Some observations on the fine structure of the myotendinous junction in myotomal muscle of the tadpole tail

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Developmental mechanism of muscle-tendon-bone complex in the fetal soft palate

OBJECTIVE

This study was performed to investigate how the palatine aponeurosis, medial pterygoid process (MPP) of the sphenoid bone, and tensor veli palatini (TVP) muscle form the pulley: muscle-tendon-bone complex.

DESIGN

Mice at embryonic day (ED) 14-17 were used as sample in this study. Azan staining was performed to observe the morphology, and immunohistochemical staining of desmin was performed to closely observe the development of the myotendinous junction. To confirm the bone formation process, immunohistochemical staining of type II collagen (col II), tartrate-resistant acid phosphatase (TRAP), and alkaline phosphatase (ALP) staining were performed. Furthermore, to objectively evaluate bone formation, the major axis and width of the MPP were measured, and osteoclasts that appeared in the MPP were counted.

RESULTS

At ED 14 and 14.5, ALP showed a reaction throughout the MPP. The col II-positive area expanded until ED 16.5, but it was markedly reduced at ED 17. The TVP initially contacted with the palatine aponeurosis at ED 16.5. The major axis and width of the MPP and the number of TRAP-positive osteoclasts were significantly increased as the TVP and palatine aponeurosis joined.

CONCLUSIONS

Therefore, in addition to the tissue units: muscle, tendon, and bone, the interaction in organogenesis promotes rapid growth of the pulley: muscle-tendon-bone complex.

Ultrastructure of sites of cholinesterase activity on amphibian embryonic muscle cells cultured without nerve

Muscle cells, derived from 1-day-old embryos of Xenopus laevis, were cultured in the absence of nerve and then examined histochemically for cholinesterase activity. Virtually all of the cells exhibited one or more discrete patches of cholinesterase activity on their surface. The patches were located not only on the lower cell surface which was apposed to the floor of the culture dish but also on the lateral and upper surfaces which were not apposed to any solid substrate. At many of the cholinesterase patches the cell surface was invaginated and the invaginations also became filled with the histochemical reaction product. Surface sites of cholinesterase activity were often associated with a thickened plasma membrane and overlying basal lamina-like material. Regions of the cell surface without cholinesterase were typically smooth and ultrastructurally unspecialized. The cholinesterase patches and their associated ultrastructural features developed in the absence of electrical and contractile activity. The similarities between the muscle specializations at cholinesterase sites on cultured, noninnervated, Xenopus muscle cells and those at the neuromuscular and myotendinous junctions of normal skeletal muscle are discussed.

Structural characteristics of the tendinous cord-papillary muscle junction in healthy and hypertensive rats

Although the myotendinous junction (MTJ) of skeletal striated muscle is well known, more detailed studies regarding the structure of the cardiac MTJ are scarce. The objective of the present study was to investigate the morphological characteristics of the MTJ in hearts of healthy and hypertensive (SHR) female rats using histological, ultrastructural and three-dimensional (SEM) methods, as well as to evaluate the expression of vinculin by immunofluorescence. In the two groups, light microscopy showed branching tendinous cords and collagen bundles penetrating the apex of the finger-like projections of the papillary muscle. SEM analysis revealed an enlarged apex of the papillary muscle in SHR which was not observed in healthy animals. The loss of force transmission appears to be compensated by the amplified connection between the papillary muscle and valvular collagen. A large number of intercalated disks close to the fiber apex, small amounts of an amorphous intercellular substance and numerous vesicles were observed in SHR. In these animals, the expression of vinculin was more marked showing a regular distribution and a pattern of transverse striations along the sarcolemma. The presence of this protein in transverse bands suggests that vinculin surrounds myofibrils in the region of the Z band. Vinculin staining was also more marked in the region of the tendinous cord-papillary muscle junctions of SHR compared to control animals. Vinculin was quantified by electrophoresis and higher amounts of this protein were observed in SHR compared to control animals.

Ultrastructural changes at the myotendinous junction induced by exercise

BACKGROUND

Although exercise is believed to reduce the risk of rupture of the myotendinous junction, exercise-induced structural changes in this region have not been studied. We examined exercise-induced ultrastructural changes in the myotendinous junction of the lower legs in rats.

METHODS

Ten adult male LETO rats were used. Five rats were randomly placed in the Exercise group; the remaining five were used as controls and placed in the non-Exercise group. Running exercise was performed every day for 4 weeks. The tibialis anterior and gastrocnemius muscles were then removed from both legs from each animal in the two groups. The specimens were subsequently examined by transmission electron microscopy (TEM). Numerous finger-like processes were observed at the myotendinous junction. The changes in frequency of branching of the finger-like process (the number of times one finger-like process branched) and the direction of the processes (the angle of the major axis of a finger-like process to the longitudinal direction of the muscle fiber) were studied. To evaluate the two indicators above, each 10 fingerlike process was randomly and separately selected from the tibialis anterior and gastrocnemius muscles of rats, providing 50 finger-like processes of both muscles for evaluation per group.

RESULTS

In terms of the frequency of branching of the fingerlike processes, the mean values obtained in the non-Exercise group were 0.04 and 0.18 times, respectively, in the tibialis anterior and gastrocnemius muscles and were 0.38 and 1.16 times, respectively, in these two muscles in the Exercise group. Regarding the direction of the finger-like processes, the values were 4.1 degrees and 3.6 degrees, respectively in the non-Exercise group and 10.4 degrees and 14.5 degrees , respectively in the Exercise group. The differences between the two animal groups were significant.

CONCLUSIONS

Morphological changes in the myotendinous junction occurred as an adaptation to tension increased by exercise.

Beta-catenin localization during Xenopus embryogenesis: accumulation at tissue and somite boundaries

beta-catenin is a cytoplasmic protein associated with cadherin adhesion molecules and has been implicated in axis formation in Xenopus (McCrea, P. D., Brieher, W. M. and Gumbiner, B. M. (1993) J. Cell Biol. 127, 477–484). We have studied its distribution in Xenopus embryos by immunofluorescence on frozen sections. Consistent with its function in cell-cell adhesion, beta-catenin is present in every cell. However, high levels are expressed in certain regions and different tissues of the embryo. No simple correlation appears to exist between the levels of beta-catenin with the expected strength of adhesion. High levels of beta-catenin were found in regions undergoing active morphogenetic movements, such as the marginal zone of blastulae and gastrulae. This suggests that high expression of beta-catenin could be involved in dynamic adhesion events. Surprisingly, beta-catenin also accumulates on plasma membranes that probably do not establish direct or strong contacts with other cells. In particular, high amounts of beta-catenin are found transiently at boundaries between tissue anlagen and at the intersomitic boundaries. This unexpected pattern of beta-catenin expression raises the possibility that this molecule participates in developmental processes, perhaps independently of its classical role in cell-cell adhesion.

Desmin organization during the differentiation of the dorsal myotome in Xenopus laevis

The reorganization of desmin-type intermediate filaments during muscle differentiation has been studied primarily in cultured cell systems. Here we describe the process of desmin reorganization during the differentiation of the dorsal myotomal muscle of the clawed frog Xenopus laevis. This muscle differs from those described previously primarily in that the desmin system forms de novo, i.e., without the presence of a pre-existing vimentin filament system. The most striking observation is that prior to myotomal segmentation and rotation desmin is concentrated at the medial and lateral tips of the myocytes. It remains concentrated in these regions following somite rotation and is located primarily to the intersomite junctions as late as the stage 33-35 tadpole. As the muscle matures (stage 30 and later) desmin becomes increasingly associated with the sarcolemma and with the Z-discs. The concentration of desmin at the nascent intersomite junction suggests that desmin is involved in coupling somites to one another in the early Xenopus embryo.

Ultrastructural comparison of slack and stretched myotendinous junctions, based on a three-dimensional model of the connecting domain

The vertebrate myotendinous junction contains junctional microfibrils, located in the lamina lucida of the basement membrane. The junctional microfibrils are thought to transmit muscular force across the junctional lamina lucida, also called the connecting domain. If true, deformation of the terminal muscle cell processes and connecting domain during force transmission would be detected as a change in spacing and/or orientation of the junctional microfibrils. This study compared connecting domain morphology in frog semitendinosus muscles fixed in two extremes of resting tension, to elucidate the mechanical properties of the myotendinous junction. An initial study of connecting domain ultrastructure revealed that junctional microfibrils are punctate or spinelike in shape, and that they are distributed in a linear, helically-oriented array on the muscle cell surface. The rows in the surface lattice are 10-15 nm in thickness, have a centre-to-centre distance between rows of approximately 24 nm, and are oriented at approximately 41 degrees with respect of the long axis of the muscle fibre. Comparison of slack and highly stretched myotendinous junctions shows no significant changes in spacing or orientation of either individual junctional microfibrils or rows in the helical surface lattice. Thus, both the connecting domain and terminal cell processes at the myotendinous junction are essentially inextensible under the loading conditions used in this study.

Morphological analysis of tendinous structure in the American alligator jaw muscles

In the American alligator, the jaw muscles show seven bundles of tendinous structure: cranial adductor tendon, mandibular adductor tendon, lamina anterior inferior, trap-shaped lamina lateralis, lamina intramandibularis, lamina posterior, and depressor mandibular tendon (originating from the musculus depressor mandibulae, m. pseudotemporalis, m. adductor mandibulae posterior, m. adductor mandibulae externus, m. intramandibularis, m. pterygoideus anterior, and m. pterygoideus posterior). These tendinous structures are composed of many collagen fibrils and elastic fibers; however, the distributions and sizes of the fibers in these tendinous components differ in comparison with those of other masticatory muscles. The differences of these properties reflect the kinetic forces or the stretch applied to each tendon by the muscle during jaw movements in spite of the simple tendon-muscle junctions. © 1993 Wiley-Liss, Inc.

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

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 x 10(5) N m-2, and for divalent cation-free muscles, 1.84 x 10(5) 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.

Induction of dystrophin localization in cultured Xenopus muscle cells by latex beads

The distribution of dystrophin in Xenopus myotomal muscle cells was examined in conventional and confocal immunofluorescence microscopy. By labeling dissociated single muscle fibers with a monoclonal or a polyclonal antibody against dystrophin, we found that dystrophin is ten times more concentrated at the myotendinous junction (MTJ) than at the extrajunctional sarcolemma. At the MTJ, dystrophin lines the membrane invaginations where myofibrils attach to the membrane. It is colocalized with talin, but is not related to the distribution of acetylcholine receptors (AChRs) which are clustered at the postsynaptic membrane in the vicinity of the MTJ in these fibers. We found that the localization of dystrophin can be induced in cultured Xenopus myotomal muscle cells by treating them with polystyrene latex beads. Dystrophin is discretely localized at the bead-muscle contacts. With electron microscopy, a sarcolemma specialization with all the salient features of the MTJ, including basal lamina-lined membrane invaginations along which myofibrils make attachment. Although these beads also induce clustering of AChRs, the patterns of dystrophin and AChR localization are distinct. The appearance of dystrophin at the bead-contacted sarcolemma is coincident with the development of the membrane invaginations. This, together with its concentration along membrane invaginations at the MTJ in vivo, suggests a role for dystrophin in the formation of this junctional specialization. Since the signal for MTJ development can be presented to cultured muscle cells in a temporally and spatially controlled manner by beads, this system offers a simple model for analyzing the mechanism of this sarcolemma specialization.

Functionally complex muscles of the cat hindlimb. II. Mechanical and architectural heterogenity within the biceps femoris

The goal of this study was to analyze the architecture of the cat biceps femoris (BF), a multifunctional hamstring muscle, and to evaluate the relationships between muscle architecture, limb position, and muscle function during natural movement. The BF muscle consists of three neuromuscular compartments: anterior (BFa), middle (BFm) and posterior (BFp). Each compartment is innervated by a separate nerve branch. Nerve branch stimulation and 2-dimensional surface EMG recordings showed that individual compartment territories were discrete and non-overlapping with well-defined borders. Comparisons of the three compartments revealed consistent differences in architecture, relationship to the skeleton, and function. The BFa crossed only the hip joint and appears to function as a pure hip extensor. The BFm had equal lever arm lengths to the hip and knee joints, appears to function as a hip extensor, and may contribute to knee flexion or femoral rotation. The BFp had a greater lever arm to the knee, functions as a knee flexor, and may contribute to hip extension, femoral rotation or ankle extension. Measurements of individual fascicles from the three compartments revealed a surprising range of lengths, 3.3-12.0 cm. Microdissection of gold-stained tissue showed that fascicles from all compartments were comprised of interdigitated, short fibers (range: 0.6-5.0 cm; average 2.14 cm) arranged in-series in fascicles, running parallel to the origin-insertion axis of each muscle compartment. In regions of fiber interdigitation, the fiber endings were round and tapered (taper lengths: 1-11 mm) although flat, tapering endings like ribbons were occasionally found. As hip and knee joint angles were varied over physiological ranges corresponding to minimal to maximal muscle length, fascicles of the three compartments changed length disproportionately. Long BFa fascicles maximally lengthened 10-18%, consistent with in vivo length measures during treadmill locomotion. However, the long BFp fascicles lengthened 25-45%, and the relatively short fascicles near the BFm/BFp border maximally lengthened 45-53%. How do these unexpectedly large length changes affect sarcomere lengths? Using laser diffraction to measure sarcomeres, static fascicle and sarcomere lengths were compared in muscles that went into rigor mortis after fixing the hip and knee joint angles. Sarcomeres within the short BFm/BFp and long BFp fascicles consistently lengthened proportionately less than the whole fascicle. It remains to be determined how and where the fascicle length changes are dissipated in the connective tissue between the interdigitated muscle fibers and whether such a series-compliance operates during the large excursions over which this muscle normally works.

Structural changes at the myogenic cell surface during the formation of myotendinous junctions

Myotendinous junctions are sites which are morphologically and molecularly specialized for force transmission between intracellular and extracellular structural proteins. In the present investigation, the formation of these specialized junctions is studied in chicken embryos from 9 days following fertilization to 1 day posthatching, using light and electron microscopy. Observations indicate that the first discernible event in myotendinous junction formation is the appearance of basement membrane at the incipient junction at 9-10 days postfertilization, concomitant with the aggregation of fibroblasts at the junctional regions of myogenic cells. Subsequently, subsarcolemmal densities appear at sites opposite basement membrane locations by 13 days postfertilization. Myofibrils insert into subsarcolemmal densities by day 15 and invaginations of the cell membrane are initiated at those insertions. Type I collagen fibers appear at the cell surface at day 17. Junctional structure at day 17 qualitatively resembles that of adult myotendinous junctions. Changes in junctional structure following day 17 are primarily increases in the amount of subsarcolemmal densities, myofibril-membrane associations, and amount of junctional membrane folding.

Intramembrane aspects of myotendinous and myomuscular junctions: a freeze-fracture study of the gill sac-muscle of the Atlantic hagfish, Myxine glutinosa

The intramembrane organization of the sarcolemma at the sites of myotendinous and myomuscular junctions was studied in the gill sac-muscle of the Atlantic hagfish, Myxine glutinosa, by using freeze-fracture replicas. At these sites rows of irregularly shaped particles (diameter approximately 6 nm) and short fibrils are present on the P face and a complementary pattern of grooves is present on the E face of the split plasma membrane. The center-to-center distance between adjacent rows of particles and grooves ranges from 12 to 20 nm. Rodlike projections being in register with the rows of particles and grooves, respectively, extend from the plasma membrane toward the extracellular space. These rodlike projections are also recognizable in thin sections, where they appear as spinelike projections (cross sections) or linear arrays (grazing sections) located in the lamina lucida of the basal lamina. The intramembrane particles are considered to be integral membrane proteins and to represent transmembrane links in a series of molecules by which intracellular actin filaments and extracellular collagen fibrils are connected across the plasma membrane. The rodlike projections are probably peripheral membrane proteins possibly connecting the plasma membrane with structural components of the basal lamina.

The human muscle-tendon junction. A morphological study during normal growth and at maturity

The myotendon junction of human paravertebral skeletal muscle was studied by light and electron microscopy. Transverse and longitudinal sections of myotendinous regions of normal multifidus muscles were examined at three chronological stages from birth to maturity. Variations in the appearance of surface extensions at the terminal ends of muscle fibers consisted of brush-like evaginations at birth and villous-like projections in the adult. Regardless of age, they were invariably covered by a prominent external lamina, and mutually interdigitated with connective-tissue elements in the adjacent tendon. Various stages of myofibrillar assembly and sarcomere alignment were evident in the muscle fiber terminus at birth. With advancing age, splitting of terminal sarcomeres at Z bands commonly gave rise to diverging myofilament bundles that attached to electron-dense patches under the sarcolemma. In these regions, leptomeric organelles were also encountered in neonatal and adolescent myotendons. At all stages, the ends of muscle fibers possessed cytological features consistent with active synthesis and secretion. Densely-packed sarcoplasmic organelles including multiple Golgi complexes, clusters of ribosomes, mitochondria, cytoplasmic vesicles, and elements of rough- and smooth-surfaced endoplasmic reticulum were prevalent. Peripheral and centrally-placed heterochromatic nuclei with prominent nucleoli were arranged singly or in groups at the ends of muscle fibers. Satellite cell profiles and unmyelinated axons in the subjacent tendon were also identified at these sites in the adult. Fibroblasts in growing tendon were plentiful, and at all stages, possessed morphological features indicative of high metabolic and secretory activities.

A morphometric analysis of the muscle-tendon junction

Transmission of the contractile force produced by skeletal muscle fibers from myofilaments to tendon fibers occurs at the muscle-tendon junction. This interface between muscle and tendon is characterized by an amplification of the membrane area across which force is transmitted, with the result that stress at the muscle-tendon interface is less than it would be if the muscle fiber ended without surface folding. The amount of stress reduction is proportional to the degree of surface amplification. Because an understanding of the mechanical properties of the muscle-tendon junction requires a quantitative appreciation of the amplification of interfacial area, and hence the reduction of stress, produced by membrane elaboration at the muscle-tendon junction, we have developed a reliable morphometric approach for quantifying this surface amplification. The approach reported here makes use of point-counting techniques applied to thin sections of murine muscle-tendon junctions, together with a statistical analysis of the data. The results indicate that the load on the cell membrane at the muscle-tendon junction is reduced approximately 1 order of magnitude by membrane amplification, compared to the load calculated to occur if the muscle fiber ended as a right cylinder. In addition, significant differences in the degree of membrane amplification have been detected among the four muscle-tendon junctions analyzed in this study. These results and methods should prove useful in future analyses of normal and abnormal muscle-tendon junctions.

Three-dimensional structure of the murine muscle-tendon junction

At the muscle-tendon junction of skeletal muscle fibers the structural interface between muscle cell and connective tissue is amplified by tapering, by indentation, and by surface folding. The precise form taken by the surface folds has been unknown due to a lack of studies on the three-dimensional geometry of the muscle-tendon junction. Analysis of this region by scanning electron microscopy, using conventional preparative techniques, is uninformative because the muscle surface is covered by connective tissue. Removal of the connective tissue from individual murine muscle fibers by incubation of fixed fibers in hot HCl, followed in some instances by treatment with collagenase, permits SEM analysis of the uncovered fiber ends. The muscle fiber end is characterized by surface specializations in the form of anastamotic cylindrical folds. Transmission electron micrographs of cross sections and of serial longitudinal sections of muscle fiber ends confirm that the SEM observations are correct.

Vinculin in subsarcolemmal densities in chicken skeletal muscle: localization and relationship to intracellular and extracellular structures

Using immunocytochemical methods we have studied the distribution of vinculin in the anterior and posterior latissimus dorsi skeletal (ALD and PLD, respectively) muscles of the adult chicken. The ALD muscle is made up of both tonic (85%) and twitch (15%) myofibers, and the PLD muscle is made up entirely of twitch myofibers. In indirect immunofluorescence, antivinculin antibodies stained specific regions adjacent to the sarcolemma of the ALD and PLD muscles. In the central and myotendinous regions of the ALD, staining of the tonic fibers was intense all around the fiber periphery. Staining of the twitch fibers of both ALD and PLD muscles was intense only at neuromuscular junctions and myotendinous regions. Electron microscopy revealed subsarcolemmal, electron-dense plaques associated with the membrane only in those regions where vinculin was localized by immunofluorescence. Using antivinculin antibody and protein A conjugated to colloidal gold, we found that the electron-dense subsarcolemmal densities in the tonic fibers of the ALD contain vinculin; no other structures were labeled. The basal lamina overlying the densities appeared to be connected to the sarcolemma by fine, filamentous structures, more enriched at these sites than elsewhere along the muscle fiber. Increased amounts of endomysial connective tissue were often found just outside the basal lamina near the densities. In tonic ALD muscle fibers, the subsarcolemmal densities were present preferentially over the I-bands. In partially contracted ALD muscle, subsarcolemmal densities adjacent to the Z-disk appeared to be connected to that structure by short filaments. We propose that in the ALD muscle, through their association with the extracellular matrix, the densities stabilize the muscle membrane and perhaps assist in force transmission.

Membrane-related specializations associated with acetylcholine receptor aggregates induced by electric fields

The localization of membrane-associated specializations (basal lamina and cytoplasmic density) at sites of acetylcholine receptor (AChR) aggregation is consistent with an involvement of these structures in receptor stabilization. We investigated the occurrence of these specializations in association with AChR aggregates that develop at the cathode-facing edge of Xenopus muscle cells during exposure to a DC electric field. The cultures were labeled with a fluorescent conjugate of alpha-bungarotoxin and the receptor distribution on selected cells was determined before and after exposure to the field. In thin sections taken from the same cells, the cathode-facing edge was characterized by plaques of basal lamina and cytoplasmic density co-extensive with sarcolemma of increased density. In sections cut in a plane similar to the fluorescence image, it was possible to demonstrate that the specializations were concentrated at areas of field-induced AChR aggregation, and at receptor clusters existing on control cells. This finding further indicates that these structures participate in AChR stabilization, and that the mechanisms involved in AChR aggregation that result from field exposure and nerve contact may be similar.

Muscle fiber termination at the tendon in the frog's sartorius: a stereological study

The force produced within skeletal muscle fibers is transmitted to the bone via a myotendinous junction. This junctional region was examined by light and electron microscopy in the sartorius muscles of three Rana temporaria. The muscle fibers tapered and inserted at an angle of about 25 degrees with the connective tissue fascia near the bone. The composition of the structures within the last 100 microns of the fiber was analyzed morphometrically. The T-system, terminal cisternae, and caveolae were the same as in the central region of the muscle fiber. However, the mitochondrial content was higher and the volume of longitudinal sarcoplasmic reticulum was lower than elsewhere in the fiber. The membrane at the end of the fiber had extensive villiform processes interdigitating with the tendon. The surface area of the membrane around the villiform processes was estimated with point-counting techniques and calculated from the stereological equations appropriate for partially anisotropic structures. The extra membrane involved in the myotendinous junction was about 32 times that of the cross-sectional area of the fiber. Part of this additional membrane contained specialized adherens junctions through which the contractile proteins of the muscle are anchored to collagen. The increased area at the myotendinous junction presumably provides greater mechanical strength than a flat termination. The high values of membrane capacitance and specific resistance measured electrophysiologically at the end of the fiber also can be attributed to the characteristics of the terminal membrane structure.

An electron microscopic study of two types of muscle fibers in the pharyngeal pad of Crussian carp, Carassius carassius

In the pharyngeal pad on the roof of the anterior pharynx of Carassius carassius two types of thin striated muscle fibers (1.5-10 micron in diameter) were found. No pattern was discerned in the orientation of muscle fibers which form a loose tissue as a whole. One of them (Z fiber) is characterized by position of triads at the level of Z lines, and in the other type (A-I fiber) they were seen at the junction of the A and I bands. Three types of intermyofibrous junction are noted between muscle fibers of the same type or between those of different types. The first type possesses ultrastructural features such as a uniform intercellular space about 90 nm which contains the basal lamina, a dense mat of the filamentous material on the sarcoplasmic aspect of the cell membrane, and a connection of myofibrils with the dense layer by thin I band filaments. The second type resembles the previous type but is distinguished from it by the lack of myofibrillar association. The third type is the nexus or gap junction. Intermyofibrous junctions of the second type are most frequently encountered (82%). Those of the first type are less frequently seen (15%), whereas the third type junctions are far less frequently seen (3%). Nerve endings at the neuromuscular junction of both types of muscle fiber contain numerous small clear vesicles suggesting their cholinergic nature.

Myotendinous junction: morphological changes and mechanical failure associated with muscle cell atrophy

The ultrastructures of the myotendinous junctions of healthy and atrophied twitch fibers from frog semitendinosus muscle are compared. The degree of plasma membrane folding at the junction of these cell types are quantified. The junctional surface area of healthy cells is increased 13.2 times by membrane folding. Calculations indicate that the junctional plasma membrane of these cells bears an average load of 2.6 X 10(4) N X m-2 during maximum isometric tension. The junctional surface area of atrophied cells is similarly increased by a factor of 6.3. The degree of junctional membrane folding does not vary with degree of cell contraction. Ultrastructural findings indicate that mechanical failure of the junction occurs just external to the junctional plasma membrane in atrophied cells. Healthy cells do not exhibit mechanical failure at the junction. These findings suggest that increased stress at the junction is associated with muscle atrophy and may cause failure at the myotendinous junction.

Structural domains of the muscle-tendon junction. 1. The internal lamina and the connecting domain

The force generated within skeletal muscle fibers of vertebrates is transmitted to the tendon at the muscle-tendon junction. Ultrastructural analysis of the murine muscle-tendon junction following a variety of experimental manipulations has produced evidence that the muscle-tendon junction can be described in terms of four principal domains (Trotter and Eberhard, 1983), two of which are discussed in the present report. Each domain is defined by the shape and orientation of its principal components, and by its position with respect to the plasma membrane. The internal lamina is composed of actin filaments, with a center to center spacing of approximately 12 nm, oriented mainly parallel to the principal vector of contractile force, and to the plasma membrane. These filaments are cross-linked into a structural unit, perhaps by the electron-dense structures which are associated with them. The internal lamina is morphologically connected to the external lamina (lamina densa) by a population of fine filaments oriented approximately perpendicular to the principal vector of contractile force. These filaments which constitute the connecting domain, are approximately 2-8 nm in diameter and are at least 50 nm long. They pass through three separate regions: the sarcoplasm between the internal lamina and the plasma membrane; the plasma membrane proper; and the extracellular space between the plasma membrane and the lamina densa. This third region is often referred to as the lamina lucida. These filaments are composed of at least three separate components in series, linked together by noncovalent interactions. The existence of these discrete structural domains implies that each has a different molecular composition and different mechanical properties.

Skeletal muscle-smooth muscle interaction: an unusual myoelastic system

The serratus superficialis metapatagialis (SSM) of pigeons is a skeletal muscle with unusual properties. It lies between the ribs and the trailing edge of the wing, where it is attached to the skin by a system of smooth muscles having elastic tendons. Wing movements during flight induce marked changes in this muscle's length. The SSM inserts onto the deep fascia, and at its termination the skeletal muscle contains large numbers of microtubules. Many myofibrils attach to leptomeric organelles, which then attach to the terminal end of the skeletal muscle fiber. The deep fascia next connects to the dermis of the skin by bundles of smooth muscles that have elastic tendons at both ends. This system allows large movements of the muscle while preventing its fibers from overstretching. The movements and presumed forces acting at this muscle make the presence of sensory receptors such as muscle spindles unlikely. Spindles are absent in this muscle.

Ultrastructural study of the muscle cell surface

In human muscle fibers and using tannic acid-glutaraldehyde fixation and freeze-fracture deepetch rotary replication, trabecular structures connecting the basal lamina to the outer leaflet of the muscle plasma membrane were observed. The trabeculae were noted at all levels of the sarcomere in regions away from the myotendinous junction. It is suggested that trabecular structures connecting the basal lamina to the plasma membrane may be a universal feature of the muscle cell surface.

Branched skeletal muscle fibers not associated with dysfunction

We report the occurrence of large clefts and branches in some amphibian vertebrate skeletal muscle fibers. Such fibers can be isolated intact from various skeletal muscles of fully mature animals and apparently are normal in other respects. Electron micrographs showed no signs of degeneration in most fibers. Electrical and mechanical studies revealed that all branches were excitable and able to contract. Branches evidently can arise in the absence of abnormal influences such as experimentally induced overload. The apparent cause may be either fusion or splitting of fibers.

Structure and function of the murine muscle-tendon junction

The muscle-tendon junctions of the extensor carpi radialis longus and brevis muscles from adult Balb C Bailey/J mice have been examined tensiometrically and ultrastructurally following removal of cellular membrane and soluble cytoplasm by exposure to nonionic detergent. As judged by the ability of the extracted muscle to generate tension upon exposure to ATP and to transmit the generated tension to the tendon, detergent extraction leaves the muscle-tendon junction functionally intact. Electron microscopic analysis of the extracted muscle-tendon junctions reveals that the relationship between the terminal myofilaments and the lamina densa of the basal lamina is retained, despite the extensive extraction of the plasma membrane. Fine filaments (2-7 nm) are seen to connect the lamina densa with an electron-dense intracellular layer into which terminal actin filaments appear to insert. These fine filaments are considered to represent an important component of the structural linkage between myofilaments and connective tissue and hence to be a significant component of the tension transmitting mechanism. Their precise nature is not known, but some part of the filaments must pass through the hydrophobic compartment of the plasma membrane and thus must be a transmembrane component of considerable tensile strength. These studies suggest that detergent-extractable membrane lipids play no significant role in the transmission of tension at the muscle-tendon junction, and that fine filaments, probably protein, are responsible for transmitting tension from myofilaments, through the plasma membrane, to the lamina densa of the basal lamina.

An ultrastructural study of multifidus muscle in progressive idiopathic scoliosis. Changes resulting from a sarcolemmal defect at the myotendinous junction

Biopsies of multifidus muscles were procured from patients with idiopathic scoliosis prior to Harrington rod instrumentation. Specimens from both convex and concave sides at the apex of the curve were examined by light and electron microscopy and compared with normal muscle samples. Abnormalities were detected on the concave aspect of the curve and the most dramatic morphological changes were noted at the myotendinous junction. Here a structural defect in the form of discontinuities in the sarcolemmal membranes of some muscle fibres was accompanied by large numbers of intimately-adhering connective tissue cells. Structural disorganization of the associated tendon occurred in conjunction with increased vascularization and with fatty cell and leukocyte infiltration. Further from the myotendon junction, structural lesions appeared more chronic and non-specific subsequent to the incipient sarcolemmal break in the affected muscle fibres. Hypertrophy, atrophy, centralization of nuclei, and disruption of sarcotubular and myofibrillar elements were noted in some muscle cells. While the aetiology of this disorder is unknown, a supposition is made that the primary change is an inherent weakness and subsequent break in the sarcolemma at the myotendon junction. This site is an important clue to the pathogenesis of this disease since it reflects morphological change in rapidly growing tissue occurring at the time of the rapid adolescent growth spurt leading to progression of the scoliotic curve.

The myotendinous junction of the smooth feather muscles (mm. pennati). A light and electron microscopic study on a myoelastic system

Smooth feather muscles (mm. pennati) consist of bundles of smooth muscle cells which are attached to the feather follicles by short elastic tendons. In addition, some muscle bundles are interrupted by elastic tendons. The elastic tendon is composed of longitudinally arranged elastic fibers which branch and wavy bundles of collagen fibrils. Smooth muscle cells of the muscle bundles are attached to each other by desmosome-like junctions and by fusion of the basal laminae. The cytoplasm of the muscle cells is characterized by conspicuous thick filaments and abundant thin and intermediate filaments. These are attached to band-like dense patches (dense bands) at the plasma membrane which are particularly broad at the tapering end of the muscle cell. The contact surface between smooth muscle cells and their elastic tendon is considerably increased (i) by deep finger-like invaginations and indentations located at the tapering muscle end, and (ii) by branching of the coarse elastic fibers into slender processes, which are attached to the richly folded surface of the muscle cell endings by peripheral microfibrils. This intimate interlocking closely resembles the myotendinous junctions in skeletal muscle. In addition to fibroblasts and fibrocytes, the myotendinous junction of the young growing chicks contains numerous so-called myofibroblasts, which are suggested to represent smooth muscle cells differentiating into fibroblasts of the developing tendon.

Fine structure of the myotendinous junction of lathyritic rat muscle with special reference to connectin, a muscle elastic protein

The fine structure of the myotendinous junction of the skeletal muscle of lathyritic rats caused by beta-aminopropionitrile was investigated. In the junction there are finger-like processes of muscle fibers, in which thin filaments were extended from the last Z lines of myofibrils and attached to the sarcolemma of the processes. By the heavy meromyosin decoration technique, these thin filaments were identified as actin filaments. In the lathyritic muscle, the thin filaments were markedly fewer in number and distributed sparsely in the sarcoplasm. The content of connectin, an elastic protein, which is localized in myofibrils and also in sarcolemma was significantly decreased in the lathyritic muscle. A possible relationship between the changes in the fine structure of the myotendinous junction and in the connectin contents is discussed.

Electron microscopic studies on the myotomes of larval lamprey, Lampetra japonica

Myotomes of the caudal one-third of the body of 26-day-old larval lampreys, Lampetra japonica, were studied by electron microscopy. Each myotome consists of horizontally stacked muscle lamellae. The myotomes are covered laterally by a single layer of flattened cells called here "lateral cells", and the other aspect is covered by an external lamina. The myotomes are midsegmentally innervated. Each muscle lamella usually contains two single cortical layers of myofibrils along the dorsal and ventral sarcolemma with a nucleus and mitochondria interposed between two layers. Numerous peripheral couplings are observed with relatively less developed triads. There are no membrane specializations to connect adjacent muscle lamellae within a myotome. Intermyotomal junctions are, however, noted between tips of cytoplasmic processes of muscle lamellae of adjoining myotomes. They resemble tight or gap junctions. No myofibrils are present in these cytoplasmic processes. Myotendinous junctions, with "terminal couplings" (Nakao, '75), are seen under development at the myoseptal ends of muscle lamellae. Lateral cells contain only ordinary organelles and no special structures such as myofibrils are found in the cytoplasm. They are connected to each other and to muscle lamellae by primitive desmosomes. They generally have no external lamina investment.