host mice received BALB/c grafts, and vice versa, leucocyte infiltration and myotube formation occurred earlier in the BALB/c muscles grafted into SJL/J hosts, than in the reverse situation with BALB/c hosts. The results show that, at least for whole muscle grafts, it is the host environment which determines the speed and outcome of the regenerative process.

Production of consistent crush lesions of murine skeletal muscle in vivo using an electromechanical device

The crush model of injury in skeletal muscle is widely used in the investigation of tissue degeneration and regeneration. Previously, such trauma has been induced by using forceps to crush the muscle, commonly applying sufficient pressure to bring the mid-arms of the forceps together. This report introduces a reliable electromechanical device designed to generate reproducible focal lesions in skeletal muscle of mice. The tibialis anterior was crushed in 17 young adult mice. Two days after injury, the muscles were examined microscopically. By morphometric analysis, it was determined that the volumes of the lesions produced were similar (mean 0.499 mm3 +/- 0.098, range 0.278 - 0.601 mm3), and that the full extent of the damaged muscle was easily distinguished and readily quantifiable. This will allow a more precise comparison in future investigations into regenerative differences between age groups, satellite cell activation and the inflammatory response.

Macrophages and dendritic cells in normal and regenerating murine skeletal muscle

Mononuclear phagocytes and MHC class II+ dendritic cells (DC) were identified in frozen sections of skeletal muscle using a panel of pan-specific antimacrophage (MOMA-2, SER-4, Mac-1, F4/80), anti-major histocompatibility complex (MHC) class II (M5/114) and anti-DC (NLDC-145, N418, M342) monoclonal antibodies. Uninjured and regenerating skeletal muscle were investigated in SJL/J and BALB/c mice, strains with known differences in muscle regenerative capacity. Resident tissue macrophages and MHC class II+ DC were present within uninjured mouse muscle. A subpopulation of DC were positive for the pan-DC markers, N418 and M342, and negative for the lymphoid DC marker NLDC-145. Following crush injury, the macrophage population increased by day 2, became marked by day 3, and had decreased by day 6. In contrast, the number of MHC class II+ cells around the injury site increased steadily after injury and remained high at day 6. The numbers of macrophages and DC detected by immunohistochemical staining were consistently higher in SJL/J than BALB/c muscles. This study confirms that macrophages are a significant component of normal murine skeletal muscle and that these cells increase dramatically after injury. Furthermore the data also reveal for the first time that DC are present in normal skeletal muscle and that MHC class II+ cells, including DC, increase after injury. The presence of DC in muscle has important implications for the understanding of the immunobiology of muscle and immune-mediated processes such as the host versus graft responses following muscle transplants and autoimmune diseases affecting this tissue.

Spontaneous myopathy in the SJL/J mouse: pathology and strength loss

Myopathy has been found to develop spontaneously in 100% of SJL/J mice between 6 and 8 months of age. Extent of muscular involvement and mouse strength were quantified in SJL/J mice and Balb/c control mice 2-16 months old. Muscle from young SJL/J mice exhibited histopathological abnormalities and occasional inflammatory infiltrate. By 6 months, 78% of SJL/J mice had developed active myopathy. By 8 months, all SJL/J mice examined had active disease with a mean of 12.9% of muscle fibers affected. Replacement of muscle fibers by fat and/or collagen began at 10 months and was pronounced by 14 months. Significant decreases in strength scores (total body pulling force) at 6 months and 10 months of age reflected the onset of active myopathy and the onset of muscle degeneration, respectively. The spontaneous onset and 100% incidence of myopathy in the SJL/J mouse line should provide a useful model for idiopathic myopathy.

Rapid death of injected myoblasts in myoblast transfer therapy

Myoblast transplantation has been proposed as a potential therapy for Duchenne muscular dystrophy (DMD). A Y-chromosome-specific probe was used to track the fate of donor male myoblasts injected into dystrophic muscles of female mdx mice (which are an animal model for DMD). In situ analysis with the Y-probe showed extremely poor survival of isolated normal male (C57B1/10Sn) donor myoblasts after injection into injured or uninjured muscles of dystrophic (mdx) and normal (C57B1/10Sn) female host mice. A decrease in the numbers of donor (male) myoblasts was seen from 2 days and was marked by 7 days after injection: few or no donor myoblasts were detected in host muscles examined at 3-12 months. There was limited movement of the injected donor myoblasts and fusion into host myofibers was rare. The results of this study strongly suggest that the failure of clinical trials of myoblast transplantation in boys with DMD may have been due to rapid and massive death of the donor myoblasts soon after myoblast injection.

MyoD is required for myogenic stem cell function in adult skeletal muscle

To investigate the function of MyoD in adult skeletal muscle, we interbred MyoD mutant mice with mdx mice, a model for Duchenne and Becker muscular dystrophy. Mice lacking both MyoD and dystrophin displayed a marked increase in severity of myopathy leading to premature death, suggesting a role for MyoD in muscle regeneration. Examination of MyoD mutant muscle revealed elevated numbers of myogenic cells; however, myoblasts derived from these cells displayed normal differentiation potential in vitro. Following injury, MyoD mutant muscle was severely deficient in regenerative ability, and we observed a striking reduction in the in vivo proliferation of myogenic cells during regeneration. Therefore, we propose that the failure of MyoD-deficient muscle to regenerate efficiently is not caused by a reduction in numbers of satellite cells, the stem cells of adult skeletal muscle, but results from an increased propensity for stem-cell self-renewal rather than progression through the myogenic program.

Hyperthyroidism impairs early repair in normal but not dystrophic mdx mouse tibialis anterior muscle. An in vivo study

The effect of hyperthyroidism on muscle repair was examined in mdx and control mice injected with triiodothyronine (T3) for 4 weeks. On day 24 of treatment, the right tibialis anterior (TA) muscle was crush-injured; 3 days later, mice received intraperitoneal [3H]thymidine to label newly synthesized DNA. One day later, muscles from both limbs were removed to study the severity of dystrophy (uncrushed muscle) and the regeneration response (crushed muscle). In uncrushed TA muscle, the area of active dystrophy (fiber damage and infiltration as a proportion of muscle cross-sectional area) was reduced by half after T3 treatment. Uncrushed muscle fiber diameter was lower in T3-treated control muscles. In crushed muscles, the diameter of new myotubes was larger in mdx mice than in controls and was reduced after T3 treatment in control regenerating muscle. In the same muscles, developmental myosin heavy chain was present in new myotubes and in small numbers of mononuclear cells (possibly differentiating myoblasts) near new myotubes and surviving fibers. Myotube density in the regenerating muscles was not changed by T3 treatment, although the number of myotube nuclei per field was decreased in control and increased in mdx T3-treated mice. Results extend previous reports of T3 effects on dystrophy and the strain difference in muscle precursor cell (mpc) proliferation. The results also suggest the hypothesis that excess T3 affects muscle regeneration either by reducing mpc proliferation or by increasing mpc fusion early in regeneration in control and mdx muscle.

The exogenous administration of basic fibroblast growth factor to regenerating skeletal muscle in mice does not enhance the process of regeneration

The effects, in vivo, of the exogenous administration of bFGF on myogenesis of regenerating skeletal muscle was assessed either morphometrically or autoradiographically in three separate models of muscle injury in mice: crush-injured, denervated, and dystrophic (mdx) muscles. The bFGF was administered at various doses and different time schedules, sometimes in combination with heparin, into injured tibialis anterior muscles of mice. Delivery of the bFGF was either by direct intramuscular injection or by the sustained release from 888polymers (Hydron or Elvax) implanted into the muscles. The bioactivity of bFGF was confirmed in vitro by measuring its ability to stimulate the proliferation of BALB/c-3T3 fibroblasts and muscle precursor cell lines. The ability of bFGF to stimulate angiogenesis in vivo was confirmed by the implantation of controlled-release polymers containing bFGF into the normally avascular cornea of rats. No measurable effect of bFGF was seen in any of the models of skeletal muscle injury under these experimental conditions, indicating that the availability of biologically active bFGF is not a limiting factor in the regeneration of skeletal muscle following injury.

Enhancement of neovascularization in regenerating skeletal muscle by the sustained release of erucamide from a polymer matrix

The angiogenic agent erucamide (cis-13-docosenamide), incorporated into a polymeric biomaterial (Elvax 40P, a copolymer of ethylene and vinyl acetate), was used to determine whether angiogenesis can be increased in the regenerating skeletal muscle, and whether the enhanced revascularization improves the new muscle formation. The angiogenic nature of this lipid was confirmed in a rat cornea-micropocket assay, prior to insertion of small strips of the polymer containing either 3 micrograms, 300 micrograms erucamide or only polymer as a control into the mid-region of crush-injured tibialis anterior (TA) muscles of forty-five adult male BALB/c mice. All TA muscles were sampled ten days after injury and analyzed morphometrically. Statistical analyses of the mean blood vessel area density in lesions from twelve perfused TA muscles (three from each of the erucamide-treated or control group), revealed a dose-dependent angiogenic effect of erucamide: a dosage of 3 micrograms increased mean blood vessel area density to 5.1% compared to 2.0% in controls, due to numerous large caliber, thin-walled vessels, whereas the mean vessel area density in both the 30-micrograms (3.5%) and 300-micrograms (1.5%) doses were similar to controls. However, at all three doses tested, erucamide did not significantly alter the degree of new muscle formation, connective tissue deposition, or removal of necrotic debris.

The genotype of bone marrow-derived inflammatory cells does not account for differences in skeletal muscle regeneration between SJL/J and BALB/c mice

This study determined whether the genotype of bone marrow-derived inflammatory cells contributes to the more pronounced leukocytic exudation and extensive new muscle formation seen in SJL/J compared with BALB/c mice after a crush-injury (Mitchell et al. 1992). Female SJL/J mice were whole-body irradiated and reconstituted with male bone marrow from the BALB/c strain, and irradiated BALB/c females reconstituted with male SJL/J bone marrow. The mice were allowed to recover for 3 weeks and the tibialis anterior muscle (in a leg which had been protected from irradiation) was injured by crushing. At 3 and 10 days after injury the extent of necrotic debris, mononuclear leukocytic infiltration and new muscle formation was assessed in the muscles. The SJL/J mice reconstituted with BALB/c bone marrow showed extensive mononuclear leukocytic infiltration and clearance of necrotic debris when compared with BALB/c mice reconstituted with SJL/J bone marrow, and these strain-specific differences mirrored those seen with control bone marrow reconstituted hosts and non-irradiated hosts. The results show that the genotype of the bone marrow-derived macrophages is not responsible for the superior regeneration of crush-injured skeletal muscle in SJL/J mice, and it appears that factors intrinsic to the muscle tissue may be of central importance.

Retarded myogenic cell replication in regenerating skeletal muscles of old mice: an autoradiographic study in young and old BALBc and SJL/J mice

The patterns of skeletal muscle precursor cell replication after crush injury were compared by the use of autoradiographic techniques, in young (4-week-old) and old (39-week-old) BALBc and SJL/J mice. Similar comparisons were made between cut and crush lesions in old BALBc muscle. Muscle precursor cell replication commenced at 18-24 h after injury in both young and old muscles from both strains of mice. In young BALBc muscle the peak of myogenic activity at 60 h was 36 h earlier than in old mice. SJL/J muscle responded more rapidly than did BALBc: in young SJL/J the peak myogenic activity was at 46 h (14 h earlier than in young BALBc muscle), and in old SJL/J muscle the peak activity at 72 h was 24 h earlier than in old BALBc muscle. In all mice (both young and old) myogenic cell replication was substantially reduced by 120 h after injury. A comparison of the timing of muscle precursor cell replication in cut and crush lesions in old BALBc mice revealed a more rapid response in the cut lesion; this difference between the lesions is comparable with data from identical lesions in 6-8-week-old BALBc mice (McGeachie and Grounds 1987). However, the peak of myogenic replication in the older mice in the present study was some 26-36 h later than in the younger 6-8-week-old mice. These experiments show that, whilst muscle precursor cell replication commences at approximately the same time (about 24 h) after injury in young and old mice, the peak level of activity is delayed by some 24-36 h in old mice. In addition, the SJL/J mouse strain responds more rapidly and prolifically to muscle injury than does the BALBc strain.

Development and postnatal regulation of adult myoblasts

The myogenic precursor cells of postnatal and adult skeletal muscle are situated underneath the basement membrane of the myofibers. It is because of their unique positions that these precursor cells are often referred to as satellite cells. Such defined satellite cells can first be detected following the formation of a distinct basement membrane around the fiber, which takes place in late stages of embryogenesis. Like myoblasts found during development, satellite cells can proliferate, differentiate, and fuse into myofibers. However, in the normal, uninjured adult muscle, satellite cells are mitotically quiescent. In recent years several important questions concerning the biology of satellite cells have been asked. One aspect has been the relationship between satellite cells and myoblasts found in the developing muscle: are these myogenic populations identical or different? Another aspect has been the physiological cues that control the quiescent, proliferative, and differentiative states of these myogenic precursors: what are the growth regulators and how do they function? These issues are discussed, referring to previous work by others and further emphasizing our own studies on avian and rodent satellite cells. Collectively, the studies presented indicate that satellite cells represent a distinct myogenic population that becomes dominant in late stages of embryogenesis. Moreover, although satellite cells are already destined to be myogenic precursors, they do not express any of the four known myogenic regulatory genes unless their activation is induced in the animal or in culture. Furthermore, multiple growth factors are important regulators of satellite cell proliferation and differentiation. Our work on the role of one of these growth factors [platelet-derived growth factor (PDGF)] during proliferation of adult myoblasts is further discussed with greater detail and the possibility that PDGF is involved in the transition from fetal to adult myoblasts in late embryogenesis is brought forward.

Hypothyroidism prolongs and increases mdx muscle precursor proliferation and delays myotube formation in normal and dystrophic limb muscle

Hypothyroidism (induced by 8 weeks of oral 0.05% propylthiouracil) heightened the phenotype of mdx mouse dystrophin-deficient myopathy to more closely resemble human Duchenne muscular dystrophy. Muscle repair after crush injury to the tibialis anterior muscle (TA) in hypothyroid mdx mice showed decreased myotube formation and delayed debris removal. To investigate whether reduced muscle precursor cell proliferation can account for the effects of hypothyroidism on repair from injury, immunocytochemistry for neural cell adhesion molecule (NCAM) on muscle precursor cells and autoradiography to detect DNA synthesis were performed in control and mdx TA. The proportions of labelled polymorphonuclear leukocyte nuclei (PMN), myotube nuclei (MN), and total mononuclear cell nuclei (TLN, the majority being muscle precursors) were counted in defined areas of regenerating TA after 2 and 4 days recovery. MN and the numbers of activated satellite cell nuclei on intact fibers were counted in surviving areas. In the same muscle, earlier phases of regeneration were observed in areas distal than proximal to the injury. At 2 days of regeneration, labelled PMN were increased in treated compared with untreated mdx TA. In distal areas at 4 days, fewer muscle precursors had recently fused to myotubes in treated than in untreated mdx. In proximal areas 4 days (relatively late in repair), TLN data suggested that muscle precursor proliferation was greater in hypothyroid compared with untreated mdx TA. NCAM immunostaining was consistent with proliferation data and confirmed that there were more muscle precursors in mdx than in control regenerating muscle. These results suggest that hypothyroidism prolongs and increases the phase of replication by mdx muscle precursors and delays precursor fusion into myotubes in regeneration.

Evidence for adenine methylation within the mouse myogenic gene Myo-D1

Previous studies have indicated that there may be uncleavable TaqI sites (TCGA) within the mouse myogenic gene, Myo-D1. Fragments of DNA bearing most of the presumed insensitive TaqI sites have been reproduced using PCR. The presence of each of the originally uncleavable TaqI sites has been confirmed and each TaqI site has been shown to be sensitive to TaqI hydrolysis in PCR-synthesized genomic DNA. Since TaqI is inhibited by methylation of the adenine residue within its recognition sequence (but not by cytosine methylation), it is suggested that specific adenine bases are methylated in the coding region of Myo-DI and maintained throughout cell division. The same TaqI recognition sequences are insensitive to digestion in genomic DNA isolated from various mouse tissues including fetus, regenerating skeletal muscle and a myogenic cell line, all of which express Myo-D1. Thus, adenine methylation is not a modification of DNA following gametic fusion nor does it appear to play a major role in regulation of Myo-D1 expression.

Initiation of satellite cell replication in bupivacaine-induced myonecrosis

To determine how and when the satellite cells are stimulated to replicate in muscle regeneration, the rat soleus muscle was examined chronologically after bupivacaine-induced myonecrosis. Bromodeoxyuridine and desmin-positive mononuclear cells, indicating the start of satellite cell replication, were seen 25 h after bupivacaine treatment when macrophages had already invaded the sarcoplasm of necrotic fiber. These findings suggest that muscle regeneration starts as early as the time at which macrophages begin to scavenge necrotic material. Proliferating myoblasts increased in number, reaching a maximum at 49 h after myonecrosis, and decreased in number 3 days after the myoblasts fused with each other form myotubes. The satellite cell proliferation after bupivacaine-induced myonecrosis began at almost the same time as in crush injury, and earlier than after muscle transplantation using whole intact or minced muscle fragments. The earlier beginning and more rapid regenerating process probably resulted from the preservation of intact satellite cells, blood vessels and peripheral nerves in the bupivacaine-induced myonecrosis.

The effects of altered metabolism (hypothyroidism) on muscle repair in the mdx dystrophic mouse

After dystrophic damage, the limb muscles of the mdx mouse recover very effectively compared to muscles in Duchenne muscular dystrophy (DMD) patients. Since thyroid hormone is required for muscle development and integrity, we examined whether a deficiency of the hormone, induced by 0.05% propylthiouracil (PTU) in drinking water over 8 weeks, would be deleterious to the myogenesis and muscle repair in control and mdx mice. Measured metabolic and growth parameters confirmed hypothyroidism in PTU-treated mice. Histological and morphometric techniques were used to study myogenesis and the repair of the tibialis anterior muscle (TA) after crush injury in mdx mice and their nondystrophic controls (C57B1/10ScSn). After 8 weeks, PTU-treated TA from mdx mice had larger crush sites and lower myotube density than TA in untreated mdx mice. In unoperated mdx TA, there was a larger proportionate area of active dystrophy and smaller fiber diameter in PUT-treated than in untreated mdx TA, which suggested that PTU increased the activity of dystrophy as well. In contrast, in control TA neither the regeneration of myotubes or fiber diameter were affected significantly by PTU. Therefore, these results suggest that mdx muscle regeneration is more affected by hypothyroidism than normal muscle repair. This may be due to the larger pool of muscle precursors in mdx than control muscle, and a possible impairment of precursor cell proliferation or fusion during myotube formation.

Skeletal muscle satellite cells

Evidence now suggests that satellite cells constitute a class of myogenic cells that differ distinctly from other embryonic myoblasts. Satellite cells arise from somites and first appear as a distinct myoblast type well before birth. Satellite cells from different muscles cannot be functionally distinguished from one another and are able to provide nuclei to all fibers without regard to phenotype. Thus, it is difficult to ascribe any significant function to establishing or stabilizing fiber type, even during regeneration. Within a muscle, satellite cells exhibit marked heterogeneity with respect to their proliferative behavior. The satellite cell population on a fiber can be partitioned into those that function as stem cells and those which are readily available for fusion. Recent studies have shown that the cells are not simply spindle shaped, but are very diverse in their morphology and have multiple branches emanating from the poles of the cells. This finding is consistent with other studies indicating that the cells have the capacity for extensive migration within, and perhaps between, muscles. Complexity of cell shape usually reflects increased cytoplasmic volume and organelles including a well developed Golgi, and is usually associated with growing postnatal muscle or muscles undergoing some form of induced adaptive change or repair. The appearance of activated satellite cells suggests some function of the cells in the adaptive process through elaboration and secretion of a product. Significant advances have been made in determining the potential secretion products that satellite cells make. The manner in which satellite cell proliferative and fusion behavior is controlled has also been studied. There seems to be little doubt that cellcell coupling is not how satellite cells and myofibers communicate. Rather satellite cell regulation is through a number of potential growth factors that arise from a number of sources. Critical to the understanding of this form of control is to determine which of the many growth factors that can alter satellite cell behavior in vitro are at work in vivo. Little work has been done to determine what controls are at work after a regeneration response has been initiated. It seems likely that, after injury, growth factors are liberated through proteolytic activity and initiate an activation process whereby cells enter into a proliferative phase. After myofibers are formed, it also seems likely that satellite cell behavior is regulated through diffusible factors arising from the fibers rather than continuous control by circulating factors.(ABSTRACT TRUNCATED AT 400 WORDS)

Age-related changes in replication of myogenic cells in mdx mice: quantitative autoradiographic studies

Cell replication in muscle was measured by tritiated thymidine (3H-TdR) incorporation and autoradiography, in mdx mice from 2-44 weeks of age. Pre-mitotic labelling (within 1 h of 3H-TdR injection) was determined in 16 mice aged from 15 to 300 days. In 30 further mdx mice, one leg was irradiated 1 h after 3H-TdR injection to block DNA synthesis. Post-mitotic labelling was measured in both legs 10-15 days later. Between 20 and 60 days of age a very high proportion (up to 2%) of muscle (satellite cell) nuclei were replicating pre-mitotically; from 80-300 days cell replication was detectable but at much lower levels. Centrally placed nuclei within muscle fibres appeared at 24 days, increased rapidly to 50% by 50-100 days, declining thereafter to 25% at 300 days. In post-mitotic samples, labelled myotubes and labelled peripheral muscle nuclei (satellite cell nuclei and myonuclei) appeared at 28 days and were present in the mdx muscles through to 310 days, indicating continued cell replication and muscle regeneration. Myogenic cell replication was both retarded and inhibited by irradiation. These data demonstrate that muscle cell replication in mdx mice commences at about 3 weeks of age, is maximal at 4-8 weeks, but continues at lower levels until at least 44 weeks.

Studies on the evolution and function of different forms of the mouse myogenic gene Myo-D1 and upstream flanking region

The product of the murine Myo-D1 gene is able to initiate the complete sequence of genetic events required for formation of skeletal muscle. Because efficiency of regeneration of skeletal muscle is more pronounced in SJL/J mice, as compared to other strains, differences in the structure of Myo-D1 and the upstream regulatory region were sought to determine whether efficiency of tissue repair was influenced by the structure of the gene itself. Analysis of the restriction-fragment length polymorphism (RFLP) of genomic DNA from SJL/J and different sub-strains of mouse indicated that there are at least three different structural forms of Myo-D1, one of which is unique to SJL/J mice and may have been derived from a double recombinational event involving founder forms of Myo-D1. The unique form of Myo-D1 in SJL/J mice also exhibits a PvuII RFLP upstream from the gene, which may reflect some form of rearrangement or variation in methylation of a potential Myo-D1-binding region. Reference to the size of fragments hybridising with the Myo-D1 probe, following digestion of genomic DNA with TaqI, suggests that in most tissues, adenine residues within Myo-D1 may be extensively methylated. Segregation of Myo-D1 allotypes with response to mechanical injury to skeletal muscle in F2 offspring derived from SJL/J and BALB/c parental strains reveals that increased efficiency of tissue repair is associated with the SJL/J type of Myo-D1 gene. These observations provide new approaches to investigation of genetic control of tissue regeneration and cellular differentiation and proliferation in general.