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

Cellular heterogeneity during vertebrate skeletal muscle development

Although skeletal muscles appear superficially alike at different anatomical locations, in reality there is considerably more diversity than previously anticipated. Heterogeneity is not only restricted to completely developed fibers, but is clearly apparent during development at the molecular, cellular and anatomical level. Multiple waves of muscle precursors with different features appear before birth and contribute to muscular diversification. Recent cell lineage and gene expression studies have expanded our knowledge on how skeletal muscle is formed and how its heterogeneity is generated. This review will present a comprehensive view of relevant findings in this field.

[Histochemical study of the sensory endings of muscle spindles in rat longissimus muscles]

Most studies concerning the structure and function of muscle spindles have utilized the hind limbs of experimental animals. However, little is known about muscle spindles of the back muscles. The purpose of this study was to investigate the sensory innervation of muscle spindles of the paravertebral muscle in the rat. The subjects were 10 normal male rats. The longissimus muscles were isolated and frozen in cooled isopentane (-160 degrees C), and serial transverse sections were made with a cryostat. Histochemical preparations were then made using nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR) stain and modified Gomori-trichrome stain. The muscle spindles in each segment were identified microscopically by observing the equatorial and polar regions. NADH-TR staining was employed to distinguish nuclear bag1, nuclear bag2, and nuclear chain intrafusal muscle fibers. A total of 20 spindle poles were surveyed. The mean polar length of intrafusal fibers as well as that of each region (A, B, and C) were measured. NADH-TR staining also demonstrated the terminal sites of sensory fibers along intrafusal fibers. All spindle poles surveyed were innervated by secondary sensory fibers in addition to primary sensory fibers. Eight spindle poles were intermediate type muscle spindles that were innervated by one primary sensory fiber and one secondary sensory fiber. Twelve spindle poles were complex type muscle spindles that were innervated by one primary sensory fiber and multiple secondary sensory fibers. The mean length of the A region was 223.1+/-37.9 microm (n=8) for intermediate type spindles and 493.8+/-157.0 microm (n=12) for complex type spindles. The length of the A region was significantly longer in the complex type spindles than in the intermediate type spindles (p<0.001). The results suggest that the innervations of secondary sensory fibers were well developed in the longissimus muscle spindles in the rat. The morphological features of muscle spindles of the longissimus muscle may represent the structural basis for qualitatively different afferent discharges that relate to the characteristic types of locomotion served by paravertebral muscles.

Proprioceptive afferents survive in the masseter muscle of trkC knockout mice

Peripheral innervation patterns of proprioceptive afferents from dorsal root ganglia and the mesencephalic trigeminal nucleus were assessed in trkC-deficient mice using immunohistochemistry for protein gene product 9.5 and parvalbumin. In trkC knockout mice, spinal proprioceptive afferents were completely absent in the limb skeletal muscles, M. biceps femoris and M. gastrocnemius, as previously reported. In these same animals, however, proprioceptive afferents from mesencephalic trigeminal nucleus innervated masseter muscles and formed primary endings of muscle spindles. Three wild-type mice averaged 35.7 spindle profiles (range: 31-41), six heterozygotes averaged 32.3 spindles (range: 27-41), and four homozygotes averaged 32.8 spindles (range: 26-42). Parvalbumin and Nissl staining of the brain stem showed approximately 50% surviving mesencephalic trigeminal sensory neurons in trkC-deficient mice. TrkC-/- mice (n = 5) had 309.4 +/- 15.9 mesencephalic trigeminal sensory cells versus 616.5 +/- 26.3 the sensory cells in trkC+/+ mice (n = 4). These data indicate that while mesencephalic trigeminal sensory neurons are significantly reduced in number by trkC deletion, they are not completely absent. Furthermore, unlike their spinal counterparts, trigeminal proprioceptive afferents survive and give rise to stretch receptor complexes in masseter muscles of trkC knockout mice. This indicates that spinal and mesencephalic trigeminal proprioceptive afferents have different neurotrophin-supporting system during survival and differentiation. It is likely that one or more other neurotrophin receptors expressed in mesencephalic trigeminal proprioceptive neurons of trkC knockout mice compensate for the lack of normal neurotrophin-3 signaling through trkC.

Slow tonic muscle fibers in the thyroarytenoid muscles of human vocal folds; a possible specialization for speech

Most of the sounds of human speech are produced by vibration of the vocal folds, yet the biomechanics and control of these vibrations are poorly understood. In this study the muscle within the vocal fold, the thyroarytenoid muscle (TA), was examined for the presence and distribution of slow tonic muscle fibers (STF), a rare muscle fiber type with unique contraction properties. Nine human TAs were frozen and serially sectioned in the frontal plane. The presence and distribution pattern of STF in each TA were examined by immunofluorescence microscopy using the monoclonal antibodies (mAb) ALD-19 and ALD-58 which react with the slow tonic myosin heavy chain (MyHC) isoform. In addition, TA muscle samples from adjacent frozen sections were also examined for slow tonic MyHC isoform by electrophoretic immunoblotting. STF were detected in all nine TAs and the presence of slow tonic MyHC isoform was confirmed in the immunoblots. The STF were distributed predominantly in the medial aspect of the TA, a distinct muscle compartment called the vocalis which is the vibrating part of the vocal fold. STF do not contract with a twitch like most muscle fibers, instead, their contractions are prolonged, stable, precisely controlled, and fatigue resistant. The human voice is characterized by a stable sound with a wide frequency spectrum that can be precisely modulated and the STF may contribute to this ability. At present, the evidence suggests that STF are not presented in the vocal folds of other mammals (including other primates), therefore STF may be a unique human specialization for speech.

Coculture of rat embryonic proprioceptive sensory neurons and myotubes

With the aim to study the cellular mechanism underlying the process of muscle spindle regeneration, dorsal root ganglia (DRG) neurons derived from 16-day rat embryos were cocultured with developing myotubes in a compartmentalized culture device. To accomplish the selective survival and neurite formation of the proprioceptive subpopulation, the neurotrophic factor, neurotrophin-3, was added to the culture medium. It appeared that the proprioceptive DRG neurons could develop specialized, Ia afferent terminal-like contacts with myotubes. However, these interactions were scarce and did not result in the induction of differentiation of the contacted myotubes into intrafusal fibers as normally occurs during in vivo development. The present coculture setup apparently lacks appropriate regulatory factors essential for the proper matching of sensory axons and intrafusal fiber precursors and the induction of a functional sensory myoneural connection.

Expression of myosin heavy chain isoforms and myogenesis of intrafusal fibres in rat muscle spindles

This review concerns the pattern of expression and regulation of myosin heavy chain (MHC) isoforms in intrafusal fibres of rat muscle spindles detected by immunocytochemistry. The three types of intrafusal fibres--nuclear bag1, nuclear bag2, and nuclear chain fibres--are unique in co-expressing several MHCs including special isoforms such as slow tonic and alpha cardiac-like MHC and isoforms typical of muscle development, such as embryonic and neonatal MHC. The distinct intrafusal fibre types appear sequentially during rat hind limb development, the nuclear bag2 precursors being first identifiable at 17-18 days in utero as the only primary myotubes expressing slow tonic MHC. Sensory innervation is required for the expression of "spindle-specific" MHC isoforms. Motor innervation contributes to the diversity in distribution of the different MHCs along the length of the nuclear bag fibres. It is suggested that unique populations of myoblasts are destined to become intrafusal fibres during development in the rat hind limb muscles and that the regional heterogeneity in MHC expression is related both to sensory and motor innervation and to the properties of the myoblast lineages. These distinct features make intrafusal fibres an attractive in situ model for investigating myogenesis, myofibrillogenesis, and the mechanisms regulating MHC expression.

Selective expression of neurotrophin-3 messenger RNA in muscle spindles of the rat

The expression of neurotrophin-3 messenger RNA was studied by in situ hybridization in rat muscle spindles from the first embryonic stages of their formation until their mature appearance in adult animals. The first expression of neurotrophin-3 messenger RNA in developing muscles was observed at E19 in the firstly formed intrafusal fiber, the nuclear bag2 fiber. High levels of neurotrophin messenger RNA were found in the equatorial region of these intrafusal fibers in thin lines of cytoplasma around and between the packed-up nuclei. From E21 on, neurotrophin-3 messenger RNA was also present in the nuclear bag1 type intrafusal fiber. The expression of neurotrophin-3 messenger RNA in nuclear chain fibers, which were found in muscle spindles from day 6 after birth, was low and insignificant in comparison to the expression in the nuclear bag fibers. After completion of muscle spindle formation around the third week after birth, high levels of neurotrophin-3 messenger RNA remained present in the intrafusal fibers throughout life. During the entire period of muscle formation, examined from E15 on, as well as in mature muscles, no neurotrophin-3 messenger RNA could be detected in extrafusal fibers by in situ hybridization. The exclusive intramuscular expression of neurotrophin-3 messenger RNA in intrafusal fibers during development as well as in mature stages suggests the involvement of neurotrophin-3 in the formation and the maintenance of muscle spindles.

Differentiation of supernumerary fibres in neonatally deefferented rat muscle spindles

The myofibrillar ATPase (mATPase) activity and the pattern of expression of several myosin heavy chain (MHC) isoforms and of M-protein (M(r) 165,000) were studied in serial cross sections of neonatally deefferented 5- to 8-week-old rat hindlimb muscle spindles with supernumerary intrafusal fibres. In a sample of 5- to 6-week-old neonatally deefferented muscle spindles cut through the A region, the average number of intrafusal fibres per spindle was 8.4 in comparison to 4.2 in control spindles. Parent fibres extended throughout the whole encapsulated portion of the spindle, whereas supernumerary fibres were found only in the A region. The diameters of the supernumerary intrafusal fibres varied from less than 1 micron up to 10 microns approximately. On the basis of the mATPase activity and the pattern of expression of MHC isoforms and of M-protein, the vast majority of the supernumerary fibres could be classified as nuclear bag2, bag1 or chain fibres. However, some supernumerary fibres with small diameters exhibited features that did not fit any of the three known intrafusal fibre types. Two major processes, namely fibre splitting versus activation and fusion of satellite cells, might account for the formation of supernumerary fibres. The data presented suggest the existence of at least two types of intrafusal satellite cells. One type of satellite cell is related to the nuclear bag fibres and gives rise to myotubes which, if they have sensory innervation, can express slow tonic MHC and, therefore, differentiate into a phenotype similar to that seen in nuclear bag fibres.(ABSTRACT TRUNCATED AT 250 WORDS)

Calbindin D-28k-immunoreactivity in rat muscle spindles during postnatal maturation and after denervation

Calbindin D-28k-immunoreactivity has been demonstrated in some of the intrafusal muscle fibres and in the capsule of adult rat muscle spindles. In this study, the immunocytochemical localization of calbindin D-28k in the muscle spindles of triceps surae muscle was studied during postnatal maturation and after denervation. In young rats calbindin D-28k-immunoreactivity was seen in a few intrafusal fibres, first at the age of 4 days. At the 7th day, three calbindin D-28k-immunoreactive fibres and one unlabelled fibre were seen in most muscle spindles, as in adult rats. The spindle capsule and perineurial sheath of nerves were first seen to exhibit calbindin D-28k immunoreactivity at the age of 14 days, and thereafter the localization of calbinding D-28k-like immunoreactivity was similar to that in adult rats. After denervation, calbindin D-28k-immunoreactivity remained in intrafusal muscle fibres and the spindle capsule for a long period. After two months of denervation, calbindin D-28k immunoreactivity could still be seen in the spindle capsule, but the intrafusal fibres were not labelled. The innervation is known to have trophic effects on the intrafusal fibres. The present findings suggest that the expression of calbindin D-28k-immunoreactivity in maturating muscle spindles may be induced by the developing innervation. The decrease of calbindin D-28k-immunoreactivity in intrafusal fibres after denervation may be due to the loss of trophic factors released by the nerves.

Aggregation of myonuclei and the spread of slow-tonic myosin immunoreactivity in developing muscle spindles

The pattern of regional expression of a slow-tonic myosin heavy chain (MHC) isoform was studied in developing rat soleus intrafusal muscle fibers. Binding of the slow-tonic antibody (ATO) began at the equator of prenatal intrafusal fibers where sensory nerve endings are located, and spread into the polar regions of nuclear bag2 and bag1 fibers but not nuclear chain fibers during ontogeny. The onset of the ATO reactivity coincided with the appearance of equatorial clusters of myonuclei (nuclear bag formations) in bag1 and bag2 fibers. Moreover, the intensity of the ATO reaction was strongest in the region of equatorial myonuclei and decreased with increasing distance from the equator of bag1 and bag2 fibers at all stages of prenatal and postnatal development. The polar expansion of ATO reactivity continued throughout the postnatal development of bag1 fibers, but ceased shortly after birth in bag2 fiber coincident with innervation by motor axons. Thus, afferents that innervate the equator might induce the slow-tonic MHC isoform in bag2 and bag1 fibers by regulating the myosin gene expression by equatorial myonuclei, and efferents or twitch contractile activity might inhibit the spread of the slow-tonic MHC isoform into the poles of bag2 but not bag1 fibers. Absence of ATO binding in chain fibers suggests that chain myotubes may not be as susceptible to the effect of afferents as are myotubes that develop into bag2 and bag1 fibers. The different patterns of slow-tonic MHC expression in the three types of intrafusal fiber may therefore result from the interaction of three elements: sensory neurons, motor neurons, and intrafusal myotubes.

Axon contacts and acetylcholinesterase activity on chicken intrafusal muscle fiber types identified by their myosin heavy chain composition

Muscle spindles of 8-week old chicken tibialis anterior muscles were examined to determine if specific intrafusal fiber types were also characterized by differences in motor innervation. Incubation with a monoclonal antibody against myosin heavy chains permitted the identification of strongly reactive, moderately reactive and unreactive intrafusal fibers. The innervation of each fiber type was evaluated in silver-impregnated sections, and in sections incubated with a monoclonal antibody against acetylcholinesterase. There was no acetylcholinesterase activity at the midequator of any fiber. At the juxtaequator and at the pole strongly reactive fibers typically exhibited fewer axon contacts and less acetylcholinesterase activity than unreactive and moderately reactive fibers. Differences were also recognized at neuromuscular junctions in the size and shape of acetylcholinesterase-positive sites. At the juxtaequator and at the pole strongly reactive fibers and moderately reactive fibers displayed significantly more small, dot-like acetylcholinesterase sites than unreactive fibers. On the contrary, the greatest number of larger, stout sites was found on unreactive fibers and the least number on strongly reactive fibers. Moderately reactive fibers took an intermediate position. The results indicate that myosin heavy chain-based chicken intrafusal fiber types are also set apart by differences in 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.

Slow-tonic MHC expression in paralyzed hindlimbs of fetal rats

Whether nerve activity and active contraction of myotubes are essential for the assembly and initial differentiation of muscle spindles was investigated by paralyzing fetal rats with tetrodotoxin (TTX) from embryonic day 16 (E16) to E21, prior to and during the period when spindles typically form. TTX-treated soleus muscles were examined by light and electron microscopy for the presence of spindles and expression of myosin heavy chain (MHC) isoforms by the intrafusal fibers. Treatment with TTX did not inhibit the formation of a spindle capsule or the expression of a slow-tonic MHC isoform characteristic of intrafusal fibers, but did retard development of spindles. Spindles of TTX-treated E21 muscles usually consisted of one intrafusal fiber (bag2) only rather than two fibers (bag1 and bag2) typically present in untreated (control) E21 spindles. Intrafusal fibers of TTX-treated spindles also had only one sensory region supplied by multiple afferents, and were devoid of motor innervation. These features are characteristic of spindles in normal E18-E19 muscles. Thus, nerve and/or muscle activity is not essential for the assembly of muscle spindles, formation of a spindle capsule, and transformation of undifferentiated myotubes into the intrafusal fibers containing spindle-specific myosin isoforms. However, activity may promote the maturation of intrafusal bundles, as well as the maturation of afferent and efferent nerve supplies to intrafusal fibers.

Non-neural and neural expression of myosin heavy chains by regenerated intrafusal fibers of rats

Expression of myosin heavy chain (MHC) isoforms was studied in rat soleus (SOL) and extensor digitorum longus (EDL) muscles which regenerated in the presence or absence of innervation. Frozen sections of two 5 day denervated SOL and EDL grafts, two 40 day denervated SOL and EDL grafts, and two reinnervated 40 day SOL and EDL grafts were processed for demonstration of motor endplates, sensory endings, myosin adenosine triphosphatase (mATPase) and for expression of 4 MHCs. No qualitative differences in MHC expression were noted between 5 day or 40 day denervated grafts of the SOL and EDL muscles. All regenerated intrafusal and extrafusal myotubes or myofibers reacted to antibodies against neonatal and fast-twitch MHCs, but not to antibodies against slow-twitch and slow-tonic MHCs in these grafts. These data indicate that MHCs expressed by regenerated intrafusal myotubes do not parallel those expressed by myotubes which give rise to the three types of intrafusal fibers during development and that MHC expression by regenerated intrafusal myotubes parallels that of regenerated extrafusal myotubes prior to innervation. However, some regenerated intrafusal fibers in 40 day nerve-intact grafts bound antibodies to slow-twitch and slow-tonic MHCs, thus expressions of these two MHCs are nerve-dependent in regenerated muscle spindles.

Neural organization of spindles in three hindlimb muscles of the rat

The neuroanatomical organization of the dynamic (bag1) and static (bag2 and chain) intrafusal systems was compared by light and electron microscopy of serial sections among 71 poles of muscle spindle in soleus (SOL), extensor digitorum longus (EDL), and lumbrical (LUM) muscles in the rat. Eighty-four percent of 195 fusimotor (gamma) axons to the spindles innervated either the dynamic bag1 fiber or the static bag2 and/or chain fibers. Sixteen percent of the gamma axons coinnervated the dynamic and static intrafusal fibers. Some of these nonselective axons were branches of effernts that also gave rise to axons selective to either the dynamic or static types of intrafusal fibers in one or more spindles. Thus activation of individual stem gamma efferents might not have a purely dynamic or purely static effect on the integrated afferent outflow from spindles of a hindlimb muscles in the rat. In addition, primary afferents in all muscles had terminations that cross-innervated the dynamic bag1 and static bag1 and/or chain intrafusal fibers in individual spindles, an arrangement that may enhance the mixed dynamic/static behavior of afferents when different intrafusal fibers are activated concurrent. Spindles of the slow SOL and fast EDL muscles had similar features, whereas differences were observed in the organization of the proximal (SOL and EDL) and distal (LUM) muscles. Spindles in LUM muscles had fewer static intrafusal fibers, a higher ratio of dynamic to static gamma axons, and a higher incidence of skeletofusimotor (beta) innervation to intrafusal fibers than spindles in the SOL or EDL muscles. Thus, the relative contribution of dynamic and static systems to muscle afferent outflow may differ among spindles located in different segments of the rat hindlimb. However, the dynamic and static intrafusal systems of spindle were less sharply demarcated in each of the three hindlimb rat muscles than in the cat tenuissimus muscle.

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.

Presence in chicken tibialis anterior and extensor digitorum longus muscle spindles of reactive and unreactive intrafusal fibers after incubation with monoclonal antibodies against myosin heavy chains

Cross and longitudinal sections from the encapsulated portions of chicken tibialis anterior and extensor digitorum longus muscle spindles were examined to determine whether their intrafusal fibers were a structurally homogeneous or heterogeneous population. The techniques used were the histochemical actomyosin (mATPase) reaction, and fluorescence immunohistochemistry employing two monoclonal antibodies, CA-83 and CCM-52, that are specific for myosin heavy chains. After incubation with antibody CCM-52, intrafusal fibers fluoresced either strongly or weakly to moderately. Antibody CA-83 was even more selective. In addition to identifying the strongly reactive category, it clearly separated the remaining fibers into unreactive and moderately reactive groups. As a whole, after incubation for mATPase, pH 9.6 preincubation, unreactive fibers stained darker than strongly reactive fibers. Moreover, the cross-sectional area of the unreactive fibers was significantly larger than that of the strongly reactive fibers. In the average-size muscle spindle with six intrafusal fibers, there were four unreactive fibers and two strongly reactive fibers. In about one-third of the receptors examined, one moderately reactive fiber was present. Taken together, the data indicate that intrafusal fibers of chicken tibialis anterior and extensor digitorum longus muscles are not structurally homogeneous. The observed variations can be better explained in terms of different fiber types than of continuous gradients within one type of fiber.

Myosin heavy chain expression in developing rat intrafusal muscle fibers

The immunocytochemical expression of several isoforms of myosin heavy chains (MHC) was determined in developing intrafusal and extrafusal fibers of the soleus muscle of prenatal and postnatal rats. At the onset of spindle assembly, both bag2 intrafusal myotubes and primary extrafusal myotubes bound a slow-twitch MHC antibody, whereas the bag1 and chain myotubes expressed a fast-twitch MHC isoform identical to that expressed by secondary extrafusal myotubes. Subsequently, developing intrafusal fibers began to express unique myosin isoforms, and ceased to express some of the myosin isoforms present initially. The initial similarity in MHC composition of intrafusal and extrafusal fibers suggests that these two kinds of mammalian muscle cell originate from a common pool of bipotential myotubes. Differences in MHC expression by intrafusal and extrafusal fibers in adult muscles might result from the effect of sensory neurons on the developing intrafusal myotubes.

Alteration of intrafusal bundle composition by nerve crush in neonatal rats

We examined the expression of myosin heavy-chain isoforms in intrafusal muscle fibers of spindles formed in gastrocnemius muscles reinnervated in the presence of exogenous nerve growth factor after nerve crush in neonatal rats. Only 50% of the experimental spindles contained intrafusal fibers that expressed a slow-tonic myosin normally expressed by at least one fiber in every rat spindle. In addition, spindles containing only bag1 and/or chain fibers, but no bag2 fibers, were observed in reinnervated muscles whereas all normal spindles contain a bag2 fiber. These data suggest that afferents retain the capacity to induce the expression of a spindle-specific myosin in a period other than during normal development of intrafusal fibers. However, a scarcity of precursor cells available to become intrafusal fibers when contacted by afferents might have resulted in the alteration of intrafusal bundle composition in some spindles of reinnervated muscles.

Role of nerve and muscle factors in the development of rat muscle spindles

The soleus muscles of fetal rats were examined by electron microscopy to determine whether the early differentiation of muscle spindles is dependent upon sensory innervation, motor innervation, or both. Simple unencapsulated afferent-muscle contacts were observed on the primary myotubes at 17 and 18 days of gestation. Spindles, encapsulations of muscle fibers innervated by afferents, could be recognized early on day 18 of gestation. The full complement of spindles in the soleus muscle was present at day 19, in the region of the neuromuscular hilum. More afferents innervated spindles at days 18 and 19 of gestation than at subsequent developmental stages, or in adult rats; hence, competition for available myotubes may exist among afferents early in development. Some of the myotubes that gave rise to the first intrafusal (bag2) fiber had been innervated by skeletomotor (alpha) axons prior to their incorporation into spindles. However, encapsulated intrafusal fibers received no motor innervation until fusimotor (gamma) axons innervated spindles 3 days after the arrival of afferents and formation of spindles, at day 20. The second (bag1) intrafusal fiber was already formed when gamma axons arrived. Thus, the assembly of bag1 and bag2 intrafusal fibers occurs in the presence of sensory but not gamma motor innervation. However, transient innervation of future bag2 fibers by alpha axons suggests that both sensory and alpha motor neurons may influence the initial stages of bag2 fiber assembly. The confinement of nascent spindles to a localized region of the developing muscle and the limited number of spindles in developing muscles in spite of an abundance of afferents raise the possibility that afferents interact with a special population of undifferentiated myotubes to form 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.

The effect of neonatal deafferentation or defferentation on myosin heavy chain expression in intrafusal muscle fibers of the rat

Muscle spindles were either deafferented or deefferented by selectively severing the sensory or motor nerve supply to neonatal soleus muscles of rats at a time when spindles are formed but when intrafusal muscle fibers are structurally and immunocytochemically immature. Experimental muscles were excised two months after nerve section. Control and experimental spindles were examined using monoclonal antibodies specific for myosin heavy chains of slow-tonic (ALD58) and fast-twitch (MF30) chicken muscles. Only intrafusal fibers bound these antibodies in intact soleus muscles. The deefferented spindles exhibited a pattern of ALD58 and MF30 binding similar to that of normal adult intrafusal fibers, whereas deafferented intrafusal fibers were unreactive with the two antibodies. Thus intact sensory innervation is essential for myosin heavy chain expression in intrafusal muscle fibers during postnatal development of rat spindles.