The effect of immobilization on myotendinous junction: an ultrastructural, histochemical and immunohistochemical study.

The effect of immobilization on the myotendinous junction of the calf muscles in the rat was studied histochemically, immunohistochemically and morphometrically with a transmission electron microscope. After 3 weeks of immobilization, the contact area between the muscle cells and tendineal collagen fibres was reduced by almost 50% in both type I (slow-twitch) and type II (fast-twitch) muscle fibres. The terminal finger-like processes of the muscle cells became shallow and cylindrical or were completely atrophied. Their basal membranes were slightly thickened. Histochemically, the most remarkable alteration in the myotendinous junction was the marked decrease in the sulphate containing glycosaminoglycans. In the basal lamina of the muscle fibres, the glycosaminoglycan and proteoglycan content was also reduced. Immunohistochemical analyses revealed that the amount of type III collagen was markedly increased on the myotendinous interface, but the amount and distribution of type I collagen was not affected by immobilization. These findings suggest that immobilization causes degenerative changes at the myotendinous junction, which, in turn, most likely decrease its tensile strength and may predispose it to rupture during activity.     

Macromolecular composition of the myotendinous junction.

The macromolecular composition of the myotendinous junction of the rat Achilles tendon was investigated. Heparan sulphate, chondroitin sulphate, and/or dermatan sulphate could be detected in the terminal processes of the muscle cells, but neither heparin nor keratan sulphate was present. The presence of hyaluronic acid was also questionable. High concentrations of sulphate containing glycosaminoglycans could be demonstrated both in the sarcolemma membranes and extracellular region. The main collagenous component in the myotendinous junction was type I collagen. Also small amounts of type III collagen was found at the myotendinous interface. In addition, high concentrations of fibronectin was present on the muscle cell surfaces of the junction. These results showed that myotendinous junction is histochemically and immunohistochemically a highly specified area rich in various polysaccharides. The high concentration of the polysaccharides in the myotendinous interface may increase the adhesive force between the muscle cell membrane and tendineal collagen fibrils and, by this way, it may be important in improving the elastic buffer capacity of the junction against loading.     

Fine structure of the myotendinous junction and “terminal coupling” in the skeletal muscle of the lamprey,Lampetra japonica

The myotendinous junction in the skeletal muscle of adult lamprey Lampetra japonica was studied with an electron microscope. Numerous finger-like sarcolemmal invaginations were present at the ends of muscle fibers to form the myotendinous junction. Parietal fibers of each muscle unit showed more closely distributed sarcolemmal invaginations than central fibers. Features of the myotendinous junction. Parietal fibers of each muscle unit showed more closely distributed sarcolemmal invaginations than central fibers. Features of the myotendinous junction generally conform to the accounts in the literature. The sarcolemmal invagination was covered on its sarcoplasmic aspect by the connecting filament layer and the dense amorphous attachment layer, and on the extracellular aspect by the intermediary layer and the external lamina with collagen fibrils arising from the myosepta. Sarcolemmal invaginations were sometimes seen to consist of a pair of sarcolemmas of adjacent muscle fibers within a muscle unit, which is characteristic to the myotendinous junction of lamprey. It is noteworthy that the connecting filament layer is much thinner than that, e.g., in the tadpole tail muscles (Nakao, '74). Furthermore, it is much thicker in the parietal fibers than in the central fibers. The sarcolemma of the terminal segment of the invagination frequently showed specific coupling with cisterns of the sarcoplasmic reticulum (terminal coupling). The external lamina is partially or completely deficient in the terminal segment of sarcolemmal invaginations which form terminal couplings so that collagen fibrils contained in the invagination appear to be in direct contact with the sarcolemma; however, definite relationships of collagen fibrils with the sarcolemma and the external lamina in the terminal segment of invagination still remain obscure. This type of coupling is considered to play a role in the coupling of excitation to contraction of muscle fibers as triads and diads.     

Divalent cation-dependent adhesion at the myotendinous junction: ultrastructure and mechanics of failure

Summary Junctional microfibrils, which span the lamina lucida of the vertebrate myotendinous junction, are thought to function in force transmission at the junction. This hypothesis has been tested by disrupting junctional microfibrils through elimination of extracellular divalent cations, and determining the effects of this treatment on the ultrastructure and mechanics of whole frog skeletal muscles passively stretched to failure. Muscles incubated in divalent cation-free solution failed exclusively in the lamina lucida of the myotendinous junction, while control muscles all failed within the muscle fibres, several millimetres away from the junction. Failure sites from divalent cation-free muscles incubated with antibodies against collagen type IV, laminin, and tenascin showed no labelling of the avulsed ends of the muscle fibres, indicating that remnants of junctional microfibrils observed on the cell surface are not composed of any of these extracellular proteins. All three proteins were present on the tendon side of the failure site, confirming that the lamina densa remains attached to the tendon. Breaking stress for control muscles was 3.47×10⁵ N m^-2, and for divalent cation-free muscles, 1.84×10⁵ N m^-2, or approximately half the control value. Breaking strain averaged 1.17 for divalent cation-free muscles and 1.39 for controls, although the difference was not significant. We conclude that junctional microfibrils are components of a divalent cation-dependent adhesion mechanism at the myotendinous junction. In addition, ultrastructural analysis of divalent cation-free fibres stretched just short of failure suggests that a second, divalent cation-independent mechanism persists along the non-junctional cell surface, and can transmit substantial passive tension from myofibrils laterally to the extracellular matrix, bypassing the failed myotendinous junction.     

Fine structure of the myotendinous junction and "terminal coupling" in the skeletal muscle of the lamprey, Lampetra japonica.

The myotendinous junction in the skeletal muscle of adult lamprey Lampetra japonica was studied with an electron microscope. Numerous finger-like sarcolemmal invaginations were present at the ends of muscle fibers to form the myotendinous junction. Parietal fibers of each muscle unit showed more closely distributed sarcolemmal invaginations than central fibers. Features of the myotendinous junction generally conform to the accounts in the literature. The sarcolemmal invagination was covered on its sarcoplasmic aspect by the connecting filament layer and the dense amorphous attachment layer, and on the extracellular aspect by the intermediary layer and the external lamina with collagen fibrils arising from the myosepta. Sarcolemmal invaginations were sometimes seen to consist of a pair of sarcolemmas of adjacent muscle fibers within a muscle unit, which is characteristic to the myotendinous junction of lamprey. It is noteworthy that the connecting filament layer is much thinner than that, e. g., in the tadpole tail muscles (Nakao, '74). Furthermore, it is much thicker in the parietal fibers than in the central fibers. The sarcolemma of the terminal segment of the invagination frequently showed specific coupling with cisterns of the sarcoplasmic reticulum (terminal coupling). The external lamina is partially or completely deficient in the terminal segment of sarcolemmal invaginations which form terminal couplings so that collagen fibrils contained in the invagination appear to be in direct contact with the sarcolemma; however, definite relationships of collagen fibrils with the sarcolemma and the external lamina in the terminal segment of invagination still remain obscure. This type of coupling is considered to play a role in the coupling of excitation to contraction of muscle fibers as triads and diads.     

The fine structure of innervated myotendinous cylinders in extraocular muscles of rhesus monkeys

Summary Many of the myelinated nerve fibres of the distal myotendinous region of rectus muscles terminate on muscle fibre tips. The terminal expansions contain aggregated, small clear vesicles and mitochondria. Neuromuscular clefts at the contacts measure 20–40 nm and are uninterrupted by a basal lamina; the sarcoplasm opposite the contacts is unmodified. Some terminals invaginate the muscle fibre tips and others contact the sides of processes formed by splitting of the tips. The muscle fibre termination, its tendon and the nerve fibre branches are encapsulated to form an end-organ averaging 125 m in length and described as a myotendinous cylinder.Approximately 350 innervated myotendinous cylinders were estimated to be present in the horizontal recti with rather fewer in the vertical rectus muscles. Many of them occur shortly before the main myotendinous junction. All muscle fibres contributing to myotendinous cylinders were identified as the compact, felderstruktur, multi-innervated variety with directly apposed myofibrils that are known to be non-twitch fibres. All felderstruktur fibre terminations examined were encapsulated but 19% of them were not innervated.The nerve terminals of myotendinous cylinders are similar to those described by Dogiel (1906) as palisade endings and it is argued that they meet the morphological criteria of sensory neuromuscular endings. Their disposition suggests a capacity to monitor felderstruktur muscle fibre contraction.     

Tendon and myo-tendinous junction in an overloaded skeletal muscle of the rat

Summary Overloading of rat plantaris muscles was produced by aseptic ablation of the synergists. The morphological changes occurring after 1 or 2 weeks were investigated at the light and electron microscopical level in the distal tendon of the plantaris and at the myotendinous junction. Sham-operated rats were prepared as controls. In the tendon, quiescent fibrocytes were replaced by activated fibroblasts displaying a vesicular nucleus with prominent nucleoli and an outstanding increase in cytomembranes, particularly the rough endoplasmic reticulum and the Golgi complex. The plasmalemma of the fibroblasts was modified by the presence of caveolae and the surbsurface cytoplasm contained many membrane-bound vacuoles. In the tendon, the collagen bundles were disrupted, resulting in the formation of empty longitudinally oriented spaces; in these spaces, as in the pericapillary areas, no inflammatory cells were observed. At the myotendinous junction, fibroblast activation was consistently observed in both control and overloaded specimens. At this level, the sarcolemma of the finger-like projections of muscle fibres presented many caveolae close to clusters of large subsurface vacuoles. These observations indicate that, at the beginning of the compensatory hypertrophy, the adaptative changes to overloading include a non-inflammatory reaction of the tendon characterized by enhanced collagen synthesis and intensive membrane renewal and recycling. From the mechanical point of view this reaction can impair the tendon resistance to stretch. At the myotendinous junction the increased membrane turnover of the sarcolemma and the fibroblast activation can be considered permanent phenomena consequent to the increased stress exerted upon the interface connecting the contractile apparatus to the stroma.     

Sensory terminals on extrafusal muscle fibres in myotendinous regions of developing rat muscles

Summary Axon terminals were observed to form neuromuscular contacts with extrafusal muscle fibres in myotendinous regions of developing rat muscles up to 5 days after birth. These neuromuscular contacts are found in fascicles of muscle fibres connected with differentiating Golgi tendon organs. Axon terminals establishing these contacts are obviously sensory, since they do not degenerate after de-efferentation performed in neonatal rats. The terminals contain mainly clear and dense core vesicles and form neuromuscular connections resembling developing motor endplates, with a cleft about 60 nm wide and basal lamina interposed between the axolemma and the sarcolemma. Each terminal, however, also forms a close contact in a restricted region where the basal lamina is missing; there the cleft is reduced to 20 nm and the axolemmal and sarcolemmal membranes are linked by desmosome-like attachment plaques. After the fifth postnatal day, axon terminals become detached from muscle fibres and are only found among collagen bundles of the tendon organ. The functional significance of these temporary neuromuscular contacts is not clear.     

Type VI collagen in mouse masseter tendon,from osseous attachment to myotendinous junction

Background and Methods: The association of masseter tendon type VI collagen with other extracellular matrix (ECM) components was examined from osseous attachment to myotendinous junction by immunohistochemistry and transmission electron microscopy with ATP treatment and enzyme digestion. Results: In the tendon proper, fibrocytes extended their processes among bundles of striated collagen fibrils and associated with adjacent cells through amorphous materials, thus forming a three-dimensional network. The amorphous or filamentous material was observed around the fibrocyte cell body and along the cell processes, where the localization of type VI collagen was confirmed by immunohistochemistry using anti-type VI collagen antibody. After treatment with 20 mM adenosine 5′-triphosphate (ATP), 100 nm periodic fibrils, an aggregated form of type VI collagen, were formed in the place where amorphous or filamentous material was present before the treatment. In myotendinous junction, the ATP-aggregated periodic fibrils were observed to associate with the external lamina of the muscle cells as well as among junctional tendon collagen fibrils. In the tendonbone boundary, ATP-aggregated periodic fibrils were observed around fibrocartilage-like cells in the uncalcifying area but not in the calcification front. Prolonged ATP treatment or hyaluronidase predigestion caused the formation of type VI collagen periodic fibrils in the area near the calcified matrix. Conclusions: The distribution of type VI collagen in mouse masseter tendon is different in different anatomical position. This may reflect the different functional demand for this collagen. © 1995 Wiley-Liss, Inc.     

Muscle spindles in rheumatoid arthritis

Summary Ultrastructural features of muscle spindles were studied in biopsy material from 100 patients suffering from classical rheumatoid arthritis. Thickening of the outer capsule, increased amount of extracellular ground substance within the inner capsule, and marked thickening of the basement membrane of capillary blood vessels supplying the muscle spindles were observed. Chronic inflammatory cells and macrophages were present within the spindles. Changes affecting the intrafusal muscle fibres were also seen. They were manifest as atrophy and degeneration of the intrafusal muscle fibres, absence of the specialised junctional complexes, and of the intercellular bridges, microladders and satellite cells. It is suggested that the changes affecting the intrafusal muscle fibres are probably secondary. Damage to the myelinated nerves was present, while the sensory and motor nerve endings were well preserved.Temporary Research Fellow the Rotterdam Centre for Rheumatic Disease Present address: Division of Pathology, Department of Obstetrics and Gynaecology, University of Chicago, Chicago, Illinois 60637, USA     

Type V and VI collagen for cohesion of dermal fibrillar structures

Type V and VI collagen were capable to joint each other and with type I and IV collagen, as well as mucopolysaccharides. This capability suggested that these collagens function for cohesion of fibrillar tissue components of dermis. This study demonstrated the locality of these types of collagen in dermis. Fresh specimens of normal skin were fixed in 2% paraformaldehyde in phosphate-buffered saline, overnight. Besides, in order to loosen the twist of collagen fibril, some pieces of the skin specimens were treated by citrate buffer pH 3.0, prior to fixation. The specimens were embedded in Technovit 4100 and the ultrathin sections were stained by antibody to type V collagen and followed by antibody to type I, III, IV and VI collagen. The immune reactant was visualized by gold particles for electron microscopic observation. Type V and VI collagen formed networks in dermis and jointed to collagen fibrils, elastic fibre and basal lamina. Type V collagen was found inside collagen fibrils, broad elastic fibres and junctions. Dermo-epidermal junction showed type V collagen on the dermal aspects of basal lamina and at the sites where anchoring filaments joint to basal lamina, while in junction of mesenchymal tissues, no precise structural components for type V collagen were identified. Type VI collagen wove with type V collagen in dermis and associated with mucopolysaccharides. In conclusion, type V collagen formed networks in dermal interfibrillar space and participated in assembling collagen fibrils and forming broad elastic fibres. Epithelial and mesenchymal cells cohered to the underlying dermal matrix in the junction by type V collagen. Type VI collagen interwove with type V collagen in the interfibrous space and associated with mucopolysaccharides. Types V and VI collagen preserved architecture of dermal matrix.     

Beta-lactam resistance amongEscherichia coli andKlebsiella species blood culture isolates in Finnish hospitals

The aim of this study was to investigate beta-lactam resistance inEscherichia coli andKlebsiella spp. blood culture isolates in Finland. Special attention was given to extended-spectrum beta-lactamases. A total of 566Escherichia coli and 108Klebsiella spp. blood culture isolates were collected from hospitals throughout Finland and their susceptibility to beta-lactam antibiotics studied. Twenty percent ofEscherichia coli and 69 % ofKlebsiella spp. strains were resistant to ampicillin. The mechanisms of resistance were studied by hybridization, isoelectric focusing and the clavulanate double-disk potentiation test. Of the ampicillin-resistantEscherichia coli strains, 83 % produced TEM-1. Of the ampicillin-resistantKlebsiella spp. strains, 43 % produced SHV-1. Only nineEscherichia coli and threeKlebsiella spp. isolates were resistant to cefuroxime (MIC 32 µg/ml), and none were resistant to third-generation cephalosporins. These data were compared with cefuroxime and third-generation cephalosporin consumption levels in Finnish hospitals. Although the use of cephalosporins is far more extensive in Finland than in other Scandinavian countries, none of the isolates produced extended-spectrum beta-lactamases. In conclusion, resistance to cefuroxime has remained rare in Finland, and cefuroxime is still an alternative to third-generation cephalosporins in the treatment of septicemia.E. Ahonen, Kajaani; M. Brander, Lappeenranta; E. Eerola, Turku; J. Eskola, Helsinki; K. Hakkarainen, Tampere; E. Herva, Oulu; L. Hirvas, Rovaniemi; P. Hirvonen, Jyväskylä; H. Jägerroos, Rovaniemi; H. Järvinen, Turku; M.L. Katila, Kuopio; M.L. Klossner, Pori; S. Kontiainen, Helsinki; J. Korpela, Lahti; M. Koskela, Oulu; A. Kostiala-Thompson, Espoo; P. Kärkkäinen, Mikkeli and Savonlinna; U. Larinkari, Kotka; O.-P. Lehtonen, Turku; O. Liimatainen, Tampere; A. Nissinen, Jyväskylä; S. Oinonen, Seinäjoki; O.-V. Renkonen, Helsinki; P. Ruuska, Oulu; H. Sarkkinen, Joensuu; and V. Vikberg, Hämeenlinna.     

Histochemical profile of muscle spindles of rat's sural muscles

The histochemical profile of intrafusal fibres of muscle spindles and muscle spindle capsule were analysed in normal sural muscles of a rat. Weak activity of oxidative enzymes (NADH diaphorase, succinic dehydrogenase, lactate dehydrogenase) and creatine phosphokinase were found in bag1 and bag2 fibres, but the oxidative enzyme activity was moderate in chain fibres. High phosphorylase activity was demonstrated in bag fibres as well as in chain fibres. No differences could be detected in adenosine desaminase activity between these various intrafusal fibres. In the outer capsule of muscle spindles, high amount of type III collagen and elastin, but only small amount of type I collagen and fibronectin could be observed.     

Neuromuscular plasticity following limb immobilization

Summary The effects of immobilization on the ultrastructure of the rat neuromuscular junction of type I and type II muscle fibres were studied both qualitatively and quantitatively. Muscle fibre areas were measured as well The plantaris muscle was immobilized in a shortened position by applying a plaster cast for three weeks. Immobilized muscles were then compared to normal litter mates. Both type I and type II immobilized muscle fibres atrophied. Endplates from type II muscle fibres exhibited greater amounts of degeneration than type I endplates. Degeneration consisted of nerve terminal disruption, exposed junctional folds, postsynaptic areas which contained little or no postjunctional folds, and subjunctional sarcoplasmic masses. In addition to degeneration, the type II endplates also exhibited regeneration in the same endplate consisting of small terminals associated with large expanses of junctional folds, several small terminals occurring within the same primary synaptic cleft, and several axons wrapped by the same Schwann cell. These observations suggest terminal axonal regeneration. Our results demonstrate that limb immobilization produces muscle atrophy as well as denervation-like changes at the neuromuscular junctions which leads to terminal axonal sprouting and an ultrastructural remodelling.     

Molecular alterations in the perijunctional region of frog skeletal muscle fibres following denervation

Summary The anatomical distribution of a frog skeletal muscle antigen was studied using immunofluorescence microscopy and a monoclonal antibody 3B6 that was produced against denervated skeletal muscle. In innervated muscles, the monoclonal antibody 3B6 stain was associated with the inner surface of the muscle plasma membrane at the endplate and myotendinous junction. After denervation, the monoclonal antibody 3B6 stain extended from the endplate laterally around the perimeter of muscle fibres and longitudinally well beyond the endplate for a total length of 600–1000 m. The monoclonal antibody 3B6 stain thus forms a cylindrical structure centred on the endplate. This observation shows that denervation produces a non-homogeneous molecular change in skeletal muscle fibres: an antigen that is present in high concentrations at innervated endplates appears in restricted perijunctional regions of denervated muscle fibres. It further suggests that perijunctional regions of denervated muscle fibres differ from the remaining non-endplate regions in molecular composition and possibly also in function.     

Collagen of slow twitch and fast twitch muscle fibres in different types of rat skeletal muscle

Summary The appearance of collagen around individual fast twitch (FT) and slow twitch (ST) muscle fibres was investigated in skeletal muscles with different contractile properties using endurance trained and untrained rats as experimental animals. The collagenous connective tissue was analyzed by measuring hydroxyproline biochemically and by staining collagenous material histochemically in M. soleus (MS), M. rectus femoris (MRF), and M. gastrocnemius (MG). The concentration of hydroxyproline in the ST fibres dissected from MS (2.72±0.35 g·mg^–1 d.w.) was significantly higher than that of the FT fibres dissected from MRF (1.52±0.33 g·mg^–1 d.w.). Similarly, the concentration of hydroxyproline was higher in ST (2.54±0.51 g·mg^–1 d.w.) than in FT fibres (1.60±0.43 g·mg^–1 d.w.), when the fibres were dissected from the same muscle, MG. Histochemical staining of collagenous material agreed with the biochemical evidence that MS and the slow twitch area of MG are more collagenous than MRF and the fast twitch area of MG both at the level of perimysium and endomysium. The variables were not affected by endurance training. When discussing the role of collagen in the function of skeletal muscle it is suggested that the different functional demands of different skeletal muscles are also reflected in the structure of intramuscular connective tissue, even at the level of endomysial collagen. It is supposed that the known differences in the elastic properties of fast tetanic muscle compared to slow tonic muscle as, e.g., the higher compliance of fast muscle could at least partly be explained in terms of the amount, type, and structure of intramuscular collagen.This study was supported by grants from the Finnish Research Council for Physical Education and Sport (Ministry of Education) and the Academy of Finnland     

Gender differences in the passive stiffness of the human gastrocnemius muscle during stretch

The aim of this study was to determine whether muscle stiffness measured in vivo was different between males and females. Distal displacement of the gastrocnemius medialis myotendinous junction was measured directly using ultrasonography during passive dorsiflexion in eight males and eight females (age range 19–28 years). Plantarflexion torque and myotendinous junction displacement were measured at 5° intervals, where 0° was with the foot at right angles to the tibia. Stiffness of the gastrocnemius medialis muscle was calculated between 0° and 25° of dorsiflexion, and defined as passive plantarflexion torque/distal displacement of the myotendinous junction (N m cm⁻¹). Relative muscle stiffness was also calculated as distal displacement relative to resting muscle length, and as passive torque relative to plantarflexion maximal voluntary contraction torque. No significant gender difference was observed in passive dorsiflexion torque, or in passive torque/maximal voluntary torque throughout the range of motion. Distal displacement of the gastrocnemius myotendinous junction was 26% more in females than in males (P < 0.05). Myotendinous junction displacement was 5.0 ± 1.4% of resting gastrocnemius medialis length in females, and 3.9 ± 0.6% in males. Over 25° of passive dorsiflexion, gastrocnemius medialis muscle stiffness was greater in males than in females by 44% (P < 0.05). In conclusion, based on the in vivo assessment of myotendinous junction displacement, passive gastrocnemius medialis muscle stiffness is greater in males than in females.     

Cyclic muscle twitch contraction inhibits immobilization-induced muscle contracture and fibrosis in rats

We investigated the effects of cyclic muscle twitch contraction caused by neuromuscular electrical stimulation (NMES) on immobilization-induced muscle contracture and fibrosis in rats. Twenty-nine rats were divided into control, immobilization, and immobilization with muscle contraction groups. The ankle joints of the immobilization and muscle contraction rats were fixed in full plantar flexion with a plaster cast for 4 weeks. In the muscle contraction group, cyclic muscle twitch contraction of the soleus muscle was induced using a commercial device (1 Hz, 4 ± 2 mA, 60 min/day, 5 times/week) with the ankle joint immobilized. The dorsiflexion range of ankle joint motion in the muscle contraction group was significantly greater than that in the immobilization group. The expressions of fibrosis-related genes (i.e., hypoxia inducible factor-1α, transforming growth factor-β1, α-smooth muscle actin, and types I and III collagen) were significantly decreased in the muscle contraction group compared to the immobilization group. The fluorescence intensities of type I and type III collagen in the perimysium and endomysium in the muscle contraction group were significantly decreased compared to the immobilization group. These results suggest that cyclic muscle twitch contraction induced by NMES might alleviate skeletal muscle fibrosis, reducing immobilization-induced muscle contracture.     

Graded Arrangement of Collagen Fibrils in the Equine Superficial Digital Flexor Tendon

By using ultramorphological and biochemical methods, we analyzed the regional differences between the three parts of the equine superficial digital flexor tendon (SDFT), namely, the myotendinous junction (MTJ), middle metacarpal (mM), and osteotendinous junction (OTJ). Cross-sectional images showed unique distributions of collagen fibrils of varying diameters in each region. Small collagen fibrils (diameter <100 nm) were distributed predominantly in the MTJ region, and the OTJ region was relatively rich in large collagen fibrils (diameter >200 nm). In the mM region, the collagen fibrils were intermediately distributed between the MTJ and OTJ. The results indicate a graded arrangement of collagen fibrils in the tendon. Type V collagen was detected preferentially in the MTJ region. Since type V collagen is believed to be one of the collagens regulating collagen fibril formation, its possible functionality in the MTJ region in terms of fibril formation and fibril arrangement in the tendon has been discussed here.     

The roles of the synaptic basal lamina and of innervation in directing the accumulation of a synaptic molecule, mAb 3B6 antigen, in regenerating skeletal muscles

Summary We have recently described a novel nonhomogeneous distribution of a muscle synaptic molecule following denervation. Monoclonal antibody (mAb) 3B6 antigen, a molecule concentrated at endplate/junctional regions and myotendinous junctions in innervated muscles, appears in denervated muscles in restricted perijunctional regions that are continuous with and centered on endplates. In the present study we examine the roles of the synaptic basal lamina and of innervation in directing the accumulation of the molecule in newly formed regenerating muscle fibres. In denervated regenerating muscle fibres, mAb 3B6 antigen was associated with the plasma membrane and localized at former junctional and perijunctional regions. In those muscle fibres which displayed the perijunctional distribution, the molecule was preferentially colocalized with and centered on former endplate areas, Altogether, a preference for the localization of mAb 3B6 at former endplate regions was observed in 86–90% of denervated regenerating myofibres. A similar preference was observed in 97–99% of innervated regenerating muscle fibres. However, whereas 85.9% of denervated regenerating muscle fibres displayed a perijunctional distribution of the molecule, only 50.5% of innervated regenerating myofibres exhibited a perijunctional distribution. In addition, mAb 3B6 antigen was detected in the cytoplasm of most of the denervated regenerating myofibres but in none of the innervated ones. These results indicate that the basal lamina directs the preferential accumulation of mAb 3B6 antigen at original synaptic sites. Innervation, which is not a prerequisite for the expression of the molecule by regenerating muscle, down-regulates its overall production and presence in perijunctional regions.