Proliferation Of Myogenic Cells Research Articles

Regeneration and myogenic cell proliferation correlate with taurine levels in dystrophin- and MyoD-deficient muscles.

This study coupled proton magnetic resonance spectroscopy (1H-NMR) and in situ hybridization plus autoradiography in a novel examination of different phenotypes of adult myogenesis that arise from genetic disruptions in mice. Study of muscle extracts from normal and dystrophin-deficient mdx limb and diaphragm muscles confirmed our previous findings linking taurine and muscle regeneration at the peak of damage and repair. 1H-NMR distinguished biochemical differences in regenerating muscles that were consistent with the extent of repair in three strains: mdx dystrophic mice; MyoD(-/-) mice that lack expression of the early myogenic regulatory gene MyoD; and a double-mutant mdx:MyoD(-/-) strain lacking expression of both MyoD and dystrophin. We tested the hypothesis that differences in spectra according to genotype and the regeneration phenotype are related specifically to proliferation by committed myogenic precursor cells. 1H-NMR distinguished the three mutant strains: Taurine was highest in mdx muscles, with the phenotype of most effective regeneration; lowest in MyoD(-/-) muscles, with the least effective formation of new muscle in repair, as reported previously; and intermediate in double-mutant muscles, now reported to show an intermediate repair phenotype. The early and late muscle precursors (mpcs) expressing myf5 and myogenin were examined for proliferation. Eighteen percent of mdx myf5-positive mpcs were proliferative, whereas myf5-positive mpcs did not proliferate in regenerating muscles that lacked MyoD expression. By contrast, whereas 30% of myogenin-positive mpcs were proliferative in mdx muscles, almost none were proliferative in MyoD(-/-) muscles, and 12% were proliferative in double-mutant muscles. Therefore, the extent of accumulated structural regeneration, taurine levels, and proliferation of late mpc (expressing myogenin) were congruent across genotypes. Proliferation by early mpc (expressing myf5) was inhibited by the lack of MyoD expression during muscle regeneration. These studies indicate the potential for 1H-NMR monitoring of muscle status in disease, regeneration, and treatment.

Proinflammatory cytokines regulate myogenic cell proliferation and fusion but have no impact on myotube protein metabolism or stress protein expression.

The objective of the present study was to evaluate the effect of the proinflammatory cytokines, tumor necrosis factor-alpha (TNF-alpha) and interleukin-1-alpha (IL-1a), on myoblast proliferation and fusion and on myocyte protein metabolism and stress protein expression. Proliferation was suppressed (p < 0.05) by both cytokines, alone and in combination, and at lower concentrations, the suppression was additive. Likewise, fusion was retarded (p < 0.05) by these cytokines alone and in combination. Myosin synthesis was not altered acutely or chronically by TNF-alpha alone or by the combination of this cytokine with IL-1alpha. Chronic exposure to TNF-alpha did not alter total cellular protein synthesis, but exposure to IL-1alpha and the cytokine combination resulted in an increase (14% to 19%, p < 0.05) in synthesis. Neither total cellular protein nor myosin degradation were influenced by either cytokine alone or by the combination. There was no detectable induction, acutely or chronically, of any of the stress proteins evaluated (HSC70, HSP70, or HSP60). These data suggest that cytokines may alter muscle growth and development prenatally and postnatally and that the changes in muscle protein metabolism during periods of immune challenge are not direct effects of TNF-alpha or IL-1alpha.

Transient expression of a winged-helix protein, MNF-beta, during myogenesis.

A novel winged-helix transcription factor, MNF-beta, is expressed coincidentally with cell cycle withdrawal and differentiation of skeletal myogenic cells. MNF-beta is closely related to the myocyte nuclear factor (MNF) protein previously described (now termed MNF-alpha), but expression of the two isoforms is differentially regulated, and they exhibit distinctive functional properties with respect to DNA binding in vitro and transcriptional regulatory activity in transient-transfection assays. A DNA sequence motif binding MNF-beta with high affinity was selected from a library of random oligonucleotides and was found to be similar to but distinct from the cognate binding site for HNF-3beta, a more distantly related winged-helix protein. The temporal pattern of MNF-beta expression and the presence of MNF binding motifs within conserved promoter elements of several genes that modulate cell cycle progression support a working hypothesis that MNF proteins may modulate proliferation of myogenic precursor cells during development and muscle regeneration.

Regulation of metabolism and growth during immune challenge: an overview of cytokine function.

Commercially reared food animals encounter serial pathogenic and nonpathogenic immune challenges throughout production. Because of the diversion of nutrients away from growth in support of immune-related processes, immune challenge is considered a major obstacle to animals' achieving their genetic potential for growth or efficiency of gain. Scientists now recognize that many metabolic processes respond directly or indirectly to proinflammatory cytokines. This cytokine-mediated "reprogramming" of metabolism is a homeorhetic mechanism that ensures an adequate supply of nutrients for proliferation of lymphocyte and macrophage populations, antibody production, and hepatic synthesis of acute phase proteins. Proinflammatory cytokines have been linked to altered nutrient uptake and utilization. Anabolic processes are interrupted, and companion catabolic activities are amplified. Furthermore, cytokines may influence prenatal growth and development, and to the extent that postnatal proliferation and differentiation of myogenic and adipogenic cells contribute to postnatal growth, cytokine regulation of these events may ultimately influence growth. The following discussion is an overview of the impact of immune challenge and proinflammatory cytokines on metabolism and growth.

Regulatory role of arginine-specific mono(ADP-ribosyl)transferase in muscle cells.

Earlier we demonstrated that meta-iodobenzylguanidine (MIBG), a specific inhibitor of arginine mono-ADP-ribosylation blocks proliferation and differentiation of chick skeletal myogenic cells in culture (Exp. Cell Res., 1992, 201:33-42). Membrane fractions from 4-day, myotube cultures of embryonic chick muscle cells were incubated with 32P-NAD+. Several proteins were labeled, but labeling of two hands of about 53 and 36 kDa appeared to be due to arginyl ADP-ribosylation. Immunoprecipitation with D3 monoclonal antibody to the intermediate filament protein desmin, SDS-PAGE and autoradiography demonstrated that the 53 kDa band contained desmin, and that this desmin is ADP-ribosylated by the endogenous arginine-specific mono(ADP-ribosyl)transferase (Exp. Cell Res., 1996, in press). Desmin is the muscle-specific intermediate filament protein, and it appears to be one of the first muscle-specific proteins expressed during terminal myogenic differentiation. We have examined whether desmin can be ADP-ribosylated in muscle cells by use of polyclonal antibodies for ADP-ribosylated arginyl residues. We have found that soluble desmin is present in 5-6 day myogenic cell cultures and that this desmin contains ADP-ribose, demonstrating that desmin is ADP-ribosylated in skeletal muscle cells. We also found that purified avian desmin contains antigenic material that reacts with these antibodies. In both cases, NaCl had no effect on the reactivity, but NH2OH did, which is consistent with an arginine-ADPR linkage. In summary, these results suggest that ADP-ribosylation is an important regulatory mechanism in differentiating muscle cells, and that the intermediate filament protein desmin is an important substrate for modification in muscle cells.

An overview of the regeneration of skeletal muscle

It has only recently been acknowledged widely that mature skeletal muscle has the ability to regenerate, although reports on this phenomenon have existed in the research literature some 40 years. The regenerative events in skeletal muscle include: phagocytosis of cellular and connective tissue debris; revascularization of the lesion; the proliferation of myogenic precursor cells; their differentiation into myoblasts; myoblasts fusion into myotubes; followed by the reestablishment of the nerve supply, and the maturation of myotubes into muscle fibers. The key cell in skeletal muscle regeneration is the satellite cell, which is a reserve myogenic cell situated in between the muscle fiber sarcolemma and its adjacent external lamina. These cells can only by identified by electron microscopy, which makes them very difficult to investigate in detail or quantitatively. However, there is a substantial body of research literature on satellite cells and relevant aspects of their activity are summarized in this review. Satellite cells provide the source of myogenic precursor cells in the regeneration of skeletal muscle, therefore, any factor which stimulates the proliferative activity of satellite cells is very important in enhancing skeletal muscle regeneration. The cellular events in regenerating skeletal muscle closely resemble those which occur in the process of developmental myogenesis, and references to these similarities and differences are briefly reviewed. Biomedical Reviews 1996; 6: 75-81.

Decreased muscle cell proliferation in chicks with a deletion in the GH receptor gene

The increase in muscle weight in neonatal animals is a consequence of increased protein accretion and DNA content. GH increases protein accretion but direct effects of GH on myogenic cell proliferation have not been demonstrated. Sex-linked dwarfism in the chick is caused by mutation or deletion in the GH receptor gene and has provided a useful model to study the physiological consequences of GH insensitivity. This study determined the consequences of GH receptor gene mutation on muscle cell proliferation in vivo. Northern and Southern blotting and PCR analysis revealed restriction fragment length polymorphism patterns and a 1.7 kb deletion of the intracellular domain of the GH receptor gene in commercial dwarf broiler chicks, similar to the Connecticut strain in which there is a dysfunctional GH receptor. Cell proliferation was measured in muscle sections from normal and dwarf chicks after incorporation of 5-bromo-2'-deoxyuridine (BrdU; 25 mg/kg) in vivo at 2, 5 and 13 days of age. Incorporation of BrdU into nuclei was measured in frozen sections, counterstained with propidium iodide to estimate the total number of nuclei by quantitative image analysis, and the labelling index was calculated. Paraffin-embedded sections of breast muscle were stained using an anti-human IGF-I polyclonal antibody. Expression of IGF-I mRNA in muscle from each genotype at 5 days of age was measured by RNAse protection assay. The labelling index was similar in 2-day-old chicks from both genotypes (normal, 20.14 +/- 2.39%; dwarf, 19.79 +/- 5.83%). By day 5 the labelling index had decreased but was significantly higher (P < 0.02) in normal (12.53 +/- 3.36%) compared with the dwarf (6.25 +/- 1.39%). By 13 days of age, there was a further decrease in labelling index but no difference between the groups (normal, 4.92 +/- 1.28%; dwarf, 4.96 +/- 1.51%). IGF-I mRNA was expressed and IGF-I peptide was identified in muscle sections but there was no difference between genotypes. The results show that cell division in breast muscle in vivo is high in neonatal chicks but it declines with increasing age. The absence of a functional GH receptor in the dwarf is associated with a greater decline in DNA synthesis and suggests that GH may directly affect a proportion of cells, since there was no difference in IGF-I mRNA or peptide.

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.

Neurons and Ecdysteroids Promote the Proliferation of Myogenic Cells Cultured from the Developing Adult Legs ofManduca sexta

During metamorphosis in the hawkmothManduca sexta,larval leg motoneurons survive the degeneration of their target muscles to innervate new muscles that form during the development of the adult legs. Observations of muscle developmentin vivosuggest that there are close interactions between motor terminals and the muscle precursor cells at the earliest stages of muscle formation and surgical denervation compromises further development of adult muscles. Here we describe a nerve/muscle coculture system that allows further exploration of this critical developmental interaction. Muscle precursor cells derived from the developing thoracic legs of early pupae and cultured in the presence of neurons assumed a spindle-like morphology and fused to form multinucleate contractile myotubes. Contractile fibers did not form in cultures of muscle precursor cells alone. In the presence of neurons the rate of bromodeoxyuridine (BrdU) incorporation into myonuclei was significantly enhanced, suggesting that neurons promote the proliferation of myogenic cells. This effect was not unique to thoracic leg motoneurons of the early pupal stage, in that larval thoracic neurons as well as neurons from the pupal brain or abdominal ganglia were also effective at enhancing BrdU incorporation and the formation of contractile muscle fibers. Medium conditioned by neurons was ineffective at promoting BrdU incorporation, and in cocultures BrdU incorporation was enhanced only in regions of physical overlap between neurons and muscle precursor cells, suggesting that a very close-range interaction was involved. Tetrodotoxin-sensitive neuronal activity was not required for the effect on muscle development, but fixed neurons were ineffective. The insect steroid hormone 20-hydroxyecdysone enhanced BrdU incorporation into the nuclei of myogenic cells in both the presence and the absence of neurons. The results suggest that both neurons and ecdysteroids play an important regulatory role in adult muscle development, at least in part by enhancing the proliferation of myogenic cells.

Transforming growth factor beta-1 facilitates establishing clonal populations of ovine muscle satellite cells

Myogenic cells isolated from lamb fetuses (approximately mid-gestation) exhibited a concentration-dependent decrease in myogenic cell proliferation in response to transforming growth factor (TGF) beta-1 (P < .001). Half-maximal inhibition of proliferation occurred at approximately .05 ng of TGF beta-1/mL and maximal inhibition of proliferation occurred at approximately .1 ng of TGF beta-1/mL. The specificity of this inhibition was confirmed by neutralization of the activity following exposure to a TGF beta antibody. The TGF beta-1 also suppressed proliferation of ovine satellite cells isolated from 5-d-old lambs (P < .0035), but to a lesser extent than observed for embryonic cells. In contrast, TGF beta-1 did not significantly suppress serum-stimulated proliferation of ovine satellite cells isolated from 30- or 150-d-old lambs. Similarly, TGF beta-1 did not suppress proliferation of skeletal muscle fibroblast-like cells isolated from either fetal lambs or 150-d-old lambs. In fact, proliferation of fibroblast-like cells derived from embryonic ovine muscle was enhanced by exposure to TGF beta-1 at all levels tested; however, a concentration-dependent response was not observed. Media transfer experiments showed that conditioning of culture media by postnatally derived cells did not render TGF beta-1 inactive. The studies described in this manuscript suggest that sensitivity of ovine myogenic cells to the antiproliferative effect of TGF-beta may vary with the stage of development.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitation of satellite cell proliferation in vivo using image analysis.

A nonisotopic, double fluorescence technique was developed to study myogenic satellite cell proliferation in posthatch turkey skeletal muscle. Labeled satellite cell nuclei were identified on enzymatically isolated myofiber segments using a mouse monoclonal antibody (anti-BrdU) followed by fluorescein-5-isothiocyanate (FITC) conjugated goat anti-mouse IgG secondary antibody. Myofiber nuclei (myonuclei+satellite cell nuclei) were counterstained with propidium iodide (PI). The myofiber segment length, myofiber segment diameter, and the number of PI and FITC labeled nuclei contained in each segment was determined using a Nikon fluorescence microscope, a SIT video camera and Image-1 software. Data collected by three different operators of the image analysis system revealed 5.0 +/- 1.4 satellite cell nuclei per 1000 myofiber nuclei and 5284 +/- 462 microns3 of cytoplasm surrounding each myofiber nucleus in the pectoralis thoracicus of 9-week-old tom turkeys. BrdU immunohistochemistry coupled with the new approach of PI staining of whole myofiber mounts is an effective combination to allow the use of an efficient semi-automated image analysis protocol.

Isolation and characterization of a cDNA clone encoding for rat CSF-1 gene. Post-transcriptional repression occurs in myogenic differentiation

A major CSF-1 (Colony-Stimulating Factor 1) mRNA 4.0 kb long was expressed during the proliferation of the L6α1 rat myogenic cells and was down-regulated after their differentiation into myotubes. A complete cDNA encoding the rat CSF-1 gene (rmCSF-1) was isolated from a cDNA library of L6α1 myoblasts and sequenced. The overall deduced amino acid sequence was 100% and 68% identical to the mouse and human CSF-1, respectively. While the previously reported mechanisms about the regulation of CSF-1 expression in TPA-treated-monocytes (Horiguchi, J., Sariban, E. and Kufe, D. (1988) Mol. Cell. Biol. 8, 3951–3954) and in fibroblasts (Falkenburg, J.H.F., Harrington, M.A., De Paus, R.A., Walsh, M.K., Daub, R., Landegent, J.E. and Broxmeyer, H.E. (1991) Blood 78, 658–665) involved a control at the transcriptional level, in contrast, the CSF-1 mRNA (half-life ∼3 h in L6α1 myoblasts) was post-transcriptionally down-regulated during myogenesis. Inhibition of protein synthesis with cycloheximide (CHX) increased differentially the half-life of CSF-1 mRNA in L6α1 myotubes compared to L6α1 myoblasts. Finally, L6α1 myoblasts were shown to synthesize a 140 kDa homodimeric form of CSF-1. Thus, these findings, together with other results, indicate that CSF-1 gene products may play a role in the normal and neoplastic proliferation of muscular cells.

Comparison of the proliferation and differentiation of myogenic satellite cells derived from Merriam's and commercial varieties of turkeys

1. 1. Myogenic satellite cells were isolated and cloned from the Pectoralis major muscles of 6-week-old Nicholas and Merriam's torn turkeys to compare in vitro properties of muscle development between turkeys with markedly different growth rates. 2. 2. Although only small differences ( P ⩽ 0.05) were noted between proliferation rates of the two cell populations in McCoy's 5A medium-15% chicken serum, satellite cells derived from the Nicholas variety were more responsive ( P ⩽ 0.05) to mitogenic stimuli from serum at all levels tested. 3. 3. When satellite cells were stimulated by low serum levels to differentiate into multinucleated myotubes, cells from the Merriam's turkey fused more rapidly ( P ⩽ 0.05).

The basic fibroblast growth factor-saporin mitotoxin impairs in vivo skeletal regeneration following injury

Basic fibroblast growth factor (bFGF) is known to promote the proliferation of myogenic cells in vitro and has been detected in regenerating muscle in vivo. We attempted to modify in vivo post-injury muscle regeneration by acting through the potential involvement of bFGF and the presence of bFGF receptors on regenerating myogenic cells. For this purpose, bFGF conjugated to saporin, a potent mitotoxin, was injected in denervated-devascularized extensor digitorum longus of mice. Cytotoxicity occurred in the regeneration process, as shown by the significantly reduced number and the smaller diameter of the regenerating myotubes, probably determined by the binding of the toxic conjugate to bFGF receptors of myogenic cells. This study describes a suitable in vivo model to develop this specifically-mediated cyotoxicity, which offers new perspectives for acting on muscle regeneration disorders.

PDGF-receptor concentration is elevated in regenerative muscle fibers in dystrophin-deficient muscle

Dystrophin-deficient muscle undergoes sudden, postnatal onset of muscle necrosis that is either progressive, as in Duchenne muscular dystrophy, or successfully arrested and followed by regeneration, as in most muscles of mdx mice. The mechanisms regulating regeneration in mdx muscle are unknown, although the possibility that there is renewed expression of genes regulating embryonic muscle cell proliferation and differentiation may provide testable hypotheses. Here, we examine the possibility that necrotic and regenerating mdx muscles exhibit renewed or increased expression of PDGF-receptors. PDGF-binding to receptors on muscle has been shown previously to be associated with myogenic cell proliferation and delay of muscle differentiation. We find that PDGF-receptors are present in 4-week-old mdx mice in muscles that undergo brief, reversible necrosis (hindlimb muscles) or progressive necrosis (diaphragm), as well as in 4-week-old control mouse muscles. Immunoblots indicate that the concentrations of PDGF-receptors in 4-week-old dystrophic (necrotic) and control muscles are similar. Prenecrotic, dystrophic fibers and control fibers possess some cell surface labeling of fibers treated with anti-PDGF-receptor and viewed by indirect immunofluorescence. Necrotic fibers in dystrophic muscle show cytoplasmic labeling for PDGF-receptors and labeling of perinuclear regions at the muscle cell surface. Adult dystrophic muscle displays higher concentrations of PDGF-receptor in both regenerated muscle (hindlimb) and progressively necrotic muscle (diaphragm) than found in controls. Anti-PDGF-receptor labeling of regenerated, dystrophic muscle is observed primarily in granules surrounding central nuclei or surrounding nuclei located at the surface of regenerated fibers. No labeling of perinuclear regions of control muscle or prenecrotic fibers was observed. Myonuclei fractionated from adult mdx hindlimb muscles contained no PDGF-receptor, indicating that PDGF-receptor-positive structures are not tightly associated with nuclei or within nuclei. L6 myoblasts show PDGF-receptor distributed diffusely on the cell surface. Stimulation of L6 myoblasts with 10 ng/ml of PDGF-BB causes receptor internalization and concentration in granules at perinuclear regions. Thus, PDGF stimulation of myoblasts causes a redistribution of PDGF-receptors to resemble receptor localization observed during muscle regeneration. These findings implicate PDGF-mediated mechanisms in regeneration of dystrophic muscle.

Myogenic Cell Proliferation and Differentiation

It is generally thought that growth factors play a major role in regulating proliferation and differentiation of myogenic cells. Cell culture studies indicate that of the known growth factors, the fibroblast growth factors, the transforming growth factor β, and the insulin-like growth factor families play the most significant roles in this process. The fibroblast growth factors stimulate proliferation and inhibit differentiation of most cultured myogenic cells. Insulin-like growth factors also stimulate proliferation of myogenic cells, but, in contrast to the fibroblast growth factors, the insulin-like growth factors also stimulate differentiation. Transforming growth factor β inhibits differentiation of cultured myogenic cells. There are conflicting reports as to its effect on proliferation. The combined effects of these growth factors in vivo may play a major role in determining the rate of proliferation and differentiation of muscle tissue.

Comparison of the proliferation and differentiation of myogenic satellite cells and embryonic myoblasts derived from the turkey

1. 1. Embryonic and posthatch turkey skeletal muscle development was compared in in vitro studies using clonal-derived embryonic myoblasts and satellite cells. 2. 2. Although population doubling times were similar between the two lines (25.4 hr for satellite cells and 26.4 hr for embryonic myoblasts), embryonic myoblasts consistently began log phase growth 24 hr earlier than satellite cells. 3. 3. Differentiation (fusion) of embryonic myoblasts was maximized by 36 hr in Dulbecco's Modified Eagle's Medium containing 1% horse serum compared with 72 hr for satellite cells. 4. 4. When administered a serum-free medium which supports proliferation of turkey satellite cells, embryonic myoblasts differentiated to form myotubes.

Erratum

Single myofibers with attached satellite cells isolated from adult rats were used to study the influence of the mature myofiber on the proliferation of satellite cells. The satellite cells remain quiescent when cultured in serum containing medium but proliferate when exposed to mitogen from an extract of crushed adult muscle. The response of satellite cells to mitogen was measured under three situations with respect to cell contact: (1) in contact with a viable myofiber and its basal lamina, (2) detached from the myofiber by centrifugal force and deposited on the substratum and (3) beneath the basal lamina of a Marcaine killed myofiber. The results show that satellite cells in contact with the plasmalemma of a viable myofiber have reduced mitogenic response. Since inhibiting growth may induce differentiation, I tested whether satellite cells proliferating on the surface of a myofiber would fuse. Although the satellite cell progeny were fusion competent, they did not fuse with the myofiber. To determine whether fusion competence of the myofiber changes with time in culture, embryonic myoblasts were challenged to fuse with myofibers that had been stripped of satellite cells and cultured for several days. The myoblasts fused with pseudopodial sprouts growing from the ends of the myofiber, but did not fuse with the original myofiber surface. These results indicate that contact with the surface of a mature myofiber suppresses proliferation of myogenic cells but the cells do not fuse with the myofiber.

Interaction between satellite cells and skeletal muscle fibers

Single myofibers with attached satellite cells isolated from adult rats were used to study the influence of the mature myofiber on the proliferation of satellite cells. The satellite cells remain quiescent when cultured in serum containing medium but proliferate when exposed to mitogen from an extract of crushed adult muscle. The response of satellite cells to mitogen was measured under three situations with respect to cell contact: (1) in contact with a viable myofiber and its basal lamina, (2) detached from the myofiber by centrifugal force and deposited on the substratum and (3) beneath the basal lamina of a Marcaine killed myofiber. The results show that satellite cells in contact with the plasmalemma of a viable myofiber have reduced mitogenic response. Since inhibiting growth may induce differentiation, I tested whether satellite cells proliferating on the surface of a myofiber would fuse. Although the satellite cell progeny were fusion competent, they did not fuse with the myofiber. To determine whether fusion competence of the myofiber changes with time in culture, embryonic myoblasts were challenged to fuse with myofibers that had been stripped of satellite cells and cultured for several days. The myoblasts fused with pseudopodial sprouts growing from the ends of the myofiber, but did not fuse with the original myofiber surface. These results indicate that contact with the surface of a mature myofiber suppresses proliferation of myogenic cells but the cells do not fuse with the myofiber.

Satellite cells of growing turkeys: Influence of donor age and sex on proliferation and differentiation in vitro

Proliferation and differentiation (fusion) of myogenic satellite cells isolated from Nicholas tom and hen turkeys at 3, 9, and 15 weeks of age and the response of a satellite cell clone to serum from these birds were examined. Responsiveness of satellite cells to culture conditions was proportional to donor age, and sex of bird had no effect on satellite cell proliferation. Fusion percentages were similar in all cultures. When a turkey satellite cell clone was exposed to serum from tom and hen turkeys at each age, an age-related decline in proliferation occurred with serum from hen but not from tom turkeys. Minimal cell fusion occurred in turkey serum and neither proliferation nor fusion was correlated with serum IGF-I levels. Cells derived from pectoralis major and anterior latissimus dorsi muscles of 15-week-old tom turkeys possessed similar proliferation and fusion properties in vitro. It appears that satellite cells retain a high proliferative activity throughout growth in turkeys, and the mitogenic properties of turkey serum may be evaluated using turkey satellite cells in culture.