Transformation is an alternative to normal skeletal muscle development

The differentiation of skeletal muscle is characterized by cessation of proliferation and fusion of single myoblasts to form non-replicating multinucleate fibers (myotubes). If termination of proliferation is an obligate requirement for further differentiation, myoblasts defective in this stage of development should fail to fuse or exhibit any further characteristics of myotubes. Furthermore, myoblasts which have lost the ability to control and cease proliferation may represent a transformed, potentially tumorigenic population. Formation of the neoplastic state may therefore be viewed as an alternate path, antithetical to the normal differentiation of skeletal muscle. To test this hypothesis, we isolated 13 clones of non-fusing cells from the myogenic L₈ line of rat myoblasts. In contrast to the L₈ line, all of the non-fusing clones maintain their proliferative capacity, do not form myotubes, nor elevated creatine kinase activity nor increased myosin, but do develop into tumors when injected into athymic mice. L₈ cells do not produce tumors in these mice. Analysis of cell growth and serum requirements, plasminogen activator, hexose transport, adhesiveness, LETS protein and growth in soft agar, indicates that these non-fusing cells are transformed and clearly distinguished from the parent L₈ cells. Whereas the L₈ line maintains a near diploid complement of chromosomes, all non-fusing clones were polyploid. In addition, 12 of 13 non-fusing clones (but not the L₈ cells) express an endogenous type C virus. Although all clones defective in differentiation formed tumors, no single in vitro characteristic was found to be a constant index of this tumorigenic capacity. We conclude that cessation of proliferation is an obligate requirement for skeletal myogenesis, that transformation is an alternative to normal skeletal muscle development and that the phenotype of these transformed cells may be quite varied.     

Sca-1 negatively regulates proliferation and differentiation of muscle cells

Satellite cells are tissue-specific stem cells critical for skeletal muscle growth and regeneration. Upon exposure to appropriate stimuli, satellite cells produce progeny myoblasts. Heterogeneity within a population of myoblasts ensures that a subset of myoblasts readily differentiate to form myotubes, whereas other myoblasts remain undifferentiated and thus available for future muscle growth. The mechanisms that contribute to this heterogeneity in myoblasts are largely unknown. We show that satellite cells are Sca-1(neg) but give rise to myoblasts that are heterogeneous for sca-1 expression. The majority of myoblasts are sca-1(neg), rapidly divide, and are capable of undergoing myogenic differentiation to form myotubes. In contrast, a minority population is sca-1(pos), divides slower, and does not readily form myotubes. Sca-1 expression is not static but rather dynamically modulated by the microenvironment. Gain-of-function and loss-of-function experiments demonstrate that sca-1 has a functional role in regulating proliferation and differentiation of myoblasts. Myofiber size of sca-1 null muscles is altered in an age-dependent manner, with increased size observed in younger mice and decreased size in older mice. These studies reveal a novel system that reversibly modulates the myogenic behavior of myoblasts. These studies provide evidence that, rather than being a fixed property, myoblast heterogeneity can be modulated by the microenvironment.     

Temporal differences in desmin expression between myoblasts from embryonic and adult chicken skeletal muscle

Desmin expression by myoblasts cultured from embryonic and adult chicken breast muscle was examined employing indirect immunofluorescence. The study was performed in conjunction with [3H]thymidine autoradiography and analysis of skeletal myosin expression in order to determine whether the desmin-expressing cells were terminally differentiated. Following 2 h of labeling with [3H]thymidine, 0.55%, 2.60%, and 15.10% of the cells in mass cultures from 10-day-old embryos, 18-day-old embryos and adults, respectively, incorporated [3H]thymidine and were desmin-positive but did not express skeletal-muscle-specific myosin. Using the same approach we determined that 0.07%, 1.25%, and 7.59% of the mononucleated cells in myogenic clones from 10-day-old embryos, 18-day-old embryos and adults, respectively, were desmin-positive, myosin-negative, [3H]thymidine-positive. We suggest that these desmin-positive, myosin-negative myoblasts are proliferating cells, and we conclude that the progeny of adult myoblasts exhibit more desmin-expressing cells of this type than embryonic myoblasts do.     

DISEASED MUSCLE CELLS IN CULTURE

(1) Cultures of differentiated muscle cells have been grown from diseased human, mouse and chick skeletal muscle, and from cardiac muscle of the myopathic hamster. (2) Methods of culture established for normal embryonic and adult skeletal muscle cells have proved suitable for cultures of diseased muscle cells. (3) Myoblasts obtained from dy^2J mouse muscle crushed in vivo before explanting fuse in culture and form morphologically normal myotubes. Studies of the effects of innervation by dy^2J spinal cord neurones on the differentiation of normal, dy^2J and dy myotubes have been inconclusive but it is probable that innervation does not play a part in the pathogenesis of this disorder. (4) Myoblasts prepared by trypsinization of embryonic dy muscle behave normally in culture and fuse to form myotubes that appear normal. It is not clear if myoblasts that migrate from explants of adult muscle in vitro fuse. Aggregates of non-fusing cells have been described, but under other culture conditions normal and abnormal forms of myotube have been observed. dy muscle fibres fail to regenerate even when cultured with normal spinal cord explants and dy nerves are without effect on regenerating normal muscle fibres. These tissue-culture studies suggest that the dy mouse mutation is a myopathic disorder. (5) Embryonic mdg myoblasts have a normal cell cycle in vitro and fuse to form well-differentiated myotubes with cross-striations. mdg myotubes have normal electro-physiological properties but do not contract spontaneously or on depolarization. The defect in the muscle of the mdg mutant appears to be a failure of excitation-contraction coupling. (6) Cells migrate earlier from explants of adult dystrophic chick muscle than from normal muscle but dystrophic chick myotubes appear morphologically normal. Myotubes prepared from embryonic dystrophic chick muscle become vacuolated and degenerate, changes that can be prevented by anti-proteases such as antipain. Lactic dehydrogenase isozyme subunit M4 is absent from dystrophic muscle in vivo but reappears in cultured myotubes. Dystrophic myotubes innervated in culture by either normal or dystrophic neurones exhibit bi-directional lcoupling and multiple innervation. These results suggest that there are changes in dystrophic myotubes and that chick muscular dystrophy is a myopathy. (7) Cardiac muscle cells from the cardiomyopathic hamster synthesize less actin and myosin than normal cells, and Z lines in dystrophic cells are irregularly arranged. The beat frequency of myopathic cardiac cells is lower than that of normal cells and declines more rapidly. Tissue-culture studies have not been made of hamster skeletal muscle. (8) Human dystrophic myotubes do not show degenerative changes in culture and have normal histochemical reactions. RNA synthesis appears normal in dystrophic myotubes but there may be changes in adenyl-cyclase activity and protein synthesis in dystrophic cells. Morphological and biochemical changes have been found in muscle cells cultured from a case of acid-maltase deficiency but phosphorylase activity re-appeared in myotubes cultured from biopsies of phosphorylase-deficient muscle. Innervation by normal mouse nerves does not induce degenerative changes in dystrophic myotubes. (9) Studies on the origins of myoblasts in explants of muscle fibres in culture suggest that in these conditions myoblasts are derived only from satellite cells and that this process may be the same in normal and diseased muscle.     

Temporal and spatial appearance of α-dystroglycan in differentiated mouse myoblasts in culture

The dystrophin-glycoprotein complex plays an important role in muscle function. One of the components of the complex, a 156-kDa cell surface glycoprotein (α-dystroglycan) binds to laminin, thereby connecting the basal lamina and muscle cells. We have examined the progressive appearance of α-dystroglycan and laminin in muscle cells that differentiate in culture. We find that nondifferentiated cultures of C2C12 myoblasts express low amounts of dystroglycan mRNA and, in contrast, this gene is prominently expressed in differentiated myotubes. Immunofluorescence analysis with a monoclonal antibody against α-dystroglycan shows its progressive appearance during myoblast differentiation into myotubes. Immunostaining with a monoclonal antibody against laminin shows that it is not present on the surface of undifferentiated myoblasts. Subsequently, laminin becomes apparent on the surface of differentiated myotubes where it codistributes with immunostained α-dystroglycan identifies a broad band of about 140–160 kDa, resembling α-dystroglycan from rabbit muscle. The composite results indicate that α-dystroglycan and laminin appear and become co-distributed on the surface of cultured C2C12 during the progression of differentiation.     

Entactin promotes adhesion and long-term maintenance of cultured regenerated skeletal myotubes.

The basal lamina protein, laminin, has been shown to promote migration and proliferation of cultured skeletal myoblasts, resulting in increased myotube formation. However, skeletal myotubes adhere poorly to a laminin substrate, and long-term cultures of skeletal myotubes on laminin have not been achieved. We have found that cultured satellite cells from bupivacaine-damaged rat skeletal muscle actively proliferate and differentiate on a diluted Matrigel substrate composed of laminin, type IV collagen, heparan sulfate proteoglycan, and entactin. Myotubes cultured on diluted Matrigel are contractile and have never been observed to detach from the culture dish; rather, myotubes generally atrophy after 2-3 weeks in culture. Antibodies directed against the various protein components of Matrigel were used to determine the role of each component in enhancing muscle differentiation. Anti-laminin impaired satellite cell adhesion, whereas antibodies against either type IV collagen or heparan sulfate proteoglycan had no effect. Anti-entactin did not inhibit attachment, proliferation, or fusion of cultured satellite cells; however, myotubes exposed to anti-entactin failed to adhere to the culture dish after spontaneous myotube contractions began. We conclude that entactin is responsible for long-term maintenance and maturation of contractile skeletal myotubes on a diluted Matrigel substrate. This is the first study to assign a biological function for entactin in myogenesis.     

Epidermal Growth Factor – based adhesion substrates elicit myoblast scattering,proliferation, differentiation and promote satellite cell myogenic activation

The biochemical properties of muscle extracellular matrix are essential for stem cell adhesion, motility, proliferation and myogenic development. Recombinant elastin-like polypeptides are synthetic polypeptides that, besides maintaining some properties of the native protein, can be tailored by fusing bioactive sequences to their C-terminal. Our laboratory synthesized several Human Elastin-Like Polypeptides (HELP) derived from the sequence of human tropoelastin. Here, we developed a novel HELP family member by fusing the elastin-like backbone to the sequence of human Epidermal Growth Factor. We employed this synthetic protein, named HEGF, either alone or in combination with other proteins of the HELP family carrying RGD-integrin binding sites, as adhesion substrate for C2C12 myoblasts and satellite cells primary cultures. Adhesion of myoblasts to HEGF-based substrates induced scattering, decreased adhesion and cytoskeleton assembly; the concomitant presence of the RGD motifs potentiated all these effects. Recombinant substrates induced myoblasts proliferation, differentiation and the development of multinucleated myotubes, thus favoring myoblasts expansion and preserving their myogenic potential. The effects induced by adhesion substrates were inhibited by AG82 (Tyrphostin 25) and herbimycin A, indicating their dependence on the activation of both the EGF receptor and the tyrosine kinase c-src. Finally, HEGF increased the number of muscle stem cells (satellite cells) derived from isolated muscle fibers in culture, thus highlighting its potential as a novel substrate for skeletal muscle regeneration strategies.     

Inhibition of the formation of myotubes in vitro by inhibitors of transglutaminase

The effects of competitive inhibitors of transglutaminase on the formation of myotubes by the fusion of myoblasts in vitro has been investigated. Myotube formation was inhibited when myoblasts from 11-day-old chick embryos were cultured in vitro in the presence of 10 mM histamine or 0.2 mM dansyl cadaverine. The inhibitions observed were reversed when the treated cells were subsequently cultured in normal medium. Glycine methyl ester also inhibited myotube formation but sarcosine methyl ester, which is not a competitive inhibitor of transglutaminase, had little if any inhibitory action. The formation of myotubes was not inhibited by cultivation in normal medium adjusted to pH 8.0-8.1, indicating that the observed effects of histamine and of dansyl cadaverine were not mediated by a lysosomotropic effect. Inhibition of myotube formation in the presence of histamine was accompanied by the production of abnormal multinucleated cells, indicating that myoblast fusion occurred in the treated cultures but that the fused cells failed to elongate into normal myotubes. Transglutaminase activity has been found in cell-free lysates of embryonic chick myoblasts and it is concluded that a transglutaminase enzyme, activated by an increase in the concentration of intracellular Ca2+, plays an important role in stabilising the cytoskeletal network of developing myotubes.     

Differentiation of activated satellite cells in denervated muscle following single fusions in situ and in cell culture

Satellite cells represent a cellular source of regeneration in adult skeletal muscle. It remains unclear why a large pool of stem myoblasts in denervated muscle does not compensate for the loss of muscle mass during post-denervation atrophy. In this study, we present evidence that satellite cells in long-term denervated rat muscle are able to activate synthesis of contractile proteins after single fusions in situ. This process of early differentiation leads to formation of abnormally diminutive myotubes. The localization of such dwarf myotubes beneath the intact basal lamina on the surface of differentiated muscle fibers shows that they form by fusion of neighboring satellites or by the progeny of a single satellite cell following one or two mitotic divisions. We demonstrated single fusions of myoblasts using electron microscopy, immunocytochemical labeling and high resolution confocal digital imaging. Sequestration of nascent myotubes by the rapidly forming basal laminae creates a barrier that limits further fusions. The recruitment of satellite cells in the formation of new muscle fibers results in a progressive decrease in their local densities, spatial separation and ultimate exhaustion of the myogenic cell pool. To determine whether the accumulation of aberrant dwarf myotubes is explained by the intrinsic decline of myogenic properties of satellite cells, or depends on their spatial separation and the environment in the tissue, we studied the fusion of myoblasts isolated from normal and denervated muscle in cell culture. The experiments with a culture system demonstrated that the capacity of myoblasts to synthesize contractile proteins without serial fusions depended on cell density and the availability of partners for fusion. Satellite cells isolated from denervated muscle and plated at fusion-permissive densities progressed through the myogenic program and actively formed myotubes, which shows that their myogenic potential is not considerably impaired. The results of this study suggest that under conditions of denervation, progressive spatial separation and confinement of many satellite cells within the endomysial tubes of atrophic muscle fibers and progressive interstitial fibrosis are the important factors that prevent their normal differentiation. Our findings also provide an explanation of why denervated muscle partially and temporarily is able to restore its functional capacity following injury and regeneration: the release of satellite cells from their sublaminal location provides the necessary space for a more active regenerative process.     

Cytoskeleton-bound mRNA for a 40-kDa polypeptide in rat L6 cells is not always translated

The relationship between attachment of mRNA to the cytoskeletal framework and its translation was examined using the mRNA for a polypeptide of 40 kDa (P-40) which is translated in rat L6 myoblasts but not in the myotubes. In both myoblasts and myotubes this mRNA was found to be associated with the cytoskeletal framework. Furthermore, the stability of the association between P-40 mRNA and the cytoskeletal framework in absence of RNA and protein synthesis was examined by using actinomycin D and NaF to block RNA and protein synthesis, respectively. In absence of RNA synthesis portions of both nontranslated P-40 mRNA and translated actin mRNA of myotubes were released into the soluble fraction. In myoblasts, however, both mRNAs remained associated with the cytoskeletal framework following inhibition of RNA synthesis. Inhibition of protein synthesis, on the other hand, had a more dramatic effect on the association between the cytoskeletal framework and P-40 mRNA in myoblasts but not in myotubes. In contrast, the association between actin mRNA and cytoskeletal framework was unaffected by inhibition of protein synthesis in both myoblasts and myotubes. The results of these studies show that the molecular nature of association between cytoskeletal framework and mRNA may differ among mRNAs and may also depend on whether the cells are dividing or are terminally differentiated. Furthermore, no direct relationship between the translation of mRNA and its attachment to the cytoskeletal framework was observed.     

Roles for the integrin VLA-4 and its counter receptor VCAM-1 in myogenesis.

Mammalian myogenesis is biphasic: primary myoblasts fuse to form primary myotubes, then secondary myoblasts align along the primary myotubes and form secondary myotubes, which comprise most of adult muscle. We provide evidence that an integrin (VLA-4) and its counter receptor (VCAM-1) have a role in secondary myogenesis. Both receptors are synthesized by cultured muscle cells: VLA-4 is induced as myotubes form, whereas VCAM-1 is present on myoblasts and myotubes. In vivo, both molecules are expressed at sites of secondary myogenesis, VLA-4 on primary and secondary myotubes, and VCAM-1 on secondary myoblasts and on regions of secondary myotubes apposed to primary myotubes. These patterns suggest that VLA-4-VCAM-1 interactions influence alignment of secondary myoblasts along primary myotubes and/or the fusion of secondary myoblasts. In support of the latter possibility, antibodies to VLA-4 or VCAM-1 inhibit myotube formation in culture.     

Establishment and conditions for growth and differentiation of a myoblast cell line derived from the semimembranosus muscle of newborn piglets

To establish an adequate model to study the proliferation and differentiation of porcine skeletal muscle in response to bioactive compounds, a pool of satellite cells was derived from the semimembranosus muscle (SM) of newborn piglets using a Percoll gradient centrifugation. The final yield amounted to 4.1 × 10⁶ cells/g muscle tissue. The percentage of muscle satellite cells has been determined by immunostaining for desmin and subsequent fluorescence analysis by flow cytometry, which revealed 95% of desmin-positive cells. For proliferation studies, satellite cell born myoblasts were seeded in gelatin-coated 96-well microplates at about 5 × 10³ cells per well. Cells were grown for 1 day in MEMα plus 10% fetal bovine serum (FBS) and 10% horse serum (HS), followed by 2 d cultivation in serum-free growth medium. For differentiation studies, myoblasts were cultured in matrigel-coated 24-well plates for 4 d with growth medium containing 10% FBS and 10% HS. At 80% confluence, cells were grown for 24 h in medium plus 10% FBS and 1 μM insulin to initiate differentiation. Subsequently, the cells were cultured in serum-free differentiation medium (SFDM) for 3 d to form myotubes. Cultures reached a maximum fusion rate of approximately 20% after 96 h. By establishing this culture system, we provide an advanced and appropriate in vitro model to study porcine skeletal muscle cell growth and differentiation including the responses to various bioactive compounds.     

Long-term culture of mouse male germline stem cells under serum-or feeder-free conditions

Spermatogonial stem cells are the only stem cells in the body that transmit genetic information to the next generation. These cells can be cultured for extended periods in the presence of serum and feeder cells. However, little is known about factors that regulate self-renewal division of spermatogonial stem cells. In this investigation we examined the possibility of establishing culture systems for spermatogonial stem cells that lack serum or a feeder cell layer. Spermatogonial stem cells could expand in serum-free conditions on mouse embryonic fibroblasts (MEFs), or were successfully cultivated without feeder cells on a laminin-coated plate. However, they could not expand when both serum and feeder cells were absent. Although the cells cultured on laminin differed phenotypically from those on feeder cells, they grew exponentially for at least 6 mo, and produced normal, fertile progeny following transplantation into infertile mouse testis. This culture system will provide a new opportunity for understanding the regulatory mechanism that governs spermatogonial stem cells.     

Interaction with normal cells suppresses the transformed phenotype of v-myc-transformed quail muscle cells

We have analyzed mixed cultures of normal mammalian fibroblastic cells and transformed quail myoblasts to investigate whether the presence of an excess of normal cells could suppress the phenotype of transformed quail cells. In such mixed cultures, only v-myc-transformed cells were growth-arrested, whereas v-src-transformed myoblasts were essentially unaffected. Growth arrest appeared to reflect reversion from the transformed state, including re-expression of the myogenic differentiation program. The v-myc-transformed myoblasts were phenotypically corrected also by differentiating normal quail myoblasts, giving rise to hybrid myotubes containing nuclei from both cell types. The differential behavior of transformed cells closely paralleled the efficiency with which they established metabolic cooperation with adjacent normal cells. Our results indicate that unrestrained proliferation associated with transformation is responsible for v-myc-induced block of myogenic differentiation.     

Microinjection of calpastatin inhibits fusion in myoblasts

Rat satellite cells (RSC) were microinjected with purified calpastatin or m-calpain, and myoblasts from a C2C12 mouse line were microinjected with purified calpastatin. Microinjection with calpastatin completely prevented fusion of myoblasts from both sources, whereas microinjection with m-calpain significantly increased the rate of fusion of cultured RSC; 44% of the nuclei of RSC cultures were in multinucleated myotubes within 48 h after microinjection with m-calpain plus labeled dextran, whereas only 15% of the nuclei were in multinucleated myotubes after microinjection with dextran alone. Western analyses indicated that neither RSC nor C2C12 myoblasts contained detectable amounts of mu-calpain before fusion. The levels of calpastatin in C2C12 myoblasts increased as cells passed from the proliferative stage to the onset of fusion, and these levels increased substantially in both the C2C12 and the RSC cells as they progressed to the late or postfusion stage. Both RSC and C2C12 myoblasts contained an 80-kDa polypeptide that was labeled with an anti-m-calpain antibody in Western blots. The results are consistent with a role of the calpain system (m-calpain in these myoblast lines) in remodeling of the cytoskeletal/plasma membrane interactions during cell fusion.     

Serine proteinase inhibitors in human skeletal muscle: Expression of β-amyloid protein precursor and α1-antichymotrypsin in vivo and during myogenesis in vitro

The balance of serine proteases and inhibitors in nerve and muscle is altered during programmed- and injury-induced remodeling. A serpin, α₁-antichymotrypsin (α₁-ACT), and Kunitz-inhibitor containing forms of the β-amyloid precursor protein (βAPP) may be important components of this balance. In the present study, we analyzed their expression in primary cultures of human myogenic (satellite) cells that mimic myogenic differentiation using Western blotting and immunocytochemistry. In vitro results were compared to in vivo results from normal adult human skeletal muscle biopsies. Using an anti-α₁-ACT polyclonal antibody, we detected a 62 kDa immunoreactive band both in cultured human myogenic cells (mononucleated myoblasts as well as multi-nucleated myotubes) and in extracts of human muscle biopsies. With a polyclonal anti-βAPP antibody we found two bands (105 and 120 kDa) in myoblasts and myotubes in culture. However, the same antibody recognized only a single band at 92 kDa in biopsies. By immunocytochemistry, both α₁-ACT and βAPP were indistinctly present on localized to the surface of myoblasts in culture. In contrast, these inhibitors were dense on myotube surfaces, where they often formed distinct aggregates and frequently co-localized. In permeabilized muscle cells, α₁-ACT and βAPP appeared to be localized to the perikarya of both myoblasts and myotubes. Confirming previous results, both α₁-ACT and βAPP were present at the neuromuscular junction in human muscle sections. These developmental changes found during in vitro myogenesis for α₁-ACT and βAPP, both serine protease inhibitors, reinforce the hypothesis that regulation of the serine proteases and serine protease inhibitors plays an important role in neuromuscular differentiation. © 1995 Wiley-Liss Inc.     

In vitro differentiation of myoblasts from skeletal muscle of rainbow trout

Substrata, plating densities and tissue culture media were compared for their effects on the proliferation and differentiation of myoblasts from skeletal muscle of rainbow trout. Mononuclear cells were isolated from the lateralis muscle of 4–11-month-old trout and plated on to glass coverslips coated with fibronectin, laminin or Matrigel. Cell proliferation was estimated by determining the density of nuclei on successive days in culture, and myoblast differentiation was detected by immunostaining cultures with the myosin-specific monoclonal antibody MF20. Mononuclear cell proliferation was highest for cells cultured on fibronectin or laminin and lowest for cells cultured on Matrigel, but the total number of nuclei in myosin-positive cells did not differ between substrata. The percentage of nuclei in myosin-positive myocytes and myotubes was significantly higher for cells cultured on Matrigel. The proportion of cells adhering to Matrigel and undergoing differentiation increased with plating density. Of three media tested, Dulbecco's Modified Eagle Medium (DMEM), RPMI 1640 (RPMI), Leibovitz's L-15 (L-15) supplemented with 1 or 10% fetal bovine serum (FBS), a significantly greater proportion of the myoblasts differentiated when cells were cultured in L-15+ 10% FBS. These results suggest that culturing trout muscle-derived cells on a substratum of Matrigel at a high density and maintaining cells in L-15+ 10% FBS provide the conditions that maximize the proportion of cells that actively synthesize muscle myosin and facilitate trout myoblast differentiation in vitro .     

Fusion between myoblasts and adult muscle fibers promotes remodeling of fibers into myotubes in vitro

Muscle satellite cells are residual embryonic myoblast precursors responsible for muscle growth and regeneration. In order to examine the role of satellite cells in the initial events of muscle regeneration, we placed individual mature rat muscle fibers in vitro along with their satellite cells. When the satellite cells were allowed to proliferate, they produced populations of myoblasts that fused together to form myotubes on the laminin substrate. These myoblasts and myotubes also fused with the adult fibers. When they did so, the fibers lost their adult morphology, and by 8 days in vitro, essentially all of them were remodeled into structures resembling embryonic myotubes. However, when proliferating satellite cells were eliminated by exposure to cytosine arabinoside (araC), the vast majority of fibers retained their adult shape. Addition of C2C12 cells (a myoblast line derived from adult mouse satellite cells) to araC-treated fiber cultures resulted in their fusion with the rat muscle fibers and restored the ability of the fibers to remodel, whereas addition of either a fibroblast cell line or a transformed, non-fusing variant of C2C12 cells, or addition of conditioned medium from C2C12 cells, failed to do so. These results imply that myoblast fusion is responsible for triggering adult fiber remodeling in vitro.     

In vitro differentiation of satellite cells isolated from normal and dystrophic mammalian muscles. A comparison with embryonic myogenic cells

Satellite cells were isolated from skeletal muscles of adult normal and dystrophic mice (C57/6J/dy strain) by sequential digestion of tissue fragments with collagenase, hyaluronidase and trypsin. These cells exhibit in culture similar behaviour to that of embryonic myoblasts, undergoing an initial duplicative period lasting about 2–3 days, followed by a shorter phase (1–2 days) of rapid cell fusion. During the duplicative phase most of the satellite cells appear round-shaped, whereas embryonic myoblasts appear typically spindle-shaped: both cell types actively incorporate [³H] thymidine. During the subsequent days of culture an increasing number of satellite cells becomes spindle-shaped; afterwards the cells contact each other and fuse into multinucleated myotubes. The majority of spindle-shaped satellite cells is unable to incorporate [³H] thymidine, thus behaving as post-mitotic cells. Concomitantly with satellite cell fusion, an increase of about 80-fold of creatine phosphokinase (CPK) specific activity is observed. Satellite cells are able to recognize co-cultured embryonic myoblasts ([³H] thymidine-labelled): hybrid myotubes containing labelled and unlabelled nuclei are formed in these experimental conditions.Satellite cells from dystrophic animals are able to differentiate in culture and do not show appreciable differences as compared to their normal counterparts. In dystrophic myotubes, however, CPK specific activity is almost twice that observed in normal myotubes.Hyman dystrophic satellite cells from biophies of adult muscle cultured in similar conditions grow and fuse into multinucleated myotubes showing a behaviour identical to normal controls.