Biomedical subjects

J R Ralphs

Publications and source records attributed to J R Ralphs.

At least 19 recordsLinked to original sources

PubMed2006

Where tendons and ligaments meet bone: attachment sites ('entheses') in relation to exercise and/or mechanical load.

Entheses (insertion sites, osteotendinous junctions, osteoligamentous junctions) are sites of stress concentration at the region where tendons and ligaments attach to bone. Consequently, they are commonly subject to overuse injuries (enthesopathies) that are well documented in a number of sports. In this review, we focus on the structure-function correlations of entheses on both the hard and the soft tissue sides of the junction. Particular attention is paid to mechanical factors that influence form and function and thus to exploring the relationship between entheses and exercise. The molecular parameters indicative of adaptation to mechanical stress are evaluated, and the basis on which entheses are classified is explained. The application of the 'enthesis organ' concept (a collection of tissues adjacent to the enthesis itself, which jointly serve the common function of stress dissipation) to understanding enthesopathies is considered and novel roles of adipose tissue at entheses are reviewed. A distinction is made between different locations of fat at entheses, and possible functions include space-filling and proprioception. The basic anchorage role of entheses is considered in detail and comparisons are explored between entheses and other biological 'anchorage' sites. The ability of entheses for self-repair is emphasized and a range of enthesopathies common in sport are reviewed (e.g. tennis elbow, golfer's elbow, jumper's knee, plantar fasciitis and Achilles insertional tendinopathies). Attention is drawn to the degenerative, rather than inflammatory, nature of most enthesopathies in sport. The biomechanical factors contributing to the development of enthesopathies are reviewed and the importance of considering the muscle-tendon-bone unit as a whole is recognized. Bony spur formation is assessed in relation to other changes at entheses which parallel those in osteoarthritic synovial joints.

Adaptation, Physiological↗

PubMed2004

Biology of fibrocartilage cells.

Fibrocartilage is an avascular tissue that is best documented in menisci, intervertebral discs, tendons, ligaments, and the temporomandibular joint. Several of these sites are of particular interest to those in the emerging field of tissue engineering. Fibrocartilage cells frequently resemble chondrocytes in having prominent rough endoplasmic reticulum, many glycogen granules, and lipid droplets, and intermediate filaments together with and actin stress fibers that help to determine cell organization in the intervertebral disc. Fibrocartilage cells can synthesize a variety of matrix molecules including collagens, proteoglycans, and noncollagenous proteins. All the fibrillar collagens (types I, II, III, V, and XI) have been reported, together with FACIT (types IX and XII) and network-forming collagens (types VI and X). The proteoglycans include large, aggregating types (aggrecan and versican) and small, leucine-rich types (decorin, biglycan, lumican, and fibromodulin). Less attention has been paid to noncollagenous proteins, although tenascin-C expression may be modulated by mechanical strain. As in hyaline cartilage, matrix metalloproteinases are important in matrix turnover and fibrocartilage cells are capable of apoptosis.

Animals↗

PubMed2002

Actin stress fibres and cell-cell adhesion molecules in tendons: organisation in vivo and response to mechanical loading of tendon cells in vitro.

Tendons consist of parallel longitudinal rows of cells separated by collagen fibres. The cells are in intimate contact longitudinally within rows, and laterally via sheet-like lateral cell processes between rows. At points of contact, they are linked by gap junctions. Since tendons stretch under load, such cell contacts require protection. Here we describe the organisation of the actin cytoskeleton and actin-based cell-cell interactions in vivo and examine the effect of cyclic tensile loading on tendon cells in vitro. Cells within longitudinal rows contained short longitudinally running actin stress fibres. Each fibre was aligned with similar fibres in the cells longitudinally on either side, and fibres appeared to be linked via adherens junctions. Overall, these formed long oriented rows of stress fibres running along the rows of tendon cells. In culture, junctional components n-cadherin and vinculin and the stress fibre component tropomyosin increased in strained cultures, whereas actin levels remained constant. These results suggest that: (1) cells are linked via actin-associated adherens junctions along the line of principal strain; and (2) under load, cells appear to attach themselves more strongly together, and assemble more of their cytoplasmic actin into stress fibres with tropomyosin. Taken together, this suggests that cell-cell contacts are protected during stretch, and also that the stress fibres, which are contractile, may provide an active mechanism for recovery from stretch. In addition, stress fibres are ideally oriented to monitor tensile load and thus may be important in mechanotransduction and the generation of signals passed via the gap junction network.

Actins↗

PubMed2002

The skeletal attachment of tendons--tendon "entheses".

Tendon entheses can be classed as fibrous or fibrocartilaginous according to the tissue present at the skeletal attachment site. The former can be "bony" or "periosteal", depending on whether the tendon is directly attached to bone or indirectly to it via the periosteum. At fibrocartilaginous entheses, the uncalcified fibrocartilage dissipates collagen fibre bending and tendon narrowing away from the tidemark; calcified fibrocartilage anchors the tendon to the bone and creates a diffusion barrier between the two. Where there are additional fibrocartilaginous specialisations in the tendon and/or bone next to the enthesis, an "enthesis organ" is created that reduces wear and tear. Little attention has been paid to bone at entheses, despite the obvious bearing this has on the mechanical properties of the interface and the clinical importance of avulsion fractures. Disorders at entheses (enthesopathies) are common and occur in conditions such as diffuse idiopathic skeletal hyperostosis and the seronegative spondyloarthropathies. They are also commonly seen as sporting injuries such as tennis elbow and jumper's knee.

Bone and Bones↗

PubMed2002

Three-dimensional reconstructions of the Achilles tendon insertion in man.

The distribution of type II collagen in sagittal sections of the Achilles tendon has been used to reconstruct the three-dimensional (3D) shape and position of three fibrocartilages (sesamoid, periosteal and enthesis) associated with its insertion. The results showed that there is a close correspondence between the shape and position of the sesamoid and periosteal fibrocartilages--probably because of their functional interdependence. The former protects the tendon from compression during dorsiflexion of the foot, and the latter protects the superior tuberosity of the calcaneus. When the zone of calcified enthesis fibrocartilage and the subchondral bone are mapped in 3D, the reconstructions show that there is a complex pattern of interlocking between pieces of calcified fibrocartilage and bone at the insertion site. We suggest that this is of fundamental importance in anchoring the tendon to the bone, because the manner in which a tendon insertion develops makes it unlikely that many collagen fibres pass across the tissue boundary from tendon to bone. When force is transmitted to the bone from a loaded tendon, it is directed towards the plantar fascia by a series of highly orientated trabeculae that are clearly visible in 3D in thick resin sections.

Achilles Tendon↗

PubMed2001

Fibrocartilage in the transverse ligament of the human atlas.

STUDY DESIGN: Immunohistochemical investigation. OBJECTIVE: To determine whether molecules typical of articular cartilage are present in the transverse ligament and whether the ligament may be a target for an autoimmune response in rheumatoid arthritis. SUMMARY OF BACKGROUND DATA: In chronic rheumatoid arthritis there is often a marked instability of the atlantoaxial complex, and the transverse ligament can show degenerative changes that compromise its mechanical function. In some rheumatoid patients there can be an autoimmune response to cartilage link protein, aggrecan, and Type II collagen. METHODS: Transverse ligaments were removed from 13 cadavers and fixed in 90% methanol. Cryosections were immunolabeled with antibodies against proteoglycans (aggrecan, link protein, and versican), glycosaminoglycans (chondroitin-4-sulfate, chondroitin-6-sulfate, dermatan sulfate, and keratan sulfate), and collagens (Types I, II, III, and VI). RESULTS: Labeling for aggrecan and link protein was characteristic of the fibrocartilages, but versican was only detected in the fibrous regions. Equally, Types I, III, and VI collagens and keratan, dermatan, and chondroitin-4-sulfates were found throughout the ligament, but labeling for Type II collagen and chondroitin-6-sulfate was restricted to the fibrocartilages. CONCLUSION: The presence of molecules typical of articular cartilage (aggrecan, link protein, and Type II collagen) in the transverse ligament explains why it can be a target for destruction in rheumatoid arthritis and also suggests that it is subject to constant compression against the dens rather than only at the extremes of movement.

Aged↗

PubMed2001

Extracellular matrix in development of the intervertebral disc.

Intervertebral discs allow bending and twisting of the spine whilst resisting compression from gravity and muscle action, and are composite structures of the peripheral annulus fibrosus enclosing the nucleus pulposus. Their development is complex, involving several different connective tissue types, yet little is known of the developing extracellular matrix (ECM). We report the ECM composition of foetal rat discs from their first appearance to birth. The earliest collagen detected was type III, which was subsequently replaced by type II in the cartilaginous inner annulus and joined by type I in the fibrous outer annulus. Type IV collagen appeared in outer annulus, associated with myofibroblast-like cells of the orienting collagenous lamellae. Laminin and fibronectin co-distributed here in later stages, although overall they had a wider distribution. Aggrecan occurred in early nucleus pulposus and then appeared in the inner annulus, in association with cartilage differentiation. Versican appeared later in the inner annulus, and also in the dorsal region of the outer annulus. Comparisons of glycosaminoglycan and proteoglycan label allowed extrapolations to be made as to likely glycosaminoglycan components of the large proteoglycans, and of other proteoglycans that may be present - thus differential distribution of aggrecan and keratan sulfate label suggested the presence of fibromodulin and/or lumican. Functionally aggrecan would confer compression resistance to cartilaginous structures. Versican may also contribute, but along with the small proteoglycans is likely to be associated with various stages of control of cell differentiation, tissue morphogenesis and collagen fibre formation in the assembly of the annulus fibrosus.

Animals↗

PubMed2001

Fibrocartilage in the transverse ligament of the human acetabulum.

Biomechanical experiments on isolated hip joints have suggested that the transverse ligament acts as a bridle for the lunate articular surface of the acetabulum during load bearing, but there are inherent limitations in such studies because the specimens are fixed artificially to testing devices and there are no modifying influences of muscle pull. Further evidence is thus needed to substantiate the theory. Here we argue that if the horns of the lunate surface are forced apart under load, the ligament would straighten and become compressed against the femoral head. It would thus be expected to share some of the features of tendons and ligaments that wrap around bony pulleys and yet previous work has suggested that the transverse ligament is purely fibrous. Transverse ligaments were removed from 8 cadavers (aged 17-39 y) and fixed in 90% methanol. Cryosections were immunolabelled with antibodies against collagens (types I, II, III, VI), glycosaminoglycans (chondroitins 4 and 6 sulphate, dermatan sulphate, keratan sulphate) and proteoglycans (aggrecan, link protein, versican, tenascin). A small sesamoid fibrocartilage was consistently present in the centre of each transverse ligament, near its inner surface at the site where it faced the femoral head. Additionally, a more prominent enthesis fibrocartilage was found at both bony attachments. All fibrocartilage regions, in at least some specimens, labelled for type II collagen, chondroitin 6 sulphate, aggrecan and link protein, molecules more typically associated with articular cartilage. The results suggest that the ligament should be classed as containing a 'moderately cartilaginous' sesamoid fibrocartilage, adapted to withstanding compression. This supports the inferences that can be drawn from previous biomechanical studies. We cannot give any quantitative estimate of the levels of compression experienced. All that can be said is that the ligament occupies an intermediate position in the spectrum of fibrocartilaginous tissues. It is more cartilaginous than some wrap-around tendons at the wrist, but less cartilaginous than certain other wrap-around ligaments, e.g. the transverse ligament of the atlas.

Acetabulum↗

PubMed2001

Entheses--the bony attachments of tendons and ligaments.

Most tendons and ligaments have fibrocartilaginous entheses where there are 4 zones of tissue at their bony attachments--dense fibrous connective tissue, uncalcified fibrocartilage, calcified fibrocartilage and bone. Such entheses leave smooth, circumscribed markings on dried bones. The uncalcified fibrocartilage dissipates the bending of collagen fibres away from the bone, ensures that a stretched tendon or ligament does not narrow too close to the bone and acts as a mini growth plate. The zone is thickest at entheses where a great deal of bending of the tendon/ligament accompanies joint movement. The calcified fibrocartilage anchors the tendon/ligament to the bone and enables it to withstand shear. Enthesis fibrocartilage may be accompanied by sesamoid and periosteal fibrocartilages that similarly protect the enthesis from wear and tear and dissipate stress. Nevertheless, each fibrocartilage can show distinctive pathological changes. A wide variety of ECM molecules has been reported in enthesis fibrocartilage, but it is best characterised by its content of type II collagen and aggrecan which account for its compression-tolerance properties.

Animals↗

PubMed2000

The mechanism of formation of bony spurs (enthesophytes) in the achilles tendon.

OBJECTIVE: To investigate the early stages in the formation of bony spurs in relation to normal enthesis development. METHODS: Histologic sections of rat Achilles tendons, stained with toluidine blue or Masson's trichrome, were examined in animals ranging from 2 weeks to 1 year of age. Further material prepared for immunohistochemistry was labeled with monoclonal antibodies for laminin and type IV collagen to highlight the presence of small blood vessels at the enthesis. Sections of small spurs from the Achilles tendons of elderly humans were also examined for comparison. RESULTS: As a part of normal development, bone grows into the Achilles tendon as the calcaneus enlarges. Ossification is preceded by vascular invasion, which occurs along rows of enthesis fibrocartilage cells. Small bony spurs develop when ossification at one point on the enthesis outstrips that on either side. CONCLUSION: Bony spurs can develop in the Achilles tendon without the need for preceding microtears or any inflammatory reaction, and they form by endochondral ossification of enthesis fibrocartilage. The increased surface area created at the tendon-bone junction may be an adaptive mechanism to ensure the integrity of the interface in response to increased mechanical loads.

Achilles Tendon↗

PubMed2000

The cell and developmental biology of tendons and ligaments.

We have sought to create, for the first time in a single comprehensive review, a modern synthesis of opinion on the cell, developmental, and molecular biology of tendons, ligaments, and their associated structures (tendon sheaths, vinculi, and retinacula). Particular attention has been paid to highlighting new data on the early development of tendons, the signaling molecules involved in their patterning, and the diversity of specialized regions (entheses, wrap-around regions, and myotendinous junctions) that characterize fully formed tendons and ligaments. We have emphasized the complexities of adult tendon and ligament cell shape and related these to their early development. The importance of gap junctions in allowing cell communication throughout an extensive extracellular matrix (ECM) has also been highlighted, particularly in relation to understanding how tendon and ligament cells respond to changes in mechanical load. Finally, we have considered the influence of growth factors and related molecules on cell proliferation and ECM synthesis.

Animals↗

PubMed1999

Fibrocartilage in the extensor tendons of the human metacarpophalangeal joints.

The extensor tendons of the fingers cross both the metacarpophalangeal (MCP) and interphalangeal joints. Previous studies have shown that where the extensor tendons replace the capsule of the proximal interphalangeal (PIP) joint, they contain a sesamoid fibrocartilage that articulates with the proximal phalanx during flexion. The fibrocartilage labels immunohistochemically for a variety of glycosaminoglycans and collagens. In the current study, we investigate the molecular composition of the extensor tendons at the level of the MCP joints. This is of particular interest because the tendon has a greater moment arm at this location (and might thus be subject to greater compression), but is separated from the joint cavity by the capsule and peritendinous tissue. Six hands were removed from elderly cadavers (39-85 years of age) and the MCP joints were fixed in 90% methanol. The extensor tendons were dissected from all fingers, cryosectioned, and immunolabelled with a panel of monoclonal and polyclonal antibodies for types I, II, III, and VI collagens, chondroitin 4 and 6 sulphates, dermatan, and keratan sulphate and aggrecan. Antibody binding was detected with the Vectastain ABC 'Elite' avidin/biotin/peroxidase kit. The extensor tendons in all the fingers had a metachromatic sesamoid fibrocartilage on their deep surface which immunolabelled for types I, III, and VI collagens, and for all glycosaminoglycans and aggrecan. Labelling for type II collagen was also seen in some fibrocartilages and was a constant feature of all index fingers. This probably relates to the greater use of that digit and the higher loads to which its tendons are subject. Chondroitin 6 sulphate and type II collagen are the most consistent markers of the fibrocartilage phenotype and most of the chondroitin 6 sulphate is probably associated with aggrecan. It is concluded that the labelling profile of the tendon fibrocartilage in the different fingers at the MCP joints is broadly similar to that at the PIP joints. Thus, the potentially greater level of compression on the extensor tendons may be counterbalanced by the lack of fusion of the tendon with the joint capsule. It is suggested that the maintenance of a similar level of fibrocartilage differentiation at two different points along the length of the extensor tendon ensures that the tensile strength is the same in the two regions and that no weak link is present.

Adult↗

PubMed1999

Role of actin stress fibres in the development of the intervertebral disc: cytoskeletal control of extracellular matrix assembly.

Orientation of collagen fibrils is a key event in the development of many tissues. In the intervertebral disc, the outer annulus fibrosus comprises lamellae of parallel collagen fibres, the direction of orientation of the long axis of which alternates in angle between lamellae. In development, this organisation is preceded by the formation of sheets of oriented fibroblasts, which then deposit the oriented lamellae. Here, using fluorescent labelling, confocal and electron microscopic techniques on developmental series, we show that the orientation of cells in lamellae is associated with the formation of adherens junctions intercellularly, involving cadherins and vinculin, and longitudinal stress fibres (label for filamentous actin and tropomyosin) intracellularly. The stress fibres direct the initial elongation of cells and control the deposition of oriented extracellular matrix via junctional complexes with the matrix involving vinculin and alpha 5 beta 1 integrins, which in turn promote the formation of oriented fibronectin at the cell surface; oriented collagen is deposited between cells at the same stages. Shortly after birth, the stress fibres disappear, probably because cells now gain orientational cues from the matrix, and are undergoing differentiation-related changes to form fibrocartilage cells. Dev Dyn 1999;215:179-189.

Actin Cytoskeleton↗

PubMed1999

Organisation of the chondrocyte cytoskeleton and its response to changing mechanical conditions in organ culture.

Articular cartilage undergoes cycles of compressive loading during joint movement, leading to its cyclical deformation and recovery. This loading is essential for chondrocytes to perform their normal function of maintenance of the extracellular matrix. Various lines of evidence suggest the involvement of the cytoskeleton in load sensing and response. The purpose of the present study is to describe the 3-dimensional (3D) architecture of the cytoskeleton of chondrocytes within their extracellular matrix, and to examine cytoskeletal responses to experimentally varied mechanical conditions. Uniformly sized explants of articular cartilage were dissected from adult rat femoral heads. Some were immediately frozen, cryosectioned and labelled for filamentous actin using phalloidin, and for the focal contact component vinculin or for vimentin by indirect immunofluorescence. Sections were examined by confocal microscopy and 3D modelling. Actin occurred in all chondrocytes, appearing as bright foci at the cell surface linked to an irregular network beneath the surface. Cell surface foci colocalised with vinculin, suggesting the presence of focal contacts between the chondrocyte and its pericellular matrix. Vimentin label occurred mainly in cells of the deep zone. It had a complex intracellular distribution, with linked networks of fibres surrounding the nucleus and beneath the plasma membrane. When cartilage explants were placed into organ culture, where in the absence of further treatments cartilage imbibes fluid from the culture medium and swells, cytoskeletal changes were observed. After 1 h in culture the vimentin cytoskeleton was disassembled, leading to diffuse labelling of cells. After a further hour in culture filamentous vimentin label reappeared in deep zone chondrocytes, and then over the next 48 h became more widespread in cells of the explants. Actin distribution was unaffected by culture. Further experiments were performed to test the effects of load on the cytoskeleton. Explants were placed in culture and immediately subjected to static uniaxial radially unconfined compressive loads of 0.5, 1, 2 or 4 MPa for 1 h using a pneumatic loading device. Loads greater than 0.5 MPa maintained the vimentin organisation over the culture period. At 0.5 MPa, the chondrocytes within the explant behaved as in free-swelling culture. The rapid change in vimentin organisation probably relates to rapid swelling of the explants--under free-swelling conditions, these reached their maximum swollen size in just 15 min of culture. The chondrocytes' response to change in tissue dimensions, and thus to their relationship to their immediate environment, was to disassemble their vimentin networks. Loading probably counteracts the swelling pressure of the tissue. Overall, this work suggests that chondrocytes maintain their actin cytoskeleton and modify their vimentin cytoskeleton in response to changing mechanical conditions.

Actins↗

PubMed1998

Distribution of collagens and glycosaminoglycans in the joint capsule of the proximal interphalangeal joint of the human finger.

BACKGROUND: The capsule of the proximal interphalangeal joint consists of the central slip of the extensor tendon dorsally, the collateral ligaments at the sides and the palmar ligament ventrally. Fibrocartilaginous menisci have been reported extending into the joint cavity and the central slip has a sesamoid fibrocartilage articulating with the proximal phalanx. This study relates ECM composition in the joint capsule to function. METHODS: Each part of the capsule from 24 fingers amputated because of trauma, carcinoma, isthaemia, fixed-flexion deformities or Dupuytren's contracture, was dissected out. Sections were prepared for routine histology or immunolabelled with a panel of monoclonal and polyclonal antibodies against collagens and glycosaminoglycans using the avidin/biotin/peroxidase procedure. RESULTS: All parts of the capsule consistently labelled for types I, III and VI collagens and for dermatan and keratan sulphate, though labelling was more pericellular in fibrocartilaginous regions. In contrast, only certain regions of the capsule in some fingers labelled for type II collagen, chondroitin 4 or 6 sulphate. The sesamoid fibrocartilage in the central slip showed the greatest degree of fibrocartilage differentiation, especially in fixed-flexion deformity fingers, and the palmar ligament the least. CONCLUSIONS: The immunolabelling patterns suggest that there is an ordered sequence of matrix changes accompanying fibrocartilage differentiation. Chondroitin sulphate-containing proteoglycans accumulate first, and type II collagen appears later. The presence or absence of type II collagen probably relates to different levels of compressive loading. No fibrocartilaginous menisci were found in normal joints and those described previously are regarded as synovial folds.

Adolescent↗

PubMed1998

Fibrocartilages in the extensor tendons of the interphalangeal joints of human toes.

The extensor tendons of the fingers and toes form part of the capsule of the interphalangeal joint and press against the proximal phalanx during flexion. Previous work on the fingers has shown that there is a "sesamoid" fibrocartilage on the deep surface of each tendon that labels immunohistochemically for a variety of glycosaminoglycans and collagens. However, we know little about the molecular composition of the tendon in the toes. This question is of special interest, because the mechanics of the interphalangeal joints differ in the upper and lower limbs-the toes balance the forefoot, distribute load during the gait cycle, and transmit the pull of larger muscles. This means that their extensor tendons are more often under higher tension than those in the fingers. Here, we report the presence of an equivalent fibrocartilage and compare its immunolabelling characteristics in all the toes. Six forefeet were removed from elderly cadavers, and the interphalangeal (IP) joints were fixed in 90% methanol. The extensor tendon and its enthesis were dissected out from the IP joint of the big toe and from the proximal interphalangeal (PIP) joint of all lesser toes, decalcified, cryosectioned, and immunolabelled with a panel of monoclonal and polyclonal antibodies for type I, II, III, and VI collagens; chondroitin 4 and 6 sulphates; and dermatan and keratan sulphate. Antibody binding was detected with the Vectastain ABC Elite avidin-biotin-peroxidase kit (Vector Laboratories, Burlingame, CA). The extensor tendon in all the toes had a metachromatic, sesamoid fibrocartilage on its deep surface that immunolabelled for all glycosaminoglycans and for type I, III, and VI collagens. Labelling for type II collagen was seen in the sesamoid fibrocartilage of all toes but was particularly characteristic of the 2nd through 5th toes. The immunolabelling patterns of the enthesis fibrocartilage were similar in all toes and to results reported previously for fingers. The normal occurrence of type II collagen in the sesamoid fibrocartilage of the 2nd through 5th toes is in contrast to our published data on the fingers. The finding can be related to the more constant loading of the tendon in the toes. The greater prominence of type II collagen in the sesamoid fibrocartilage of the 2nd through 5th toes could be related to a difference in joint position during walking between the 1st toe and the 2nd through 5th toes--the PIP joints of the latter are usually more flexed than the IP joint of the former.

Adult↗

PubMed1998

Characterization of collagens and proteoglycans at the insertion of the human Achilles tendon.

This study provides a unique correlation between a molecular biological and biochemical analysis of the extracellular matrix (ECM) macromolecules in one half of 28 human Achilles tendons with an immunohistochemical study of the other. Both the insertion site and the mid-tendon were studied. The insertion (enthesis) is characterized by three distinctive fibrocartilages, two in the tendon (enthesial and sesamoid) and one on the heel bone (periosteal). Thus, its structure contrasts markedly with the fibrous character of the mid-tendon. RT-PCR analyses were performed on RNA extracted from mid-tendon and from the tendon fibrocartilages to investigate transcription of collagens and proteoglycans. Western blotting was also used to identify and characterize these macromolecules, and immunohistochemistry to localize their distribution. The results demonstrate striking differences in the ECM between the mid-tendon and its insertion. Types I, III, V and VI collagens, decorin, biglycan, fibromodulin and lumican were found in both the mid-tendon and the fibrocartilages, although their precise distribution often differed with site. mRNA for type II collagen was constantly present in the fibrocartilages, but it was only found in the mid-tendon of one specimen. The patterns of distribution for versican and aggrecan mRNA were complimentary - versican mRNA was present in the mid-tendon and absent from the fibrocartilages, while aggrecan mRNA was present in the fibrocartilages and absent from the mid-tendon. The range and distribution of ECM molecules detected in the Achilles tendon reflect the differing forces acting on it - the mid-tendon largely transmits tension and is characterized by molecules typical of fibrous tissues, but the fibrocartilages must also resist compression and thus contain, in addition, molecules typical of cartilage.

Achilles Tendon↗

PubMed1998

Regional differences in cell shape and gap junction expression in rat Achilles tendon: relation to fibrocartilage differentiation.

Tendon cells have complex shapes, with many cell processes and an intimate association with collagen fibre bundles in their extracellular matrix. Where cells and their processes contact one another, they form gap junctions. In the present study, we have examined the distribution of gap junction components in phenotypically different regions of rat Achilles tendon. This tendon contains a prominent enthesial fibrocartilage at its calcaneal attachment and a sesamoid fibrocartilage where it is pressed against the calcaneus just proximal to the attachment. Studies using DiI staining demonstrated typical stellate cell shape in transverse sections of pure tendon, with cells withdrawing their cell processes and rounding up in the fibrocartilaginous zones. Coincident with change in shape, cells stopped expressing the gap junction proteins connexins 32 and 43, with connexin 43 disappearing earlier in the transition than connexin 32. Thus, there are major differences in the ability of cells to communicate with one another in the phenotypically distinct regions of tendon. Individual fibrocartilage cells must sense alterations in the extracellular matrix by cell/matrix interactions, but can only coordinate their behaviour via indirect cytokine and growth factor signalling. The tendon cells have additional possibilities--in addition to the above, they have the potential to communicate direct cytoplasmic signals via gap junctions. The formation of fibrocartilage in tendons occurs because of the presence of compressive as well as tensile forces. It may be that different systems are used to sense and respond to such forces in fibrous and cartilaginous tissues.

Achilles Tendon↗