Systematic variation in myosin expression along extraocular muscle fibres of the adult rat

Summary Monoclonal antibodies (McAB) specific for fast (C14) and slow (S58) myosin, and a myosin antigenically similar to neonatal/embryonic myosin in mammals (ALD180), were used to characterize the myosin distribution in orbital layer fibres of rat extraocular muscles (EOM) in relation to innervation patterns. The orbital layer is composed of both singly-innervated (SIF) and multiply-innervated (MIF) fibres. The SIFs have the characteristics of twitch fibres, while the MIFs, in addition to possessing many small endings characteristic of tonic fibres, also have an en-plaque-like innervation in the endplate band resembling that of the adjacent SIFs. Myosin expression in MIFs and SIFs is unusual and varies systematically along the length of the fibres. Both SIFs and MIFs label with ALD180, but this labelling is absent in both fibre types in the endplate band region, where all fibres label with C14. Distally and also proximally to the endplate band, SIFs label with both ALD180 and C14, while the MIFs, innervated by many small, superficial endings in these regions, label with ALD180 only. This pattern of myosin expression could also be demonstrated in isolated fibres. The results are discussed in relation to the hypothesis that both populations of orbital layer fibres express constitutively both fast and the neonatal-like myosin, and that superimposed on this constitutive expression twitch or tonic innervation acts locally to selectively suppress either neonatal-like or fast myosin, respectively.     

Myosin isozymes in avian skeletal muscles. II. Fractionation of myosin isozymes from adult and embryonic chicken pectoralis muscle by immuno-affinity chromatography

Chicken pectoralis consists primarily of large white fibres, which react exclusively with antibodies prepared against adult fast myosin. There is, however, a small region of uniformly red fibres which responds to antibodies against adult slow myosin as well as adult fast myosin. The myosin extracted from this red region is also heterogeneous as shown by the presence of both slow and fast light chains. By means of immunoadsorbents, it has been possible to separate the 'red myosin' into a 'fast' component and a 'slow' component. These two fractions have been characterized with respect to their light and heavy chain content by one-dimensional and two-dimensional gel electrophoresis. The myosin heavy chain was reduced to the smaller fragments required for electrophoresis by proteolytic degradation. We conclude from the electrophoretic patterns that the 'fast' and 'slow' myosin components from the pectoralis red region closely resemble the myosin from the white region of the pectoralis and the myosin from the slow anterior latissimus dorsi (ALD) muscle. The demonstration of a 'slow myosin' in adult pectoralis muscle raises the possibility that the crossreactivity of embryonic pectoralis myosin with anti-slow (ALD) myosin antibodies might be due to the presence of such slow components in embryonic chicken muscle. Direct isolation of a slow component from embryonic pectoralis was achieved by immunoadsorption, as described for adult mixed muscle myosin. Analysis of the subunit composition by gel electrophoresis shows an enrichment in adult-type slow light chains, but the heavy chain pattern is quite distinct from that of adult slow heavy chain. These studies suggest that several myosin isozymes exist in embryonic chicken pectoralis, but that none is identical to those myosins found in the different fibres of the adult pectoralis muscle.     

The onset and progress of transformation of avian slow into fast muscles under neural influence

The slow anterior latissimus dorsi (ALD) muscles of newly hatched chickens were transposed and cross0innervated by the mixed, predominantly fast superior brachialis nerve, and investigated 2 to 15 months after the operation. Two months after the operation, myosin ATPase activity of the cross-innervated ALD muscles was still as low as in the control ALD, although the ultrastructure and the histochemical ATPase activity already showed a mixed fibre-type pattern with a predominance of fast -type fibres around the site of nerve implantation. The change of myosin properties of thw whole cross-innervated ALD did not occur until the third month after the operation. At that time, the myosin ATPase activity increased about 2.5 times and light chains of myosin of the fast type appeared in the electrophoretic pattern. The myosin ATPase activity attained 62% of the activity found in the control fast posterior latissimus dorsi muscles at three months; subsequently it remained at about this level reaching 68% 18 months after the operation. The results indicate that approximately two thirds of the cross-innervated ALD muscle fibres became changed towards the fast type under neural influence, whereas about one third remained slow, being re-innervated by the slow-type motor fibres of the implanted nerve.     

Transient appearance of a fast myosin heavy chain epitope in slow-type muscle fibres during stretch hypertrophy of the anterior latissimus dorsi muscle in the adult chicken

Summary Myosin expression during hypertrophy of the chicken anterior latissimus dorsi (ALD) muscle was investigated by immunocytochemical procedures using monoclonal antibodies to the fast and slow isoforms of the myosin heavy chain (myosin HC). Antifast antibody 1F9 bound to the adult fast HC of pectoralis muscle and cross-reacted with the HC found in early developing muscle. Antislow antibody 3D1 bound exclusively to the HC of slow myosin 2 (SM2). Stretch hypertrophy of the ALD was produced by attaching a weight to the wing; there was no evidence for a change in fibre number in the muscle. Between 4 and 6 days of stretch there appeared a dramatic increase in the number of fibres staining with the antifast antibody which reached a peak between 12 and 19 days. By this time between 28 and 52% of the fibres in the stretched ALD stained to varying degrees with the antifast antibody compared with 1% in the unstretched control ALD. Most antifast-stained fibres in the stretched muscle also stained with the antislow antibody; the contralateral control muscle showed mostly antislow staining except for the very small number of strongly antifast-stained fibres. By 50 days in some birds and by 80 days in all birds antifast staining had returned to normal. Analysis of the isomyosin composition of the ALD by native gel electrophoresis did not reveal a significant increase in fast myosin content of the hypertrophied muscle even though immunocytochemical staining may have suggested otherwise.     

Muscle fibre type composition in the lateral muscle of olive flounder Paralichthys olivaceus

In this paper, a combined-method study has been made on the lateral muscle of the teleost olive flounder Paralichthys olivaceus in just-hatched and adult stages. In just-hatched stage, both slow and fast muscle fibres were detected: (1) in situ hybridization analysis indicated that slow and fast myosin heavy chain genes were specifically expressed in the superficial and deep part of the myotomal muscle, respectively; (2) immunohistochemistry analysis showed that fibres in the deep part reacted with anti-fast myosin antibody F310; (3) western blot analysis detected a weak expression of slow myosin and a strong expression of fast myosin. In adult stage, the slow and fast muscle fibres had their own distribution characteristics: (1) hematoxylin/eosin staining showed the histological characteristics of the muscle fibre composition; (2) histochemical observations showed that the deep muscle fibres, and some fibres near the epidermis, contain alkali-stable myofibrillar ATPase activity; (3) immunohistochemistry analysis indicated that all the deep muscle fibres reacted with F310 antibody and some fibres in the superficial layer of muscle also reacted with F310; (4) western blot analysis showed that fast myosin was expressed both in the blended muscles (the mix of superficial and deep muscles) and deep muscles, while slow myosin was mainly expressed in the blended muscles. These findings suggested that both slow and fast muscle fibres existed in the musculature of the olive flounder in just-hatched and adult stages. Notably, the adult fast fibres also exist in the superficial layer of the muscle.     

Myosin isozyme expression in response to stretch-induced hypertrophy in the Japanese quail

When skeletal muscle is subjected to stretch it undergoes a rapid increase in muscle mass. However, the effect of stretch on the native myosin isozyme content of muscle has received attention only recently. Using the Japanese quail to investigate stretch-induced hypertrophy, we demonstrated an increase in the expression of fast myosin in the predominantly slow anterior latissimus dorsi muscle (ALD). The fast myosin content of the control quail ALD is not sufficient to be quantified on native myosin pyrophosphate gels. After 33 days of stretch, the fast myosin content (N = 10) averaged 16 +/- 11% in the stretched muscles and reached a maximum of 40%. Mean hypertrophy in the stretched muscle, as indicated by muscle weight, was 247 +/- 91% (range, 168-378%). Fast myosin was consistently expressed in muscles with hypertrophy greater than 250%. Muscle fiber size from the stretched muscles contained a greater number of fibers with small cross-sectional areas than was observed in controls. These results indicate that substantial remodeling occurs in the stretched ALD muscle of the Japanese quail.     

Freeze-fractured sarcoplasmic reticulum in adult and embryonic fast and slow muscles

Summary There is evidence to suggest that 8 nm calcium transport particles in the sarcoplasmic reticulum are involved in the regulation of twitch properties in adult muscles. We have studied ultrastructural characteristics of the sarcoplasmic reticulum in relation to previously defined physiological changes that take place in the normal course of development. The fast twitch posterior latissimus dorsi (PLD) and the slow tonic anterior latissimus dorsi (ALD) of the chicken were compared using the procedure of freeze-fracture. In the adult PLD, the sarcoplasmic reticulum was composed of longitudinal tubules, which gave rise to fenestrated cisternae at the centre of the H band and to terminal cisternae that form triads regularly at each A-I junction. In most of the fibres (85%), 8 nm intramembrane particles were closely packed in the concave fracture face (P-face). In the ALD, a tubular network with an open circular pattern extended the entire length of the A band and usually throughout the I band as well. Dyads or triads, which were infrequent, were often oriented obliquely. The density of intramembrane particles was low in the majority of the fibres, but there was a significant minority population (30%) in which particle density was relatively high. At 10 daysin ovo, when speed of contraction in both the ALD and PLD is slow, there was a circular configuration of sarcoplasmic reticulum components in both muscles, and particle density was low. Surprisingly, at 18 daysin ovo, when the rate of tension development and relaxation have reached nearly adult values in the fast PLD, this muscle, like the ALD, continued to exhibit a circular arrangement of sarcoplasmic reticulum tubules. The density of P-face particles, although greater than at 10 days, was still low relative to the adult PLD. Estimated values for the 18-day PLD were similar to those calculated for the adult slow muscle. Our observations, along with those of other investigators, suggest that abundant intramembrane particles may be related to the fast twitch properties of the adult PLD. However, they indicate that neither the pattern of membranes typical of the adult fast muscle nor the high content of calcium transport particles is required for the differentiation of fast twitch characteristics.     

Myosin isoform transitions during development of extra-ocular and masticatory muscles in the fetal rat

Summary The late fetal development of rat extra-ocular and masticatory muscles was examined by myosin immunohistochemistry. The pattern of slow and neonatal myosin isoform expression in primary and secondary myotubes in these muscles was generally similar to that seen by others in limb muscles. We observed a consistent difference between the Sprague-Dawley and Wistar rats in the degree of maturity reached by all muscles studied at a particular age. In both strains, extra-ocular muscles were also about one day in advance of the masticatory muscles. Thus, secondary myotubes were first seen at E17 in Wistar extraocular muscles, at E18 in Sprague-Dawley extra-ocular muscles and Wistar masticatory muscles, and at E19 in Sprague-Dawley masticatory muscles. There was a strikingly early and complete type differentiation of primary myotubes in extraocular muscles, and tonic myosin first appeared before birth in presumptive extrafusal tonic fibres in the orbital layer of the oculorotatory muscles. Throughout the late fetal period, retractor bulbi was composed of fast myotubes only, but these myotubes were not arranged in classical clusters. In the masticatory muscles at E17/E18 some slow primary myotubes started to express tonic myosin, and these presumptive spindle bag2 fibres were located only in regions of the muscles known to contain spindles in the adult. Presumptive bag1 fibres appeared about a day later (initially without tonic myosin), and in the region of the spindle cluster in anterior deep masseter extrafusal secondary myotube production appeared to be suppressed.     

Independent development of contractile properties and myosin light chains in embryonic chick fast and slow muscle

1. The contractile speeds and tetanus/twitch ratios of the slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles were studied during embryonic development and correlated with the type of myosin light chains present in these muscles as studied by one and two dimensional polyacrylamide gel electrophoresis. 2. At a time when the contractions of PLD were slow, i.e. in 15 day old embryos, the myosin light chains in this muscle were of the fast type. The slow contraction of this muscle may be due to incomplete and slow activation of the contractile elements. The tetanus/twitch ratio of muscles from 15 day old embryos is low and increases sharply with age. This increase could be due to the maturation of the internal membrane system, and occurs at about the same time as the increase in the speed of contraction. 3. ALD muscles contract slowly during all stages of development, although their tetanus/twitch ratio also increases with age. At 13 days they contain a mixture of fast and slow type myosin light chains and with increasing age the proportion of the slow type myosin light chains increases at the expense of the fast type. The slow time course of contraction of ALD is consistent with the presence of slow type myosin light chains. 4. The possibility that the synthesis of the slow type myosin light chains in ALD is induced by early motor activity in chick embryos is discussed.     

The effect of load on the phenotype of the developing rat soleus muscle

In newborn Wistar rats the load on the soleus muscle was reduced by removing the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles. Eighteen days later the soleus muscles were removed from both the operated and control legs and examined physiologically and histologically. The time course of twitch contraction of the soleus on the operated side was not significantly different from that of control muscles, but the muscles developed less tension. The decreased tension was consistent with a smaller number of muscle fibres. Histochemical and immunocytochemical examination showed that in the operated muscle, fewer fibres reacted with an antibody against slow myosin, while the number of fibres that reacted for alkali-preincubated ATPase, indicative of neonatal or adult fast myosin, was increased. Some fibres expressed both types of myosin. These findings suggest that a reduced load delays the phenotypic expression of slow myosin isoform in the developing soleus muscle.     

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

Summary 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.     

A freeze-fracture study of the anterior and posterior latissimus dorsi muscles of the chicken

The sarcolemma, sarcoplasmic reticulum (SR), and T system of the anterior (tonic) and posterior (fast twitch) latissimus dorsi muscles of the chicken have been examined by the freeze-fracture technique, and quantitative data on the P and E fracture faces have been obtained. The fractured plasma membranes reveal (a) profiles of surface caveolae, (b) randomly distributed intramembranous particles ranging in size from 40–100 Å in diameter, and (c) orthogonal assemblies composed of groups of 60 Å particles in close association, and differences with respect to all three structures are present between the tonic (ALD) and fast twitch (PLD) muscles. In the ALD muscle, the surface caveolae are more uniformly distributed and have smaller openings than in the PLD muscle; the former muscle also has a two-fold higher caveolae density than the latter muscle. The intramembranous particles are more numerous in the ALD than in the PLD muscle in both fracture faces, but the orthogonal assemblies are fewer. The functional significance of these differences in the two fiber types are discussed. The fractured membranes of the SR have intramembranous particles (IMP's) approximately 80 Å in diameter, with a two-fold higher packing density in the PLD than in the ALD muscle. This difference is present in both the longitudinal and cisternal components of the SR. In addition, there are collar-like expansions (CLE's) in the SR of the ALD muscle which are particularly poor in intramembranous particles. These particles are considered to represent Ca²⁺ transport ATP-ase, and the reduced density of IMP's could be a significant factor in the low calcium uptake and slow relaxation characteristics of the ALD muscle.     

Subcellular localization of newly incorporated myosin in rabbit fast skeletal muscle undergoing stimulation-induced type transformation

Summary Immunogold labelling was used to study the distribution of newly synthesized slow muscle myosin (SM) at the ultrastructural level as it replaced fast muscle myosin (FM) in rabbit muscles undergoing stimulation-induced type transformation. Control fast muscle was labelled strongly with antibody to FM and control slow muscle with antibody to SM; label was confined to the A-band. Well-defined differences in the distribution of label within the A-band suggested that the monoclonal antibodies used corresponded to epitopes on different parts of the myosin molecule; this was confirmed by Western blots of subfragments prepared from FM and SM. After 4 weeks of continuous stimulation at 10 Hz, fibres of the tibialis anterior muscle reacted with antibodies to both isoforms; after 6 weeks, labelling was obtained only with antibody to SM. After a 7-week period of stimulation and 3 further weeks of recovery, fibres again reacted with both antibodies. In all positively-labelled sections, the distribution of gold particles was characteristic of the antibody and independent of the origin or history of the fibres. This observation supports the conclusion that newly synthesized myosin is capable of being incorporated throughout the length and cross-section of the A-band.     

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. Anat Rec 256:146–157, 1999. © 1999 Wiley‐Liss, Inc.     

Histochemical dichotomy of extrafusal and intrafusal fibers in an avian slow muscle

The avian anterior latissimus dorsi (ALD) is unique amongst vertebrates since it has been considered to be a true slow (or tonic) skeletal muscle. While the structure and function of the extrafusal fibers in the ALD have been intensively investigated, the intrafusal fibers of its muscle spindles and their relationship to the surrounding extrafusal fibers in this muscle have been virtually neglected. Serial frozen sections of this muscle from normal adult domestic chickens were tested for two separate enzymes: myosin ATPase after preincubations at differing pH (Brooke and Kaiser, '69) and NADH-tetrazolium reductase, a mitochondrial-bound oxidative enzyme. Both enzyme reactions were able to detect two distinct categories of extrafusal fibers in this muscle, as well as two classes of intrafusal fibers in its muscle spindles. Neither of the extrafusal fiber-types reacted like typical fast (or twitch) fibers for myosin ATPase; they did not show a characteristic reversal in their relative staining patterns throughout the alkaline (pH 9.4) to acid (pH 4.6 and 4.3) preincubation range. The majority of fibers (type 1) were significantly larger in their cross-sectional size, consistently stained lightly for ATPase, and showed high NADH-TR activity. They represented about 84% of the total fiber population (n = 3540 ± 75). The other set of extrafusal fibers (type 2) constituted the remaining 16% of the total fiber population. They were smaller in diameter, exhibited high myosin ATPase activity, and reacted less intensely for the oxidative enzyme. The histochemical characteristics of the two kinds of intrafusal fibers were profoundly different from those observed in the extrafusal fibers. Their crosssectional fiber diameters were not significantly different from each other, and they exhibited a reversal in their staining reactions for myosin ATPase following acid preincubation for this enzyme. The results of this study concur with recently published reports of extrafusal fiber heterogeneity in the slow ALD muscle of the chicken. In addition, this work clearly demonstrates a histochemical dichotomy amongst the intrafusal fibers of muscle spindles in this muscle.     

Comparative analysis of satellite cell properties in heavy- and lightweight strains of turkey

The growth of muscle during postnatal development results partly from the proliferation of satellite cells and their fusion with muscle fibres. We analysed the properties of satellite cells in a heavyweight (HW) turkey strain characterized by high body weight and a fast growth rate, and in a lightweight farm strain (LW) characterized by low body weight and a slow growth rate. Satellite cell activation was then examined in stretched-overloaded anterior latissimus dorsi (ALD) muscle by weighting one wing in young turkeys from both strains. As early as day 1 of stretching for HW and day 2 for LW, small embryonic-like fibres expressing ventricular cardiac myosin heavy chain (MHC) isoform were observed. Following four days of stretching, the number of nascent fibres had increased in both strains but was significantly greater in HW than LW ALD muscle. The proliferation and differentiation capacities of satellite cells from HW and LW strains were investigated in culture. As judged by in vitro measurements of 3H-thymidine incorporation and DNA content, satellite cells of HW turkey exhibited a greater proliferative capability than those of LW turkey. No differences in the temporal appearance of muscle markers (desmin, MHC isoforms) were noted in vitro between the two strains. These data confirm our in vivo observations indicating that selection based on growth rate does not modify muscle fibre maturation. Our in vivo and in vitro observations suggest that variations in the postnatal muscle growth pattern between HW and LW strains may be related to a difference in the capacity of their satellite cells to proliferate. This revised version was published online in August 2006 with corrections to the Cover Date.     

The urethral striated sphincter in adult male rat

This study reports the morphology of the urethral sphincter in adult male rats, mainly the histological aspects, the features of the endplates, and the heavy myosin chain distribution in the striated fibres. First, the prostate is entirely out of the striated sphincter, which is surprising when compared to man. Second, the urethral striated sphincter consists of two lateral fascicles separated by an anterior and a posterior strip of connective tissue, which extend from the prostatic urethra (i.e. the part of the urethra which runs though the prostate) to the bulb of the penis. An additional third fascicle of striated muscle (SM) covers the caudal part of the anterior connective strip of the membranous urethra (i.e. the urethra which extends from its prostatic part to the bulb of the penis). In the membranous urethra, the striated sphincter surrounds directly the urethral lumen without intercalated smooth muscle. In urethral cross sections, the endplates detected by α-bungarotoxin, which binds to nicotinic receptors, are clustered in the postero-lateral part of the lateral fascicles. The cross-sectional area of the urethral striated fibres shows a bimodal distribution: the largest fibres are located at the periphery of the sphincter and these fibres express only fast myosin heavy chains (MHC) as shown by immunochemistry. The smallest fibres are less numerous and are situated near the lumen co-expressing fast and slow MHC. All the striated fibres express desmin and dystrophin as SM fibres do. Taken together, these results suggest that the urethral striated fibres in male rat present the same characteristics as those of the skeletal muscles. The predominance of fast fibres is consistent with phasic contractions playing a role not only during micturition and urinary continence but also probably during ejaculation.     

Tenotomy of the avian anterior latissimus dorsi muscle. I. Effect of age on fiber-type transformation and regeneration from the stump in chicks

The chick's anterior latissimus dorsi muscle (ALD) was tenotomized at its origin at either 1 day or 4 weeks of age, and investigated histochemically and ultrastructurally at intervals thereafter to determine whether muscle fiber-type transformation from a slow to a twitch type is greater in young birds than older birds. No transformation of fiber type occurred in either procedure, but a new muscular connection regenerated between the scar tissue at the end of the original tenotomized stump and the former origin. This regenerated muscle had a mosaic pattern of fiber types, as demonstrated by myofibrillar ATPase activity, and contained predominantly fast fibers, as contrasted with controls or the tenotomized portion, which contained predominantly slow tonic muscle fibers. The regenerated portion contained muscle spindles. The original portion of the tenotomized muscle was indistinguishable from the control muscle. These responses of the chick ALD to tenotomy are quite different from those in the pigeon, which are reported in the following study.     

Adaptation in myosin expression of avian skeletal muscle after weighting and unweighting

Summary Stretch-induced hypertrophy of the quail anterior latissimus dorsi is associated with decreased slow myosin 1 and increased slow myosin 2 expression and a small increase in expression of fast myosins. Because reduced neural activity has also been shown to accelerate expression from slow myosin 1 to slow myosin 2, we tested whether the increased expression of slow myosin 2 would be maintained when stretch was removed during a time when muscle activity would not be expected to increase. Quail anterior latissimus dorsi muscles were examined after 0–30 days of stretch overload and after 30 or 60 days of unweighting following 30 days of stretch. As expected, slow myosin 2 expression increased and slow myosin 1 expression decreased after 14–30 days of stretch. Novel findings were that slow myosin 1 and slow myosin 2 returned to control levels after unweighting. Furthermore, the expression of developmental and fast myosin heavy chains were evident by day 7, and maintained throughout wing unweighting. These data are consistent with the hypothesis that alterations in fast and developmental myosin expression result from formation and subsequent maintenance of new fibres during hypertrophy and regression. The relative amount and expression of myosin appears dependent upon mechanical stretch in the anterior latissimus dorsi muscle.     

Structure, distribution and innervation of muscle spindles in avian fast and slow skeletal muscle

Muscle spindles in 2 synergistic avian skeletal muscles, the anterior (ALD) and posterior (PLD) latissimus dorsi, were studied by light and electron microscopy to determine whether morphological or quantitative differences existed between these sensory receptors. Differences were found in the density, distribution and location of muscle spindles in the 2 muscles. They also differed with respect to the morphology of their capsules and intracapsular components. The slow ALD possessed muscle spindles which were evenly distributed throughout the muscle, whereas in the fast PLD they were mainly concentrated around the single nerve entry point into the muscle. The muscle spindle index (number of spindles per gram wet muscle weight) in the ALD was more than double that of its fast-twitch PLD counterpart (130.5±2.0 vs 55.4±2.0 respectively, n=6). The number of intrafusal fibres per spindle ranged from 1 to 8 in the ALD and 2 to 9 in the PLD, and their diameters varied from 5.0 to 16.0 μm and 4.5 to 18.5 μm, respectively. Large diameter intrafusal fibres were more frequently encountered in spindles of the PLD. Unique to the ALD was the presence of monofibre muscle spindles (12.7% of total spindles observed in ALD) which contained a solitary intrafusal fibre. In muscle spindles of both the ALD and PLD, sensory nerve endings terminated in a spiral fashion on the intrafusal fibres in their equatorial regions. Motor innervation was restricted to either juxtaequatorial or polar regions of the intrafusal fibres. Outer capsule components were extensive in polar and juxtaequatorial regions of ALD spindles, whereas inner capsule cells of PLD spindles were more numerous in juxtaequatorial and equatorial regions. Overall, muscle spindles of the PLD exhibited greater complexity with respect to the number of intrafusal fibres per spindle, range of intrafusal fibre diameters and development of their inner capsules. It is postulated that the differences in muscle spindle density and structure observed in this study reflect the function of the muscles in which they reside.