Fibrous framework of human skeletal muscles, fasciae and tendons

By means of scanning and transmissive electron microscopy, the construction of the fibrous framework of the human skeletal muscles, fasciae and tendons has been investigated and its morphofunctional analysis has been performed. The fibrous framework of the endomysium is presented as a complexly organized system of anastomosing fibers of the connective tissue, forming a net-like construction. The fibrous structures of the framework are united into a whole construction by connecting fibers and fibrils. Different types of structural interconnection of collagenous fibers with sarcolemma are revealed. The structure of the fibrous framework both in different muscles and within one muscle has certain peculiarities. The main constructive element of the fascial fibrous framework make large anastomosing collagenous fibers, their architectonics is stabilized by connective fibers and fibrils. The construction of the tendinous fibrous framework is characterized by a pronounced anisotropia of the largest collagenous fibers and a developed network of connective structures both on the surface and inside the collagenous fibers. Structural mechanisms, interconnecting muscles and tendons, are demonstrated. Presence of anastomoses between the fibrils in the composition of the collagenous fibers in the fascia and Achilles tendon are stated. Together with the peculiarities existing, the general principle of the structural organization of the fibrous framework of the muscle system is the net-like constructure dependent on presence of anastomoses and elements of the connective system between the fibrous structures. Depending on the organ's function, the construction of the network acquires certain specific morphological forms.     

Spatial arrangement of white muscle fibers and myoseptal tendons in fishes

We describe the arrangement of white muscle fibers and tendinous myoseptal structures and the relation of these structures to each other in order to provide an anatomical framework for discussions and experimental research on fish swimming mechanics. For the three major craniate groups, the petromyzontids, myxinids and gnathostomes, we identify three conditions that differ remarkably. Myxinids are characterized by asymmetrical myosepta with long cones. Within a single myoseptum these are connected by collagenous fibers that are almost oriented longitudinally. Distinct tendons are absent in myxinid myosepta. Petromyzontid myosepta lack cones and distinct myoseptal tendons, whereas gnathostomes bear cones and distinct tendinous structures: the lateral band, epineural (epipleural) tendon and myhabdoid tendon. Myoseptal fibers of petromyzontids and myoseptal tendons of gnathostome myosepta are firmly anchored in the skin. Myxinids lack firm myoseptal-skin-connections. Their muscular arrangement is neither comparable to that of petromyzontids nor to that of gnathostomes. The latter two bear archlike arrangements of muscle fibers spanning several segments that are hypothesized to play a role during bending. In gnathostomes, archlike helical muscle fiber arrangements (HMFAs) are present that span the length of several body segments and are multiply intersected by myosepta. Hence, a series of tendinous lateral bands of myosepta is embedded in HMFAs. The posterodorsally oriented HMFAs are underlain by posteroventrally oriented crossing muscle fibers (CMFs). Bending may be generated by contraction of the muscle fibers belonging to an HMFA and the simultaneous counteraction of CMFs. Moving caudally, this anterior muscle fiber arrangement gradually changes, eventually becoming the posterior muscle fiber arrangement. This pattern suggests that the function of the myomeres will also change. Three additional putative roles of myoseptal tendons can be deduced from their relations to white muscle fibers in gnathostomes (and in part in petromyzontids): (1) Posterior transmission of anteriorly generated muscular forces via lateral bands and/or myorhabdoid tendons. These tendons are more robust posteriorly. Anterior and posterior cones appear to play an important role in force transmission. (2) Pulling on collagen fibers of the skin via lateral bands and myorhabdoid tendons, suggesting a transmission of muscular forces that puts the skin into tension. (3) Resisting radial expansion of contracting muscle fibers by epineural (epipleural) tendons. By the latter two mechanisms modulation of body stiffness is likely to be achieved.     

Review the role of terminal domains during storage and assembly of spider silk proteins

Fibrous proteins in nature fulfill a wide variety of functions in different structures ranging from cellular scaffolds to very resilient structures like tendons and even extra-corporal fibers such as silks in spider webs or silkworm cocoons. Despite their different origins and sequence varieties many of these fibrous proteins share a common building principle: they consist of a large repetitive core domain flanked by relatively small non-repetitive terminal domains. Amongst protein fibers, spider dragline silk shows prominent mechanical properties that exceed those of man-made fibers like Kevlar. Spider silk fibers assemble in a spinning process allowing the transformation from an aqueous solution into a solid fiber within milliseconds. Here, we highlight the role of the non-repetitive terminal domains of spider dragline silk proteins during storage in the gland and initiation of the fiber assembly process.     

Morphological and histochemical organization of the flexor carpi radialis muscle in the cat.

Most studies concerning the structure and function of skeletal muscle have utilized the hind limb of the experimental animal. However, it has been shown that the number of behavioral tasks performed by the cat's forelimb is greater than that of the hind limb. In addition, the forelimb muscles exhibit a functional complexity not observed in hind-limb musculature. The purpose of this study was to investigate the distribution of fast-twitch and slow-twitch muscle fibers and muscle spindles in the flexor carpi radialis muscle (FCR) and to correlate the distributional patterns in these structures with muscle tendon architecture and muscle function. It was found that the FCR, a wrist flexor, contains 37% slow-twitch fibers and 63% fast-twitch fibers. However, the slow-twitch fibers were concentrated in the deep region located between the tendons of origin and insertion, while the fast-twitch-glycolytic fibers were concentrated more peripherally. Muscle spindles were associated with the slow-twitch region and were never found in the region containing high concentrations of fast-twitch-glycolytic fibers. Fast-twitch-oxidative-glycolytic fibers were uniformly distributed throughout the muscle. It is proposed that the association of muscle spindles with slow-twitch fibers and the differential distribution of muscle fibers into slow-twitch and fast-twitch regions might allow these regions to function independently of one another when called upon to perform complex behavioral tasks.     

Myoseptal architecture of sarcopterygian fishes and salamanders with special reference to Ambystoma mexicanum

During axial undulatory swimming in fishes and salamanders muscular forces are transmitted to the vertebral axis and to the tail. One of the major components of force transmission is the myoseptal system. The structure of this system is well known in actinopterygian fishes, but has never been addressed in sarcopterygian fishes or salamanders. In this study we describe the spatial arrangement and collagen fiber architecture of myosepta in Latimeria, two dipnoans, and three salamanders in order to gain insight into function and evolution of the myoseptal system in these groups. Salamander myosepta lack prominent cones, and consist of homogenously distributed collagen fibers of various orientations that never form distinct tendons. Fiber orientations are difficult to homologize with those of fish myosepta. The myosepta of Latimeria and dipnoans (Protopterus and Neoceratodus) illustrate that major changes in architecture occurred in the sarcopterygian clade (loss of horizontal septum), in the rhipidistian (dipnoans + tetrapods) clade (loss of epineural and epipleural tendon), and in tetrapods (loss of lateral tendons and myoseptal folding). When compared to fishes, the myosepta of wholly aquatic salamanders (Ambystoma mexicanum, Amphiuma tridactylum, Necturus maculosus) do not have the lateral tendons we suppose serve to transfer muscular forces posteriorly. We propose that alternative structures (most conspicuously present in Ambystoma) perform this function: posteriorly the relative amount of connective tissue increases considerably, and myosepta are disintegrated to horizontal lamellae of connective tissue. The structures thought to be involved in modulation of body stiffness in fishes during swimming are also absent in salamanders. Our data also have implications for the hypothesis that salamander hypaxial myosepta are designed to increase shortening amplification of the hypaxial muscle fibers. The posterior hypaxial myosepta of all three salamander species possess only mediolaterally directed collagen fibers, which would indeed amplify the shortening of the associated muscle.     

Inflammatory cells do not decrease the ultimate tensile strength of intact tendons in vivo and in vitro: protective role of mechanical loading.

Although inflammatory cells and their products are involved in various pathological processes, a possible role in tendon dysfunction has never been convincingly confirmed and extensively investigated. The goal of this study was to determine whether or not an acute inflammatory process deprived of mechanical trauma can induce nonspecific damages to intact collagen fibers. To induce leukocyte accumulation, carrageenan was injected into rat Achilles tendons. We first tested the effect of leukocyte recruitment on the concentrations or activities of matrix metalloproteinases and tissue inhibitors of matrix metalloproteinases. Second, we analyzed at the biochemical, histological, and biomechanical levels the impact of leukocyte invasion on tendons. Finally, collagen bundles isolated from rat-tail tendons were exposed in vitro to mechanical stress and/or inflammatory cells to determine if mechanical loading could protect tendons from the leukocyte proteolytic activity. Carrageenan-induced leukocyte accumulation was associated with an increased matrix metalloproteinase activity and a decreased content of tissue inhibitors of matrix metalloproteinases. However, hydroxyproline content and load to failure did not change significantly in these tendons. Interestingly, mechanical stress, when applied in vitro, protected collagen bundles from inflammatory cell-induced deterioration. Together, our results suggest that acute inflammation does not induce damages to intact and mechanically stressed collagen fibers. This protective effect would not rely on increased tissue inhibitors of matrix metalloproteinases content but would rather be conferred to the intrinsic resistance of mechanically loaded collagen fibers to proteolytic degradation.     

Electron-microscopic localization of type II,IX, and V collagen in the organ of Corti of the gerbil

Summary The presence of types II, IX and V collagen was probed in the organ of Corti of the adult gerbil cochlea by use of immunocytochemistry at the light- and electron-microscopic levels. Type II collagen is found in the connective tissues of the osseous spiral lamina and spiral limbus. In the region of the sensory hair cells it is present in the tectorial membrane and antibodies bind to the thick unbranched radial fibers. Type IX collagen co-localizes with type II collagen in the tectorial membrane, where antibodies bind to the thick unbranched radial fibers. Type V collagen is present in the connective tissue of the spiral limbus, the osseous spiral lamina, the eighth nerve, and the tectorial membrane. In the tectorial membrane, the staining with antibodies to type V collagen is more diffuse than that seen for types II and IX collagen and antibodies to type V bind to the thin, highly branched fibers in which the thick fibers are embedded. The results indicate that collagens characteristic of cartilage are localized in the organ of Corti. Within the tectorial membrane, types II and IX collagen form heterotypic thick fibers embedded in a reticular network of type V collagen fibers. These collagens form a highly structured matrix which contributes to the rigidity of the tectorial membrane and allow it to withstand the physical stresses associated with transmission of the stimuli necessary for sensory transduction.     

Evolution of high-performance swimming in sharks: transformations of the musculotendinous system from subcarangiform to thunniform swimmers

In contrast to all other sharks, lamnid sharks perform a specialized fast and continuous "thunniform" type of locomotion, more similar to that of tunas than to any other known shark or bony fish. Within sharks, it has evolved from a subcarangiform mode. Experimental data show that the two swimming modes in sharks differ remarkably in kinematic patterns as well as in muscle activation patterns, but the morphology of the underlying musculotendinous system (red muscles and myosepta) that drives continuous locomotion remains largely unknown. The goal of this study was to identify differences in the musculotendinous system of the two swimming types and to evaluate these differences in an evolutionary context. Three subcarangiform sharks (the velvet belly lantern shark, Etmopterus spinax, the smallspotted catshark, Scyliorhinus canicula, and the blackmouth catshark, Galeus melanostomus) from the two major clades (two galeans, one squalean) and one lamnid shark, the shortfin mako, Isurus oxyrhinchus, were compared with respect to 1) the 3D shape of myomeres and myosepta of different body positions; 2) the tendinous architecture (collagenous fiber pathways) of myosepta from different body positions; and 3) the association of red muscles with myoseptal tendons. Results show that the three subcarangiform sharks are morphologically similar but differ remarkably from the lamnid condition. Moreover, the "subcarangiform" morphology is similar to the condition known from teleostomes. Thus, major features of the "subcarangiform" condition in sharks have evolved early in gnathostome history: Myosepta have one main anterior-pointing cone and two posterior-pointing cones that project into the musculature. Within a single myoseptum cones are connected by longitudinally oriented tendons (the hypaxial and epaxial lateral and myorhabdoid tendons). Mediolaterally oriented tendons (epineural and epipleural tendons; mediolateral fibers) connect vertebral axis and skin. An individual lateral tendon spans only a short distance along the body (a fraction between 0.05 and 0.075 of total length, L, of the shark). This span is similar in all tendons along the body. Red muscles insert into the midregion of the lateral tendons. The shortfin mako differs substantially from this condition in several respects: Red muscles are internalized and separated from white muscles by a sheath of lubricative connective tissue. They insert into the anterior part of the hypaxial lateral tendon. Rostrocaudally, this tendon becomes very distinct and its span increases threefold (0.06L anteriorly to 0.19L posteriorly). Mediolateral fibers do not form distinct epineural/epipleural tendons in the mako. Since our morphological findings are in good accordance with experimental data it seems likely that the thunniform swimming mode has evolved along with the described morphological specializations.     

From head to tail: the myoseptal system in basal actinopterygians

Experimental studies indicated that myomeres play several functional roles during swimming. Some of the functions in question are thought to change rostrocaudally, e.g., anterior myomeres are thought to generate forces, whereas posterior myomeres are thought to transmit forces. In order to determine whether these putative functions are reflected in myoseptal morphology we carried out an analysis of the myoseptal system that includes epaxial and hypaxial myosepta of all body regions for the first time. We combined clearing and staining, microdissections, polarized light microscopy, SEM technique, and length measurements of myoseptal parts to reveal the spatial arrangement, collagen fiber architecture, and rostrocaudal gradients of myosepta. We included representatives of the four basal actinopterygian clades to evaluate our findings in an evolutionary and in a functional context. Our comparison revealed a set of actinopterygian groundplan features. This includes a set of specifically arranged myoseptal tendons (epineural, epipleural, lateral, and myorhabdoid tendons) in all epaxial and postanal hypaxial myosepta. Only preanal hypaxial myosepta lack tendons and exclusively consist of mediolateral fibers. Laterally, myosepta generally align with the helically wound fibers of the dermis in order not to limit the body's maximum curvature. Medially, the relationship of myosepta to vertebrae clearly differs from a 1:1 relationship: a myoseptum attaches to the anterior margin of a vertebra, turns caudally, and traverses at least three vertebrae in an almost horizontal orientation in all body regions. By this arrangement, horizontal multiple layers of myosepta are formed along the trunk dorsal and ventral to the horizontal septum. Due to their reinforcement by epineural or epipleural tendons, these multiple layers are hypothesized to resist the radial expansion of underlying muscle fibers and thus contribute to modulation of body stiffness. Rostrocaudally, a dorsoventral symmetry of epaxial and hypaxial myosepta in terms of spatial arrangement and collagen fiber architecture is gradually developed towards the postanal region. Furthermore, the rostrocaudal extension of myosepta measured between anterior and posterior cones gradually increases. This myoseptal region is reinforced by longitudinal fibers of lateral tendons. Furthermore, the percentage of connective tissue in a cross section increases. These morphological data indicate that posterior myosepta are equipped for multisegmental force transmission towards the caudal fin. Anteriormost myosepta have reinforced and elongated dorsal posterior cones. They are adequately designed to transmit epaxial muscular forces to the neurocranium in order to cause its elevation during suction feeding.     

Distribution of sensory receptors in the flexor carpi radialis muscle of the cat

The structures and distribution of encapsulated muscle receptors were examined in serial transverse sections of flexor carpi radialis in the adult cat. Four types of receptors (muscle spindles, Golgi tendon organs, paciniform, and Pacinian corpuscles) were identified. Their structures resembled those encountered in other limb muscles. Pacinian corpuscles were rare and occurred only in the external fascial coat of the muscle near its origin. The other three receptor types were distributed in an uneven but consistent pattern throughout the muscle. As noted previously (Gonyea and Ericson, '77), spindles were largely confined to a deep muscle region comprising less than 20% of the muscle volume, located directly between the long tendon of origin and the tendon of insertion. This region contains the largest proportion of type SO muscle fibers (Gonyea and Ericson, '77). Tendon organs and paciniform corpuscles were concentrated along the tendons that lined the spindle-rich muscle region. This region appeared to be composed of extrafusal fibers that were shorter and of more oblique pinnation than those in other regions. The localization of muscle receptors to the "oxidative" core of the muscle in its direct line of pull may have functional implications for afferent input to the spinal cord which are discussed. In addition, the possibility is raised that there are more paciniform corpuscles in flexor carpi radialis (and possibly other muscles) than previously thought.     

Biomimetic approaches to tendon repair

The linear organization of collagen fibers in tendons results in optimal stiffness and strength at low strains under tensile load. However, this organization makes repairing ruptured or lacerated tendons extremely difficult. Current suturing techniques to join split ends of tendons, while providing sufficient mechanical strength to prevent gapping, are inadequate to carry normal loads. Immobilization protocols necessary to restore tendon congruity result in scar formation at the repair site and peripheral adhesions that limit excursion. These problems are reviewed to emphasize the need for novel approaches to tendon repair, one of which is the development of biomimetic tendons. The objective of the empirical work described here was to produce biologically-based, biocompatible tendon replacements with appropriate mechanical properties to enable immediate mobilization following surgical repair. Nor-dihydroguaiaretic acid (NDGA), a di-catechol from creosote bush, caused a dose dependent increase in the material properties of reconstituted collagen fibers, achieving a 100-fold increase in strength and stiffness over untreated fibers. The maximum tensile strength of the optimized NDGA treated fibers averaged 90 MPa; the elastic modulus of these fibers averaged 580 MPa. These properties were independent of strain rates ranging from 0.60 to 600 mm/min. Fatigue tests established that neither strength nor stiffness were affected after 80 k cycles at 5% strain. Treated fibers were not cytotoxic to tendon fibroblasts. Fibroblasts attached and proliferated on NDGA treated collagen normally. NDGA-fibers did not elicit a foreign body response nor did they stimulate an immune reaction during six weeks in vivo. The fibers survived 6 weeks with little evidence of fragmentation or degradation. The polymerization scheme described here produces a fiber-reinforced NDGA-polymer with mechanical properties approaching an elastic solid. The strength, stiffness and fatigue properties of the NDGA-treated fibers are comparable to those of tendon. These fibers are biocompatible with tendon fibroblasts and elicit little rejection or antigenic response in vivo. These results indicate that NDGA polymerization may provide a viable approach for producing collagenous materials that can be used to bridge gaps in ruptured or lacerated tendons. The tendon-like properties of the NDGA-fiber would allow early mobilization after surgical repair. We predict that timely loading of parted tendons joined by this novel biomaterial will enhance mechanically driven production of neo-tendon by the colonizing fibroblasts and result in superior repair and rapid return to normal properties.     

Immunocytochemical localization of thrombomodulin in the aqueous humor passage of the rat eye

 This report describes the distribution and localization of thrombomodulin (TM) in the rat eye by light and electron microscopic immunocytochemistry. In addition to the endothelium of the entire vasculature, TM was found on the non-vascular structures lining the cavities of the posterior and anterior chambers and the limbus. TM was localized on the basal, lateral, and apical plasma membranes of the inner and outer ciliary epithelium, and the posterior iris epithelium in which there was no polarized expression of TM. In the anterior chamber, TM was localized on the luminal surface of the corneal endothelium, but was negative on the anterior border layer of the iris, which is composed of a discontinuous layer of fibroblasts and collagen fibers. Thus, TM was present at sites of cell-to-cell contact. TM was also present on the endothelia of the trabecular meshwork and the Schlemm’s canal in the limbus. TM was localized not only on the luminal plasma membrane, but also on the cytoplasmic giant vacuoles in the endothelial cells of the Schlemm’s canal. These findings extend the importance of anticoagulant mechanisms to the systems of secretion, circulation, and drainage of the aqueous humor. Accepted: 18 March 1997     

Polymyxin-B-binding sites in the cochlea as demonstrated by polymyxin-B/gold labeling

Summary A polymyxin-B/bovine-serum-albumin/gold complex was used as a probe to detect the binding sites of polymyxin B on thin sections of cochlea embedded in Spurr's resin. The binding sites were found to be mainly located on the stereocilia, the cuticular plate of hair cells, the head plate of Deiters' cells, the tonofilaments in pillar cells and Deiters' cells, fibrous structures in the spiral limbus, the tectorial membrane and the basilar membrane and neural elements such as nerve endings, fibers, and the myelin sheath. The mitochondria, plasma mimbrane, and chromatin of the nuclei of the cells observed also exhibited binding. Our results suggest that phospholipids, glycoconjugates, cytoskeletal proteins and nucleic acids are responsible for this binding activity.     

Polymyxin-B-binding sites in the cochlea as demonstrated by polymyxin-B/gold labeling

A polymyxin-B/bovine-serum-albumin/gold complex was used as a probe to detect the binding sites of polymyxin B on thin sections of cochlea embedded in Spurr's resin. The binding sites were found to be mainly located on the stereocilia, the cuticular plate of hair cells, the head plate of Deiters' cells, the tonofilaments in pillar cells and Deiters' cells, fibrous structures in the spiral limbus, the tectorial membrane and the basilar membrane and neural elements such as nerve endings, fibers, and the myelin sheath. The mitochondria, plasma membrane, and chromatin of the nuclei of the cells observed also exhibited binding. Our results suggest that phospholipids, glycoconjugates, cytoskeletal proteins and nucleic acids are responsible for this binding activity.     

Tendon crimps and peritendinous tissues responding to tensional forces

Tendons transmit forces generated from muscle to bone making joint movements possible. Tendon collagen has a complex supramolecular structure forming many hierarchical levels of association; its main functional unit is the collagen fibril forming fibers and fascicles. Since tendons are enclosed by loose connective sheaths in continuity with muscle sheaths, it is likely that tendon sheaths could play a role in absorbing/transmitting the forces created by muscle contraction. In this study rat Achilles tendons were passively stretched in vivo to be observed at polarized light microscope (PLM), scanning electron microscope (SEM) and transmission electron microscope (TEM). At PLM tendon collagen fibers in relaxed rat Achilles tendons ran straight and parallel, showing a periodic crimp pattern. Similarly tendon sheaths showed apparent crimps. At higher magnification SEM and TEM revealed that in each tendon crimp large and heterogeneous collagen fibrils running straight and parallel suddenly changed their direction undergoing localized and variable modifications. These fibril modifications were named fibrillar crimps. Tendon sheaths displayed small and uniform fibrils running parallel with a wavy course without any ultrastructural aspects of crimp. Since in passively stretched Achilles tendons fibrillar crimps were still observed, it is likely that during the tendon stretching, and presumably during the tendon elongation in muscle contraction, the fibrillar crimp may be the real structural component of the tendon crimp acting as shock absorber. The peritendinous sheath can be stretched as tendon, but is not actively involved in the mechanism of shock absorber as the fibrillar crimp. The different functional behaviour of tendons and sheaths may be due to the different structural and molecular arrangement of their fibrils.     

Changes in bones of the foot and development of inserting tendons of the lower leg musculature in tibial aplasia

Skeleton, muscles and tendons were investigated in 3 lower legs with aplasia of the tibia. The tibiofibular joint appears to be rather a syndesmosis than a diarthrosis. In all 3 cases, talus and calcaneus are connected by a synostosis. Also in the other parts of the skeleton of the feet, synarthroses with different extensions are found. The muscles, normally originating from the tibia, have shifted their origin to the fibula. The proximal part of the inserting tendons under the retinacula is quite normal. Also, their distal part in the region of the metatarsalia and of the digits exhibits no alterations. In the region of the tarsus in which already alterations in bone formation are found, the tendons of the flexor muscles of the toes exhibit some variations. Often the muscles show a new, strong insertion into the bones of the tarsus. The normal connection between the proximal and distal parts of the tendons is sometimes still indicated by a thin bundle of collagenous fibers. In all 3 cases, the tibialis anterior muscle inserts into the distal part of the tendons of the flexor muscles of the toes, indicating that there are also connections between tendons of muscles from different blastemas.     

The effect of cryopreservation or heating on the mechanical properties and histomorphology of rat bone-patellar tendon-bone

The effects of cryopreservation on tendon allograft have been reported, but remain unclear, particularly the potential effects on mechanical properties and histological changes by ice crystal formation. There are also few studies about effects of heating for sterilization of tendon. We evaluated the effect of cryopreservation or heating on the mechanical properties and histomorphology of rat bone-patellar tendon-bones (BTBs). BTBs were processed by cryopreservation at −80°C for 3 weeks, or heating at 80°C for 10 min. Tensile testing and histomorphological examination were performed. The cryopreservation of tendons showed less influences on their mechanical properties. When cryopreserved BTBs in frozen state were fixed by freeze-substitution method, many spaces were observed in interfibrillar substances. These results suggest that the collagen fibers of cryopreserved tendons were histomorphologically affected by ice crystals. The heating of tendons completely destroyed the collagen fibers of the tendons and is therefore thought to be inappropriate for the sterilization of BTBs.     

The myoseptal system in Chimaera monstrosa: collagenous fiber architecture and its evolution in the gnathostome stem lineage

Recent studies have revealed the 3D morphology and collagen fiber architecture of myosepta in teleostome fishes. Here we present the first data set on the myoseptal structure of a representative of the chondrichthyan clade. We investigate the series of myosepta in the ratfish Chimaera monstrosa (Holocephali) from the anterior to the posterior body using microdissections of cleared and stained specimens, polarized light microscopy of excised myosepta, and histology. The features of the myoseptal system of Chimaera are compared to data from closely related vertebrate groups and are mapped onto a phylogenetic tree to further clarify the characteristics of the myoseptal series in the gnathostome ancestor. The 3D morphology and collagen fiber architecture of the myoseptal series in C. monstrosa resembles that of Teleostomi (Actinopterygii+Sarcopterygii) with regard to several features. Our comparative analysis reveals that some of them have evolved in the gnathostome stem lineage. (1) A series of epineural and epaxial lateral tendons (LTs) along the whole body, and a series of epipleural and hypaxial LTs in the postanal region evolved in the gnathostome stem lineage. (2) The LTs increase in length towards the posterior body (three-fold in Chimaera). Data on Chimaera and some comparative data on actinopterygian fishes indicate that LTs also increase in thickness towards the posterior body, but further data are necessary to test whether this holds true generally. (3) Another conspicuous apomorphic gnathostome feature is represented by multi-layer structures of myosepta. These are formed along the vertebral column by converging medial regions of successive sloping parts of myosepta. (4) The dorsalmost and ventralmost flanking parts of myosepta bear a set of mediolaterally oriented collagen fibers that are present in all gnathostomes but are lacking in outgroups. Preanal hypaxial myosepta are clearly different from epaxial myosepta and postanal hypaxial myosepta in terms of their collagen fiber architecture. In Chimaera, preanal hypaxial myosepta consist of an array of mediolaterally oriented collagen fibers closely resembling the condition in other gnathostome groups and in petromyzontids. Only one series of tendons, the myorhabdoid tendons of the flanking parts of myosepta, have evolved in the stem lineage of Myopterygii (Gnathostomata+Petromyzontida). Similar to LTs, the tendons of this series also increase in length towards the posterior body. In combination with other studies, the present study provides a framework for the design of morphologically based experiments and modeling to further address the function of myosepta and myoseptal tendons in gnathostomes.     

The innervation pattern of the human Achilles tendon: studies of the normal and tendinosis tendon with markers for general and sensory innervation

Pain-free normal Achilles tendons and chronic painful Achilles tendons were examined by the use of antibodies against a general nerve marker (protein gene-product 9.5, PGP9.5), sensory markers (substance P, SP; calcitonin gene-related peptide, CGRP), and immunohistochemistry. In the normal tendons, immunoreactions against PGP9.5 and against SP/CGRP were encountered in the paratendinous loose connective tissue, being confined to nerve fascicles and to nerve fibers located in the vicinity of blood vessels. To some extent, these immunoreactions also occurred in the tendon tissue proper. Immunoreaction against PGP9.5 and against SP/CGRP was also observed in the tendinosis samples and included immunoreactive nerve fibers that were intimately associated with small blood vessels. In conclusion, Mechanoreceptors (sensory corpuscles) were occasionally observed, nerve-related components are present in association with blood vessels in both the normal and the tendinosis tendon.