Primary, secondary and tertiary myotubes in developing skeletal muscle: a new approach to the analysis of human myogenesis

Measurement of membrane potential and myoplasmic [Ca2+] in developing rat myotubes at rest and in response to stimulation

In this study, the membrane potential and cytosolic [Ca2+] were measured in rat myotubes developing in culture from days 6-14. It was found that as the myotubes developed in culture, the resting membrane potential (RMP) became more negative during days 6-8, and then did not significantly change until after day 13, when it started to become less negative. The mean RMP measured at days 8-13 was -59 +/- 1 mV (n = 70). The amplitude of action potentials elicited in the myotubes by anode break stimulation increased in size during development (range: 47.5-119 mV) and this closely correlated with the development of a more negative RMP. Cytosolic [Ca2+] was measured in the rat myotubes using the Ca2+ indicator Fura-2, and no significant change in the resting [Ca2+] was observed during development (days 6-14). Ca2+ responses triggered by action potentials varied from small slow increases in [Ca2+] that failed to return to the baseline to rapid [Ca2+] transients. The size of the [Ca2+] transients positively correlated with both the observed increase in the RMP during development and the size of the action potential. Larger [Ca2+] transients also had more rapid rates of [Ca2+] decay, indicating a tandem increase in the ability of the sarcoplasmic reticulum to release and resequester Ca2+ during development of rat myotubes. Repetitive stimulation (10 Hz) of the myotubes exhibiting small [Ca2+] transients produced a step-like rise in [Ca2+]. Many myotubes exhibiting larger [Ca2+]transients could not be stimulated at 10 Hz by anode break stimulation due to the presence of action potentials with large hyperpolarisations. However, when these myotubes were depolarised at 10 Hz, they produced a tetanic Ca2+ response similar to that seen in adult skeletal muscle.

Formation of new myotubes occurs exclusively at the multiple innervation zones of an embryonic large muscle

This work examines the general principle of whether production of embryonic muscle fibres is invariably linked to sites of innervation, as we have previously reported in small rodent muscles (Duxson et al. [1989] Development 107:743-750). The experimental strategy has been to make a detailed electron microscopic analysis of the formation of new myotubes in a large muscle having multiple, discrete innervation zones. The particular model system is the guinea pig sternomastoid muscle, a strap-like, parallel-fibred muscle with four distinct endplate bands, both in the embryo and the adult. Primary myotubes in the developing muscle extended from tendon to tendon of the muscle and were innervated at each of the multiple endplate zones. Each point of innervation of the primary myotubes was a focus around which many new secondary myotubes formed, and each secondary myotube was approximately centred on one of the innervation sites of its supporting primary myotube. This confirms our previous report, in rat IVth lumbrical muscle, of an invariable association between sites of formation of new secondary myotubes and sites of innervation. We suggest that, in vivo, nerve terminals either directly induce the initial myoblast fusions which give rise to new secondary myotubes, or induce some precondition for fusion. An alternative hypothesis is that a common patterning influence in the muscle localizes both innervation and secondary myotube formation to the same zone. The pattern of secondary myotube production in the embryo has important implications for the size and final architecture of muscles in larger animals, and some of these are discussed.

Up-regulation of a novel integrin alpha-chain (alpha mt) on human fetal myotubes

Integrin expression and distribution was studied in cloned human fetal G6 myoblasts and myotubes. Immunoprecipitation of beta 1 integrins from surface iodinated and metabolically labeled G6 cells typically showed a five-fold induction of a beta 1 integrin associated protein upon differentiation. Under non-reducing conditions this beta 1 associated protein migrated as 145 kD. No such beta 1 associated protein was observed in the myogenic L8 rat cell line, before or after differentiation. The beta 1 integrin associated cell surface protein present in G6 myotubes remained associated with the beta 1 subunit in the presence of 1% Triton X-100 and 0.5 M NaCl. Like integrin alpha-chains, the protein dissociated from the beta 1 integrin subunit at low pH. Immunoprecipitation of G6 myotubes further indicated the presence of alpha 1, alpha 3, alpha 5, and alpha v integrins, and small amounts of alpha 4 and alpha 6 integrins. Immunodepletion with integrin alpha-chain antibodies to alpha 1, alpha 3, alpha 4, alpha 5, alpha 6, and alpha v integrin chains could not deplete the beta 1 integrin associated protein, indicating that it did not interact with any of these known integrin heterodimers. Upon treatment with reducing agents, the beta 1 integrin associated protein migrated in SDS-PAGE as a 155 kD protein. The decreased mobility in SDS-PAGE upon reduction is a feature shared with alpha 1, alpha 2, and alpha 9 integrin alpha-chains. Antibodies to alpha 1 immunoprecipitated an integrin heterodimer distinct from the 155 kD protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Differentiation and growth of muscle in the fish Sparus aurata (L): I. Myosin expression and organization of fibre types in lateral muscle from hatching to adult

Post-hatching development of lateral muscle in a teleost fish, Sparus aurata (L) was examined. At hatching only two fibre types were present, several layers of mitochondria-poor, myofibril-rich deep muscle fibres surrounded the notochord and were covered by a superficial monolayer of mitochondria-rich, myofibril-poor A third ultrastructurally distinct fibre type first appeared as one or two fibres located just under the lateral line at 6 days post-hatching. This type, which gradually increased in number during larval life, contained a slow isoform of myosin, identified by mATPase staining and immunostaining with myosin isoform-specific antibodies. Deep muscle fibres--the presumptive fast-white type--contained a fast myosin, and superficial monolayer fibres an isoform similar but not identical to that in adult pink muscle fibres. The only fibres present during larval life which showed a clear change in myosin expression were the superficial monolayer fibres, which gradually transformed into the slow type post-larvally. Pink muscle fibres first appeared near the end of larval life. Both slow and pink muscle fibres remained concentrated around the horizontal septum under the lateral line during larval life, expanding outwards towards the apices of the myotomes only after metamorphosis. Between 60 and 90 days very small diameter fibres with a distinct mATPase profile appeared scattered throughout the deep, fast-white muscle layer, giving it a 'mosaic' appearance, which persisted into adult life. A marked expansion in the slow muscle layer began at the same time, partly by transformation of superficial monolayer fibres, but mainly by addition of new fibres both on the deep surface of the superficial monolayer and close to the lateral line. The order of appearance of these fibre types, their myosin composition, and the significance of the superficial monolayer layer are discussed and compared to muscle fibre type development in higher vertebrates.

Differentiation and growth of muscle in the fish Sparus aurata (L): II. Hyperplastic and hypertrophic growth of lateral muscle from hatching to adult

Post-hatching growth of lateral muscle in a teleost fish, Sparus aurata (L) was studied morphometrically to identify and quantify muscle fibre hyperplasia and hypertrophy, and by in vivo nuclear labelling with 5-bromo-deoxyuridine to identify areas of myoblast proliferation. Muscle fibre types were identified principally by myosin ATPase histochemistry and immunostaining, and labelled nuclei were identified at light and electronmicroscope level by immunostaining with a specific monoclonal antibody. Hyperplastic growth was slow at hatching, but then increased to a maximum at the mid-point of larval life. Larval hyperplastic growth occurred by apposition of new fibres along proliferation zones, principally just under the lateral line and in the apical regions of the myotome, but also just under the superficial monolayer at intermediate positions. The first of these zones gave rise to slow and pink muscle fibres, in a process which continued through into postlarval life. The other zones added new fibres to the fast-white muscle layer in a process which was exhausted by the end of larval life. Post-larvally, between 60 and 90 days posthatching, a new hyperplastic process started in the fast-white muscle as nuclei proliferated and new muscle fibres were formed throughout the whole layer. This process resulted in a several-fold increase in the number of fast-white fibres over a few weeks, and then waned to very low levels in juveniles. Hyperplasia by apposition continued for some time postlarvally on the deep surface of the superficial monolayer, but at this stage gave rise to slow fibres only. Hypertrophic growth occurred at all ages, but was the dominant mechanism of muscle growth only in the juvenile and adult stages. Mechanisms giving rise to these different growth processes in fish muscle are discussed, and compared with muscle development in higher vertebrates.

Pattern of muscle fiber type formation in the pig

The aim of this study was to analyze the temporal sequence of expression of the myosin isoforms in the populations of muscle fibers in the pig and to bring more information on the origin of the strikingly different pattern of fiber composition and distribution between the deep medial red (oxido-glycolytic) and superficial white (glycolytic) portions of semitendinosus (ST) muscle. Muscle samples were taken from 49-, 55-, 75-, 90-, 103-, and 113- (birth) day-old fetuses, from 6-, 11-, 21-, 35-, 50-, and 80-day-old piglets, and from a 3-year-old pig. Our results confirm the sequential formation of primary and secondary generation fibers. The use of immunohistochemistry and heterologous monoclonal antibodies (mAb) directed against specific myosin heavy chain (MHC) isoforms revealed a different pattern of gene expression between the two portions of the ST muscle for both generations of fibers. By 75 days of gestation (dg), primary myotubes from the deep medial portion stained positively for the anti-slow MHC mAb and negatively for the adult anti-fast MHC, whereas the opposite was observed in the superficial portion. Secondary fibers never expressed slow MHC until late gestation. Instead, they expressed an adult fast MHC isoform as soon as they formed in the deep medial portion and later on in the superficial portion. From late gestation to the first 3 postnatal weeks, slow MHC began to be expressed in a subpopulation of secondary fibers. These fibers were in the direct vicinity of primary myotubes in the deep medial portion, whereas their location could not be established in the superficial portion. The remaining secondary fibers matured to type IIA in the direct vicinity of these type I fibers and to type IIB at the periphery of the islets. In both portions of the muscle, a subpopulation of secondary fibers, the first ones to express slow MHC, also transitorily expressed a MHC that was identical or closely related to the alpha-cardiac MHC during the early postnatal period. A third generation of small diameter fibers was observed shortly after birth and reacted with the anti-fetal MHC mAb; their destiny remains to be established.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of chronic electrical stimulation on myosin heavy chain expression in satellite cell cultures derived from rat muscles of different fiber-type composition

Myotube cultures were established from satellite cells of three rat muscles of different fiber-type composition, slow-twitch soleus, diaphragm, and fast-twitch tibialis anterior (TA). Effects of chronic electrical stimulation were studied by exposing these cultures for up to 13 days to a stimulus pattern consisting of 250 ms impulse trains of 40 Hz, repeated every 4 s. Changes in myosin expression were assessed at the mRNA level by Northern blotting and in situ hybridization. Expression of slow myosin at the protein level was analysed by immunoblotting and immunohistochemistry with two antibodies, one specific to adult slow myosin, the other reacting with developmental and adult slow myosin heavy chain (MHCI) isoforms. In all three myotube cultures stimulation enhanced the mRNA and protein expression of a developmental isoform of slow myosin (MHCI). However, the three myotube cultures differed in the extent of the increase in MHCI. It was greatest in soleus-derived myotubes, least in TA-derived myotubes, and intermediate in diaphragm-derived myotubes. In addition to the increase in slow myosin, long-term stimulation led to an isoform switch, as indicated by an increase in myotubes reacting with the antibody specific for the adult MHCI. Our results suggest that enhanced contractile activity promotes the expression of the slow phenotype predetermined in satellite cells of slow-twitch, type I fibers. The different extents of increased slow myosin expression may thus be explained as reflecting different percentages of type I fibers and consequently of slow-type satellite cells in the corresponding donor muscles.

Fast myosin heavy chains expressed in secondary mammalian muscle fibers at the time of their inception

Mammalian skeletal muscle is generated by two waves of fiber formation, resulting in primary and secondary fibers. These fibers mature to give rise to several classes of adult muscle fibers with distinct contractile properties. Here we describe fast myosin heavy chain (MyHC) isoforms that are expressed in nascent secondary, but not primary, fibers in the early development of rat and human muscle. These fast MyHCs are distinct from previously described embryonic and neonatal fast MyHCs. To identify these MyHCs, monoclonal antibodies were used whose specificity was determined in western blots of MyHCs on denaturing gels and reactivity with muscle tissue at various stages of development. To facilitate a comparison of our results with those of others obtained using different antibodies or species, we have identified cDNAs that encode the epitopes recognized by our antibodies wherever possible. The results suggest that epitopes characteristic of adult fast MyHCs are expressed very early in muscle fiber development and distinguish newly formed secondary fibers from primary fibers. This marker of secondary fibers, which is detectable at the time of their inception, should prove useful in future studies of the derivation of primary and secondary fibers in mammalian muscle development.

Comparison of the foetal development of fibre types in four bovine muscles

The pattern of expression of different types of myosin heavy chains and the development of different generations of muscle cells during foetal life were studied in four bovine muscles with widely varying characteristics, the Masseter, Longissimus thoracis, Cutaneus trunci and Diaphragma. Different complementary techniques were performed: immunocytochemistry, electrophoresis, immunoblotting and ELISA. Monoclonal antibodies against different myosin heavy chain isoforms were used. The results confirmed the existence of at least two generations of cells during foetal development in cattle. A first generation, which appeared at a very early stage, gave rise to adult type I fibres. A second generation, made up of different cell populations, gave rise to adult fast type IIA and IIB fibres, and to type IIC. In the slow muscles, it also seemed to give rise to type I fibres. The beginning of myogenesis was characterized in the different cell generations by the expression of transitory myosin forms that are not found in the adult. Type 1 myosin heavy chain was observed from 90 days whereas the fast types, 2a and 2b, were present from 210 to 230 days, at which stage the foetal form disappeared. Muscles that have greatly different contractile characteristics in the adult exhibit also different profiles of differentiation: the Diaphragma was the first to develop, followed by Cutaneus trunci, Longissimus thoracis and Masseter.

The pituitary-muscle axis in mdx dystrophic mice

As myogenesis, muscle growth and differentiation and growth factor expression are influenced by thyroid and growth hormone (GH) levels, it is important to investigate the possibility that altered activity of the pituitary-muscle axis prevents the lethal progression of mdx dystrophy and/or contributes to the muscle fiber hypertrophy of limb muscles. The ultrastructure of pituitary and thyroid tissues in age-matched control and mdx mice at 2 and 12 months of age was examined. Pituitary GH, and serum thyroid stimulating hormone (TSH), thyroid hormone (T4), and creatine kinase (CK) levels were measured. Mdx thyroid gland structure was similar to age-matched control glands. Mdx thyroid gland weighed significantly more than in age-matched controls, but was unchanged relative to body weight. TSH and T4 levels were not different from levels in control mice. High CK levels reflected the active dystrophy in mdx muscles. Somatotrophs in mdx pituitaries were hypertrophied in comparison to controls, indicating increased secretory activity, and pituitary GH was slightly but significantly greater in old mdx female mice compared to age-matched female controls. These observations rule out hypopituitary or hypothyroid function as a reason for the low impact of dystrophin deficiency in mdx muscles. Results suggest a contribution by raised GH to the fiber hypertrophy in mdx limb and heart muscle, which might also assist the large capacity for limb muscle regeneration in mdx mice.

CD24, a signal-transducing molecule expressed on human B lymphocytes, is a marker for human regenerating muscle

The expression of the CD24 molecule, a glycoprotein expressed at the surface of most B lymphocytes and differentiating neuroblasts, was studied in developing nerve and muscle (after 16 weeks of gestation), normal adult and various diseased human muscles using immunohistochemistry and Western blot analysis. Immunohistochemical studies demonstrated that: (1) in developing muscles, fibers did not express CD24, whereas only some mesenchymal areas, also expressing neural cell adhesion molecule (N.CAM) and vimentin, and developing nerves were positive; (2) in normal adult muscles, CD24 immunoreactivity was observed only in some unmyelinated nerve fibers--intra and extra fusal muscle fibers, satellite cells and neuromuscular junctions were negative; and (3) in all diseased muscles studied here, CD24 expression was always associated with a subpopulation of regenerative fibers. These fibers also expressed vimentin, desmin, developmental myosin heavy chain, N.CAM and its polysialylated isoforms (PSA-N.CAM). The number of CD24-positive fibers was always lower than that of PSA-N.CAM-positive fibers. Denervated fibers and vacuolated muscle fibers never expressed CD24. Western blot analysis indicated that the apparent molecular mass of CD24 antigen was different between muscle and developing nervous tissues, suggesting that CD24 glycosylation is tissue specific. Since the molecule was not expressed in developing human muscle fibers, it strongly suggests that regenerative and fetal myotubes are different with respect to the CD24 molecule expression.

Developmental studies of the expression of myosin heavy chain isoforms in cultured human muscle aneurally and innervated with fetal rat spinal cord

To study the influence of innervation of human muscle fiber type differentiation, we performed immunohistochemical studies using three monoclonal antibodies (McAbs) to myosin heavy chain (MHC) on cultured human muscles at different developmental stages. McAbs QM 355 (McAb-1), E 35-3 (McAb-2) and SM 1-11-2 (McAb-3) bound to fiber types I, IIA, IIB and IIC, types IIA, IIB and IIC, and type I, respectively. At the mononucleated cell stage the majority was immunonegative to the three McAbs; however, a few myoblasts were immunopositive to the McAb-1. They were also weakly stained with McAb-2 but not with McAb-3. In aneurally cultured myotubes (AMs), all myotubes were stained with the McAb-1 and 92.1% of AMs were positive to the McAb-2, whereas only a few (0.9%) AMs were immunopositive to the McAb-3. In contracting muscle fibers in an innervated area (CMis), which were co-cultured with fetal rat spinal cord explants, the percentage of the McAb-3-positive CMis was significantly increased (8.3%; P < 0.01) compared with that of AMs (0.9%). The double staining with the McAbs-2 and -3 clearly showed that slow MHC-positive muscle fibers without fast MHC only appeared in CMis. This is the first report of the neuronal influence on the expression of human adult slow MHC isoform derived from adult human satellite cells in vitro.

Developmentally regulated expression of three slow isoforms of myosin heavy chain: diversity among the first fibers to form in avian muscle

At least three slow myosin heavy chain (MHC) isoforms were expressed in skeletal muscles of the developing chicken hindlimb, and differential expression of these slow MHC isoforms produced distinct fiber types from the outset of skeletal muscle myogenesis. Immunohistochemistry with isoform-specific monoclonal antibodies demonstrated differences in MHC content among the fibers of the dorsal and ventral premuscle masses and distinctions among fibers before splitting of the premuscle masses into individual muscles (Hamburger and Hamilton Stage 25). Immunoblot analyses by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of myosin extracted from the hindlimb demonstrated the presence throughout development of different mobility classes of MHCs with epitopes associated with slow MHC isoforms. Immunopeptide mapping showed that one of the MHCs expressed in the embryonic limb was the same slow MHC isoform, slow MHC1 (SMHC1), that is expressed in adult slow muscles. SMHC1 was expressed in the dorsal and ventral premuscle masses, embryonic, fetal, and some neonatal and adult hindlimb muscles. In the embryo and fetus SMHC1 was expressed in future fast, as well as future slow muscles, whereas in the adult only the slow muscles retained expression of SMHC1. Those embryonic muscles destined in the adult to contain slow fibers or mixed fast/slow fibers not only expressed SMHC1, but also an additional slow MHC not previously described, designated as slow MHC3 (SMHC3). Slow MHC3 was shown by immunopeptide mapping to contain a slow MHC epitope (reactive with mAb S58) and to be structurally similar to a MHC expressed in the atria of the adult chicken heart. SMHC3 was designated as a slow MHC isoform because (i) it was expressed only in those muscles destined to be of the slow type in the adult, (ii) it was expressed only in primary fibers of muscles that subsequently are of the slow type, and (iii) it had an epitope demonstrated to be present on other slow, but not fast, isoforms of avian MHC. This study demonstrates that a difference in phenotype between fibers is established very early in the chicken embryo and is based on the fiber type-specific expression of three slow MHC isoforms.

Early stages of myogenesis in a large mammal: formation of successive generations of myotubes in sheep tibialis cranialis muscle

The generation of myotubes was studied in the tibialis cranialis muscle in the sheep hindlimb from the earliest stage of primary myotube formation until a stage shortly before muscle fascicles began to segregate. Primary myotubes were first seen on embryonic day 32 (E32) and reached their maximum number by E38. Small numbers of secondary myotubes were first identified at E38, and secondary myotube numbers continued to increase during the period of study. The ratio of adult muscle fibre to primary myotube numbers was approximately 70:1, making it seem unlikely that every later generation myotube used a primary myotube as scaffold for its formation, as described in small mammals. By E62, some secondary myotubes were supporting the formation of a third generation of myotubes. Experiments with diffusible dye markers showed that primary myotubes extended from tendon to tendon of the muscle, whereas most adult fibres ran for only part of the muscle length, terminating with myo-myonal attachments to other muscle fibres in a series arrangement. Acetylcholinesterase (AChE) and acetylcholine receptor (AChR) aggregations appeared in multiple bands across the muscle shortly after formation of the primary generation of myotubes was complete. The number of bands and their pattern of distribution across the muscle as they were first formed was the same as in the adult. Primary myotubes teased from early muscles had multiple focal AChE and AChR deposits regularly spaced along their lengths. We suggest that the secondary generation of myotubes forms at endplate sites in a series arrangement along the length of single primary myotubes, and that tertiary and possibly later generations of myotubes in their turn use the earlier generation myofibres as a scaffold. Although the fundamental cellular mechanisms appear to be similar, the process of muscle fibre generation in large mammalian muscles is more complex than that described from previous studies in small laboratory rodents.

Myosin heavy chain composition of single fibres and their origins and distribution in developing fascicles of sheep tibialis cranialis muscles

The myosin heavy chain (MHC) composition of single muscle fibres in developing sheep tibialis cranialis muscles was examined immunohistochemically with monoclonal antibodies to MHC isozymes. Data were collected with conventional microscopy and computerized image analysis from embryonic day (E) 76 to postnatal day (PN) 20, and from adult animals. At E76, 23% of the young myofibres stained for slow-twitch MHC. The number of these fibres considerably exceeded the number of primary and secondary myotubes. By E100, smaller fibres, negative for slow-twitch MHC, encircled each fibre from the initial population to form rosettes. A second population of small fibres appeared in the unoccupied spaces between rosettes. Small fibres, whether belonging to rosettes or not, did not initially express slow-twitch MHC, expressing mainly neonatal myosin instead. These small fibres then diverged into three separate groups. In the first group most fibres transiently expressed adult fast myosin (maximal at E110-E120), but in the adult expressed slow myosin. This transformation to the slow MHC phenotype commenced at E110, was nearing completion by 20 postnatal days, and was responsible for approximately 60% of the adult slow twitch fibre population. In the other two groups expression of adult fast MHC was maintained, and in the adult they accounted for 14% (IIa MHC) and 17% (IIb MHC) of the total fibre numbers. We conclude that muscle fibre formation in this large muscle involves at least three generations of myotube. Secondary myotubes are generated on a framework of primary myotubes and both populations differentiate into the young myofibres which we observed at E76 to form rosettes. Tertiary myotubes, in turn, appear in the spaces between rosettes and along the borders of fascicles, using the outer fibres of rosettes as scaffolds.

The relationship of nerve to myoblasts and newly-formed secondary myotubes in the fourth lumbrical muscle of the rat foetus

The formation of normal numbers of skeletal muscle fibres depends on functional innervation of the muscle before and during the period of secondary myotube formation, but little has been known about the physical relationship between nerve terminals and the myoblasts and secondary myotubes over the critical period. This paper reports the results of a serial-section electron microscopic study of the IVth lumbrical muscle of the rat hindlimb, studied on embryonic day 20 (E20), a time when all secondary myotubes are less than 24 h old, and new ones are rapidly forming. Most myoblasts lying within the endplate region of the muscle received some direct neural contact; in almost all cases, the contact originated from an extension of a differentiated nerve terminal present at the endplate of an adjacent primary myotube. At six of 15 neural contact sites on myoblasts, primitive synaptic specialization was present. The newly-formed secondary myotubes were also directly, although sparsely, innervated in nine of ten instances. One secondary myotube was never seen to be innervated, despite extensive serial tracing. Nerve terminals passing to secondary myotubes were also principally derived from the innervation zone of the earlier-formed primary myotubes. Primary myotubes were profusely innervated by multiple axons. The results suggest that most nerve terminals are initially accommodated on the primary generation of myotubes, but progressively transfer to pre-fusion myoblasts or to secondary myotubes as these appear. In general, very young secondary myotubes are innervated by only a single axon, rather than being polyneuronally innervated. The existence of some secondary myotubes which lack any direct innervation suggests that intimate nerve contact may not be obligatory for formation of new secondary myotubes.

Indirect myosin immunocytochemistry for the identification of fibre types in equine skeletal muscle

The histochemical ATPase method for muscle fibre typing was first described by Brooke and Kaiser in 1970. However, problems have been found with the subdivision of type II fibres using this technique. To determine whether indirect myosin immunocytochemistry using anti-slow (5-4D), anti-fast (1A10) and anti-fast red (5-2B) monoclonal antibodies with cross reactivity for type I, II and IIa fibres, respectively, in a number of species, could identify three fibre types in equine skeletal muscle, data on fibre type composition and fibre size obtained using the two different techniques were compared. Results indicate that different myosin heavy chains can coexist in single equine muscle fibres. Type I and type II fibres were identified by immunocytochemistry, but subdivision of type II fibres was not possible. Although the percentage of type I and type II fibres was not significantly different for the two techniques, a few fibres reacted with both the 1A10 and 5-4D antibodies.

Immunocytochemical characterisation of two generations of fibers during the development of the human quadriceps muscle

We have carried out a comprehensive study of the formation of muscle fibers in the human quadriceps in a large series of well dated human foetuses and children. Our results demonstrate that a first generation of muscle fibers forms between 8-10 weeks. These fibers all express slow twitch myosin heavy chain (MHC) in addition to embryonic and foetal MHCs, vimentin and desmin. Between 10-11 weeks, a subpopulation of these fibers express slow tonic MHC, being the first primordia of muscle spindles. Extrafusal fibers of a second generation form progressively and asynchronously around the primary fibers between 10-18 weeks, giving the muscle a very heterogeneous aspect due to different degrees of organization of their proteins. By 20 weeks, these second generation fibers become homogeneous and thereafter undergo a process of maturation and differentiation when they eliminate vimentin, embryonic and foetal MHCs to express either slow twitch or fast MHC. The differentiation of these second generation fibers into slow and fast depends upon different factors, such as motor innervation or level of thyroid hormone. Around the intrafusal first generation fibers, additional subsequent generations of fibers are also progressively formed. Some differ from the extrafusal second generation fibers by expressing slow tonic MHC, others by continuous expression of foetal MHC. The differentiation of intrafusal fibers is probably under the influence of both sensory and motor innervation.

Rat muscle spindle immunocytochemistry revisited

The expression of myosin heavy chain isoforms in muscle spindle fibres has been the subject of a number of immunocytochemical studies, some of them with discordant results. In order to assess whether these discrepancies are due to differences in the specificity and sensitivity of the antibodies used, we have compared the reactivity of rat muscle spindle fibres to two pairs of antibodies presumed to be directed against slow tonic (ALD 19 and ALD 58) and neonatal (NN5) and neonatal/fast (MF30) myosin heavy chains. Adult, developing and neonatally de-efferented muscle spindles from the rat hind limb muscles were studied in serial cross-sections processed for the peroxidase-antiperoxidase method. Important differences in the staining profiles of intrafusal fibres were noted when ALD 19 and ALD 58 were compared. ALD 19 stained the muscle spindle precursors from the seventeenth day in utero, whereas ALD 58 only did so by the twentieth day of gestation. In adult spindles ALD 19 stained the nuclear bag1 fibres along their entire length, whereas ALD 58 did not stain these fibres towards their ends. ALD 19 stained the nuclear bag2 fibres along the A, B and inner C region, but ALD 58 stained these fibres only in the A and the inner B regions. ALD 19 stained some nuclear chain fibres along a short equatorial segment, whereas ALD 58 did not stain the nuclear chain fibres at all. NN5 stained the nascent nuclear bag1 and chain fibre precursors at earlier stages of development than MF30. Clear differential staining between primary and secondary generation of both extra- and intrafusal myotubes was seen with NN5, whereas MF30 stained all myotubes alike. However, in postnatal spindles, MF30 was a very good negative marker of nuclear bag1 fibres. The staining profile of the adult fibres with NN5 and MF30 was rather similar. The staining pattern of neonatally de-efferented bag fibres obtained with ALD 19 and ALD 58 was practically identical and it differed from that of control spindles, confirming that motor innervation participates in the regulation of the expression of slow tonic MHC along the length of the nuclear bag2 fibres, as we have previously shown with ALD 19. The distinct staining patterns obtained with ALD 19 versus ALD 58 and with NN5 versus MF30 reflect differences in antibody sensitivity and specificity. These differences account, in part, for the discrepancies in the results of previous studies on muscle spindles, published by Kucera and Walro using ALD 58 and MF30, and by us using ALD 19 and NN5.

Coordination of skeletal muscle gene expression occurs late in mammalian development

The acquisition of specialized skeletal muscle fiber phenotypes during development is investigated by systematic measurement of the accumulation of 21 contractile protein mRNAs during hindlimb development in the rat and the human. During early myotube formation in both species there is no coordination of expression of either fast or slow contractile protein isoform genes, but rather some slow, some fast, and some cardiac isoforms are expressed. Some isoforms are not detected at all in early myotubes. From Embryonic Day 19 in the rat, and after 14 weeks in the human, a strong bias toward fast isoform expression is evident for all gene families examined. This results in the establishment of a coordinated fast isoform phenotype at birth in the rat, and by 24 weeks in the human fetus. Unexpectedly, during secondary myotube formation in the rat we observe sudden rises and falls in contractile protein gene output. We interpret these fluctuations in terms of periods of myoblast proliferation followed by synchronized fusion into myotubes. The data presented indicate that each contractile protein gene has its own determinants of mRNA accumulation and that the different myoblast populations which contribute to the developing limb are not intrinsically programmed to produce particular coordinated phenotypes with respect to the non-myosin heavy chain contractile proteins.

Arrangement of fiber types within fascicles of human vastus lateralis muscle

A total of 106 fascicles at 6 predetermined areas of the vastus lateralis muscle from 9 healthy men, aged 18 to 40 years, were analyzed. Fibers in a fascicle were divided into layers according to their relation to the perimysium. In each layer the proportions of type 1 and 2, subdivided into 2a, 2b, and 2c fibers were determined and normalized by the fiber type proportion in the whole fascicle. A consistent arrangement of fiber types within the fascicles was obtained, regardless of subject, sampling site, fiber type proportion, and fascicle size. A high proportion of 2b fibers on the border, a prevalence of type 1 fibers in the layer beneath, and a rather uniform distribution of 2a fibers in all layers are the main characteristics of the distribution of fibers in a fascicle. Developmental processes in fiber type differentiation most probably constitute the basis for fiber type arrangement, which can further be influenced by local factors.

Biphasic expression of slow myosin light chains and slow tropomyosin isoforms during the development of the human quadriceps muscle

Using a two-dimensional electrophoresis technique coupled with sensitive silver staining, we have investigated the chronology of appearance of the myosin light chain and tropomyosin isoforms during early stages of human quadriceps development. Our results show that slow myosin light chains and the slow tropomyosin isoform are not detected at 6 weeks of gestation. These isoforms transiently appear between 12.5 weeks and 15 weeks of gestation and then disappear. The slow myosin light chains are re-expressed at 31 weeks of gestation and the slow tropomyosin isoform later at 36 weeks of gestation, and normally remained expressed into the adulthood. Our study thus reveals a biphasic expression of the slow myosin light chains and the slow tropomyosin isoform in developing human quadriceps muscle.

Distinct contractile protein profile in congenital myotonic dystrophy and X-linked myotubular myopathy

The contractile proteins present in muscle biopsies taken from infants suffering either from congenital myotonic dystrophy or X-linked myotubular myopathy were compared using biochemical and immunocytochemical techniques. Two-dimensional gel analysis has revealed that in all cases of X-linked myotubular myopathy the pattern of expression of myosin light chains, tropomyosin and troponin was roughly similar to that of normal age matched control muscle. However, biopsies from infants affected by congenital myotonic dystrophy demonstrated a predominance of most fast contractile protein isoforms. Non-denaturing gel electrophoresis confirmed the presence of both fast and slow myosin isoforms in X-linked myotubular myopathy. Fetal myosin was also present but in amounts higher than that found in normal muscles of the same age. In congenital myotonic dystrophy fetal and fast myosin were the predominant isoforms detected by native gel electrophoresis. These results were confirmed by immunocytochemistry and Western blot analysis using antibodies specific for the different myosin isoforms.

Origin of intrafusal muscle fibers in the rat

The expression of several isoforms of myosin heavy chain (MHC) by intrafusal and extrafusal fibers of the rat soleus muscle at different stages of development was compared by immunocytochemistry. The first intrafusal myotube to form, the bag2 fiber, expressed a slow-twitch MHC isoform identical to that expressed by the primary extrafusal myotubes. The second intrafusal myotube to form, the bag1 fiber, expressed a fast-twitch MHC similar to that initially expressed by the secondary extrafusal myotubes. At subsequent stages of development, the equatorial and juxtaequatorial regions of bag2 and bag1 intrafusal myofibers began to express a slow-tonic myosin isoform not expressed by extrafusal fibers, and ceased to express some of the MHC isoforms present initially. Myotubes which eventually matured into chain fibers expressed initially both the slow-twitch and fast-twitch MHC isoforms similar to some secondary extrafusal myotubes. In contrast, adult chain fibers expressed the fast-twitch MHC isoform only. Hence intrafusal myotubes initially expressed no unique MHCs, but rather expressed MHCs similar to those expressed by extrafusal myotubes at the same chronological stage of muscle development. These observations suggest that both intrafusal and extrafusal fibers develop from common pools of bipotential myotubes. Differences in MHC expression observed between intrafusal and extrafusal fibers of rat muscle might then result from a morphogenetic effect of afferent innervation on intrafusal myotubes.

Regeneration of cat posterior temporalis muscle in culture

Cat posterior temporalis muscle has a rapid speed of contraction associated with a unique superfast myosin isoform. Superfast myosin expression appears to be an intrinsic property of the muscle fibres and satellite cells, though in culture they failed to express superfast myosin. We have, therefore, cultured this muscle in a system which had previously been shown to encourage the expression of an adult phenotype. The presence of nerve cells resulted in effective regeneration of cat posterior temporalis muscle and even the formation of functional neuromuscular junctions. However, superfast myosin was not found even in mature, contracting, innervated cultures. Thyroid hormone, a known regulator of myosin isoform expression, also failed to elicit superfast myosin expression. Different culture conditions may allow a different outcome, but under circumstances in which mouse muscle expresses an adult phenotype, cat posterior temporalis muscle fails to do so.

Immunocytochemical demonstration of myosin heavy chain expression in human muscle

Three new monoclonal antibodies are shown by immunocytochemical techniques to recognise the adult fast, slow and neonatal myosin heavy chain (MHC) isoforms in adult and fetal human muscle. In fetal muscle of 17-20 weeks of gestation, slow MHC was present only in primary myotubes. Secondary myotubes contained neonatal MHC with different levels of fast and some embryonic MHC. We confirmed the presence of tertiary myotubes in the fetal muscle (Draeger et al. (1987) J. Neurol. Sci., 81: 19-43) and show that these contained fast, neonatal and possibly some embryonic MHC. Fast MHC was therefore present in secondary and tertiary myotubes at least as early as 17 days of gestation.

Postnatal expression of myosin heavy chains in muscle spindles of the rat

The immunocytochemical expression of two myosin isoforms in intrafusal muscle fibers was examined in soleus muscles of neonatal (zero to six days postpartum) and adult rats. Monoclonal antibodies specific for myosin heavy chains of the slow-tonic anterior latissimus dorsi (ALD58) and fast-twitch pectoralis (MF30) muscles of the chicken were used. In adults ALD58 bound to the intracapsular regions of bag1 and bag2 fibers and MF30 bound to the intracapsular regions of bag2 and chain fibers. The extracapsular regions of intrafusal fibers and all extrafusal fibers did not react to ALD58 or MF30. Bag1 and bag2 fibers of neonatal rats expressed immature myosin patterns but chain fibers did not. The adult pattern of immunoreactivity of intrafusal fibers developed by the fourth postnatal day, when the patterns of motor but not sensory innervation in the spindle are still immature. Data suggest that the expression and maintenance of the specific anti-myosin immunoreactivity of intrafusal fibers during postnatal development of rat spindles is dependent upon sensory but not motor innervation. Moreover, afferents might regulate the gene expression responsible for synthesis of myosins isoforms specific to intrafusal fibers only in those myonuclei located within the capsule, but not in the myonuclei in extracapsular regions of intrafusal fibers.

Nonuniform expression of myosin heavy chain isoforms along the length of cat intrafusal muscle fibers

The expression of four myosin heavy chain (MHC) isoforms, avian slow-tonic (ATO) or neonatal-twitch (ANT) and mammalian slow-twitch (MST) or fast-twitch (MFT) in intrafusal fibers was examined by immunocytochemistry of spindles in the tenuissimus muscle of adult cats. The predominant MHCs expressed by nuclear bag fibers were ATO and MST, whereas the MHCs prevalent in nuclear chain fibers were ANT and MFT. The expression of these isoforms of MHC was not uniform along the length of intrafusal fibers. In general, both bag and chain fibers expressed avian MHC in the intracapsular region and mammalian MHC in the extracapsular region. The nonuniform expression of MHCs observed along the length of bag and chain fibers implies that different genes are activated in myonuclei located in the intracapsular and extracapsular regions of the same muscle fiber. Regional differences in gene activation might result from a greater effect of afferents on myonuclei located near the equator of intrafusal fibers then on myonuclei outside the spindle capsule.

Sequential studies of a childhood myopathy: a clinical, histochemical and morphometric investigation

An unusual inherited progressive distal myopathy of early childhood onset is described in two sisters from a consanguineous Asian family. Motor milestones were normal but gait deteriorated slowly thereafter with development of generalized hypotonia and muscle weakness particularly in the wrist extensors and hand muscles. Muscle biopsies obtained at the ages of 6 and 10 years respectively (Case 1) showed significant differences. At 6 years muscle morphology and histochemical appearance were normal although type I fibres predominated (79%) and a substantial pool of 'undifferentiated' fibres (12%) was present. By 10 years there was a significant reduction in type I fibres (-13%) and in 'undifferentiated' fibres (-10%) with a concomitant increase in type II fibres (+23%). Fibre size and shape were normal at the age of 6 years but no further fibre growth was evident 4 years later. The older sister (Case 2, age 13 years) was similarly affected. The possibility of this progressive myopathy being caused by loss of neural control at two separate stages of development is discussed. The importance of performing sequential morphometric studies of muscle biopsies from patients with unusual childhood myopathies is emphasized.

Myosin heavy chain composition of muscle fibers in spinal muscular atrophy

Muscle biopsies from 20 cases of spinal muscular atrophy (SMA), mostly diagnosed as Werdnig-Hoffmann (W-H) disease, were examined for myosin heavy chain (HC) composition. The fetal, fast, and slow heavy chains were characterized in the isolated muscle myosin, and in myosin of single, chemically skinned fibers, by electrophoresis in SDS-6% polyacrylamide gels and by immunoblot techniques, using specific antibodies directed to each main type of myosin HC. The fiber distribution of myosin HC isozymes was further investigated on muscle cryostat sections by an indirect immunofluorescent technique. Fetal myosin HC was found to be expressed in a subpopulation of severely atrophic fibers, alone or together with the slow form of myosin HC. Triangulated fibers of intermediate size contained fetal and fast myosin or fast myosin alone. The hypertrophic fibers were characterized by the predominant expression of slow myosin HC; but in some of these fibers, also low amounts of HC fetal were found to be expressed. These findings are discussed in relation to developmental transitions of myosin heavy chains in human muscle.

The expression of slow myosin during mammalian somitogenesis and limb bud differentiation

The developmental pattern of slow myosin expression has been studied in mouse embryos from the somitic stage to the period of secondary fiber formation and in myogenic cells, cultured from the same developmental stages. The results obtained, using a combination of different polyclonal and monoclonal antibodies, indicate that slow myosin is coexpressed in virtually all the cells that express embryonic (fast) myosin in somites and limb buds in vivo as well as in culture. On the contrary fetal or late myoblasts (from 15-d-old embryos) express in culture only embryonic (fast) myosin. At this stage, muscle cells in vivo, as already shown (Crow, M.T., and F.A. Stockdale. 1986. Dev. Biol. 113:238-254; Dhoot, G.K. 1986. Muscle & Nerve. 9:155-164; Draeger, A., A.G. Weeds, and R.B. Fitzsimons. 1987. J. Neurol. Sci. 81:19-43; Miller, J.B., and F.A. Stockdale. 1986. J. Cell Biol. 103:2197-2208), consist of primary myotubes, which express both myosins, and secondary myotubes, which express preferentially embryonic (fast) myosin. Under no circumstance neonatal or adult fast myosins were detected. Western blot analysis confirmed the immunocytochemical data. These results suggest that embryonic myoblasts in mammals are all committed to the mixed embryonic-(fast) slow lineage and, accordingly, all primary fibers express both myosins, whereas fetal myoblasts mostly belong to the embryonic (fast) lineage and likely generate fibers containing only embryonic (fast) myosin. The relationship with current models of avian myogenesis are discussed.

Immunocytochemical and electrophoretic analyses of changes in myosin gene expression in cat posterior temporalis muscle during postnatal development

Changes in myosin gene expression during the postnatal development of the homogeneously superfast kitten posterior temporalis muscle were examined using immunocytochemical techniques supplemented by pyrophosphate gel electrophoresis and gel electrophoresis-derived enzyme linked immunosorbent assay (GEDELISA) of myosin isoforms. The antibodies used were polyclonals directed against the heavy chains of superfast and foetal myosins and monoclonals against the heavy chains of slow and fast myosins. The fibres of the posterior temporalis in the newborn kitten stained almost uniformly with the anti-foetal myosin antibody and the largest of these fibres stained strongly for superfast myosin. A subpopulation of fibres staining for superfast myosin also stained lightly for slow myosin. These slow staining fibres were evenly distributed in the centres of muscle fibre bundles, reminiscent of primary fibres in limb fast muscle. During subsequent development, slow myosin staining disappeared and superfast myosin replaced foetal myosin so that by 50 days the muscle was virtually homogeneously superfast as in the adult. Fast myosin was never expressed at any stage. It is proposed that fibres staining transiently for slow myosin are superfast primary fibres which are homologous to fast primary fibres recently described in regions of limb muscles devoid of slow fibres in the matured animal. Other jaw-closing muscles have significant populations of slow fibres in the mature animal and it is postulated that there exists in these muscles a second class of jaw primary fibres, the slow primary fibres, in which slow myosin synthesis would be sustained in the adult. It is suggested that the myogenic cells of jaw-closing and limb muscles are of two distinct types preprogrammed to express different muscle genes.

Immunocytochemical and electrophoretic analyses of changes in myosin gene expression in cat limb fast and slow muscles during postnatal development

Changes in myosin synthesis during the postnatal development of the fast extensor digitorum longus (EDL) and the slow soleus muscles of the kitten were examined using immunocytochemical techniques supplemented by pyrophosphate gel electrophoresis and gel electrophoresis-derived enzyme linked immunosorbent assay (GEDELISA) of myosin isoforms. The antibodies used were monoclonals against heavy chains of slow and fast myosins and a polyclonal against foetal/embryonic myosin. In both muscles in the newborn kitten, there was a population of more mature fibres which stained strongly for slow but weakly for foetal/embryonic myosin. These fibres were considered to be primary fibres. They formed 4.8% of EDL fibres and 26% of soleus fibres at birth, and continued to express slow myosin in adult muscles. The less mature secondary fibres stained strongly for foetal/embryonic myosin, and these could be divided into two subpopulations; fast secondaries in which foetal/embryonic myosin was replaced by fast myosin, and slow secondaries in which the myosin was replaced by slow myosin. At 50 days the EDL had a large population of fast secondaries (83% of total fibres) and a small population of slow secondaries which gradually transformed into fast fibres with maturity. The vast majority of secondary fibres in the soleus were slow secondaries, in which slow myosin synthesis persisted in adult life. There was a restricted zone of fast secondaries in the soleus, and these gradually transformed into slow fibres in adult life. It is proposed that the emergence of primary fibres and the two populations of secondary fibres is myogenically determined.

Atypical persisting fibres in explants of human muscle cocultured with embryonic nerve cells

When samples of human muscle are cocultured with embryonic mouse spinal cord, the muscle fibres usually regenerate to form a bundle of new myotubes which become innervated and develop cross-striations and contractions. However, we have noticed that in some cultures of 6 biopsies of human muscle, there were fibres which "persisted" in the cultures for long periods of time without being replaced by regenerated myotubes. At both the light and electron microscopic levels, they appeared to be mature muscle with well-organized myofibrils, intact plasma membranes and closely-apposed basal laminae. At least some of the fibres contained adult myosin heavy chains. They were only found in freshly-cultured samples, and appeared to be associated with younger patients, but were not associated with any particular muscle condition. The nature of these persisting fibres is discussed, and we emphasize the need for sequential observation of cultures to ensure that fibres which have regenerated in culture are distinguished from those which have persisted from the original biopsy.

Ultrastructural identification of type 1 fibres in human skeletal muscle. Immunogold labelling of thin cryosections with a monoclonal antibody against slow myosin

Existing methods for the ultrastructural identification of fibre types in human skeletal muscle are fallible. This has prompted us to develop a reliable immunoelectron microscopic approach for the identification of human skeletal muscle fibre types. Here we report the unambiguous electron microscopic identification of human type 1 muscle fibres, achieved by combining cryoultramicrotomy with colloidal gold immunocytochemical labelling, using a monoclonal antibody (N0Q7.5.4D) which is specific for the heavy chain of the slow myosin isoform of human skeletal muscle. This method for the identification of muscle fibre types and determination of myosin isoform distributions may have important applications in the ultrastructural study of pathological muscle and in the analysis of myofibrillar assembly during myogenesis.