The transmembrane heparan sulphate proteoglycan syndecan-4 is involved in establishment of the lamellar structure of the annulus fibrosus of the intervertebral disc

The annulus fibrosus of the intervertebral disc unites adjacent vertebral bodies along the length of the spine and provides tensile resistance towards compressive, twisting and bending movements arising through gait. It consists of a nested series of oriented collagenous lamellae, arranged in cross-ply circumferentially around the nucleus pulposus. The organisation of oriented collagen in the annulus is established during foetal development by an identical arrangement of oriented fibroblasts that are precisely organised into cell sheets, or laminae. These provide a template for ordered deposition of extracellular matrix material on cell surfaces, by means of a poorly understood mechanism involving the actin cytoskeleton. In this study, we investigate the role of two cell surface heparan sulphate proteoglycans (HSPGs), glypican-6 and syndecan-4, in the matrix assembly process in the developmental rat intervertebral disc. We compare their expression patterns with those of heparan sulphate and the interactive, cell-surface adhesive glycoprotein, fibronectin, and relate these to the stage-specific collagenous architectures present within the annulus at both light and electron microscopic levels. We show that both proteoglycans are strongly associated with the development, growth and aging of the intervertebral disc. Furthermore, the immunohistochemical labelling patterns suggest that syndecan-4, in particular, plays a potentially-significant role in annulus formation. We propose that this HSPG mediates interaction between the actin cytoskeleton and nascent extracellular matrix in the lamellar organisation of annulus tissue. These data add considerably towards an understanding of how cells organise and maintain complex, oriented extracellular matrices and has particular clinical relevance to the fields of tissue engineering and repair.

Enhancing inflammatory and chemotactic signals to regulate bone regeneration

Stem cells have become the fundamental element in regenerative medicine due to their inherent potential to differentiate into various cell types, and the ability to produce various bioactive molecules, including growth factors, cytokines and extracellular matrix molecules. In vivo, the secretion of tropic factors is modulated by chemotactic and inflammatory factors. In this study, we analysed the influence of a 2 h stimulation of mesenchymal stem cells (MSCs) with interleukin-1β (IL1β), granulocyte-colony stimulating factor (GCSF), stromal cell-derived factor 1 (SDF1) and stem cell factor (SCF). Our results demonstrated that this short stimulation exerts pronounced effects on the expression of multiple cytokine genes and proteins in MSC cells 48 and 72 h later. IL1β strongly promoted the secretion of a wide range of proteins with chemotactic, proinflammatory and angiogenic properties, whereas SCF regulated the expression of proteins involved in proliferation, chondrogenesis and ECM regulation. This demonstrates that the changes in secretome can be directed towards a desired final functional outcome by selection of the most appropriate cytokine. Moreover, the expression pattern of Wnt signalling pathway components suggested the differential regulation of this pathway by IL1β and SCF. Altogether, the robust paracrine action of MSCs can be achieved within a just 2 h treatment, which would be feasible within the operating theatre during a single surgical procedure. These results suggest that integrating inflammatory modulation in bone tissue engineering, by modifying the MSC secretome by way of a short stimulus, would provide a more targeted approach than administering unmodified MSCs alone.

A phenotypic comparison of osteoblast cell lines versus human primary osteoblasts for biomaterials testing

Immortalized cell lines are used more frequently in basic and applied biology research than primary bone-derived cells because of their ease of access and repeatability of results in experiments. It is clear that these cell models do not fully resemble the behavior of primary osteoblast cells. Although the differences will affect the results of biomaterials testing, they are not clearly defined. Here, we focused on comparing proliferation and maturation potential of three osteoblast cell lines, SaOs2, MG-63, and MC3T3-E1 with primary human osteoblast (HOb) cells to assess their suitability as in vitro models for biomaterials testing. We report similarities in cell proliferation and mineralization between primary cells and MC3T3-E1. Both, SaOs2 and MG-63 cells demonstrated a higher proliferation rate than HOb cells. In addition, SaOs2, but not MG-63, cells demonstrated similar ALP activity, mineralization potential and gene regulation to HOb's. Our results demonstrate that despite SaOs-2, MG63, and MC3T3 cells being popular choices for emulating osteoblast behavior, none can be considered appropriate replacements for HOb's. Nevertheless, these cell lines all demonstrated some distinct similarities with HOb's, thus when applied in the correct context are a valuable in vitro pilot model of osteoblast functionality, but should not be used to replace primary cell studies.

Collagen fibrillogenesis in the development of the annulus fibrosus of the intervertebral disc

The annulus fibrosus of the intervertebral disc is a complex, radial-ply connective tissue consisting of concentric lamellae of oriented collagen. Whilst much is known of the structure of the mature annulus, less is known of how its complex collagenous architecture becomes established; an understanding of which could inform future repair/regenerative strategies. Here, using a rat disc developmental series, we describe events in the establishment of the collagenous framework of the annulus at light and electron microscopic levels and examine the involvement of class I and II small leucine rich proteoglycans (SLRPs) in the matrix assembly process. We show that a period of sustained, ordered matrix deposition follows the initial cellular differentiation/orientation phase within the foetal disc. Fibrillar matrix is deposited from recesses within the plasma membrane into compartments of interstitial space within the outer annulus - the orientation of the secreted collagen reflecting the initial cellular orientation of the laminae. Medially, we demonstrate the development of a reinforcing 'cage' of collagen fibre bundles around the foetal nucleus pulpous. This derives from the fusion of collagen bundles between presumptive end-plate and inner annulus. By birth, the distinct collagenous architectures are established and the disc undergoes considerable enlargement to maturity. We show that fibromodulin plays a prominent role in foetal development of the annulus and its attachment to vertebral bodies. With the exception of keratocan, the other SLRPs appear associated more with cartilage development within the vertebral column, but all become more prominent within the disc during its growth and differentiation.

Chondroitin sulphate sulphation motif expression in the ontogeny of the intervertebral disc

Chondroitin sulphate chains on cell and extracellular matrix proteoglycans play important regulatory roles in developing systems. Specific, developmentally regulated, sulphation motifs within the chondroitin glycosaminoglycan structure may help bind, sequester or present bioactive signalling molecules to cells thus modulating their behaviour. Using monoclonal antibodies 3B3(-), 4C3, 6C3 and 7D4, we have mapped the distribution of different chondroitin sulphation epitopes in a rat intervertebral disc developmental series. The sulphation epitopes had complex, dynamic and specific distributions in the disc and vertebral tissues during their differentiation, growth and ageing. At embryonic day [E]15, prior to disc differentiation, 4C3 and 7D4 occurred within the cellular disc condensations whilst 6C3 was present in the notochordal sheath. At E17, post disc differentiation, 4C3 and 7D4 occurred within the nucleus pulposus, inner annulus and vertebral bodies; 3B3(-) in the nucleus, inner annulus, annulus/vertebral body interface and perichondrium; and 6C3, ventrally, within the perichondrium. At E19, 3B3(-), 4C3 and 7D4 became further restricted to the nucleus, inner annulus, annulus/vertebral body interface and perichondrium. Prior to birth, all four epitopes occurred within the inner annulus and nucleus, with 6C3 and 7D4 also occurring within the future end-plate. Postnatal expression of the sulphation epitopes was more widespread in the disc and also within the growth plate. At 4 months, the epitopes were associated with chondrocyte clusters within the nucleus; and at 24 months, with annular lesions. Overall, our data suggests that differential sulphation of chondroitin correlates with significant events in development, growth and aging of the rat intervertebral disc.

Characterization of Candida albicans infection of an in vitro oral epithelial model using confocal laser scanning microscopy

INTRODUCTION

Oral candidosis presents as several distinct forms and one of these, chronic hyperplastic candidosis, is distinguished by penetration of the epithelium by Candida. The aim of this study was to use confocal laser scanning microscopy to examine invasion of the oral epithelium by Candida albicans from different oral conditions and to determine whether inherent strain differences exist that could relate to infection type. Reverse transcription-polymerase chain reaction was also used to detect products from virulence gene families.

METHODS

C. albicans (n = 19) was used to infect reconstituted human oral epithelium, which was incubated for 12 h. One half of the reconstituted human oral epithelium was then fixed and stained with concanavalin A-Alexa 594, pan-cytokeratin antibody-Alexa 488 and Hoechst nucleic acid dye. RNA was extracted from the remaining tissue for reverse transcription-polymerase chain reaction targeting secreted aspartyl proteinase, phospholipase and agglutinin-like sequence genes of C. albicans.

RESULTS

Confocal laser scanning microscopy revealed strain-dependent tissue invasion, with differences evident in surface colonization, C. albicans morphology and the extent and pattern of tissue penetration. Hyphae were seen to directly penetrate epithelial cells and migrate between keratinocytes with yeast budding also evident in the reconstituted human oral epithelium. A relationship between 'high tissue invasion' and expression of secreted aspartyl proteinase genes 4-6 was noted. Interestingly, four of the five 'high invaders' originated from chronic hyperplastic candidosis.

CONCLUSIONS

Confocal laser scanning microscopy permitted high resolution analysis of reconstituted human oral epithelium invasion by C. albicans and identified strain differences in the invasion process. Association between extensive hyphal morphology, direct epithelial penetration and high surface colonization were made with the 'highly invasive' strains.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Fibrocartilage in tendons and ligaments--an adaptation to compressive load

Where tendons and ligaments are subject to compression, they are frequently fibrocartilaginous. This occurs at 2 principal sites: where tendons (and sometimes ligaments) wrap around bony or fibrous pulleys, and in the region where they attach to bone, i.e. at their entheses. Wrap-around tendons are most characteristic of the limbs and are commonly wider at their point of bony contact so that the pressure is reduced. The most fibrocartilaginous tendons are heavily loaded and permanently bent around their pulleys. There is often pronounced interweaving of collagen fibres that prevents the tendons from splaying apart under compression. The fibrocartilage can be located within fascicles, or in endo- or epitenon (where it may protect blood vessels from compression or allow fascicles to slide). Fibrocartilage cells are commonly packed with intermediate filaments which could be involved in transducing mechanical load. The ECM often contains aggrecan which allows the tendon to imbibe water and withstand compression. Type II collagen may also be present, particularly in tendons that are heavily loaded. Fibrocartilage is a dynamic tissue that disappears when the tendons are rerouted surgically and can be maintained in vitro when discs of tendon are compressed. Finite element analyses provide a good correlation between its distribution and levels of compressive stress, but at some locations fibrocartilage is a sign of pathology. Enthesis fibrocartilage is most typical of tendons or ligaments that attach to the epiphyses of long bones where it may also be accompanied by sesamoid and periosteal fibrocartilages. It is characteristic of sites where the angle of attachment changes throughout the range of joint movement and it reduces wear and tear by dissipating stress concentration at the bony interface. There is a good correlation between the distribution of fibrocartilage within an enthesis and the levels of compressive stress. The complex interlocking between calcified fibrocartilage and bone contributes to the mechanical strength of the enthesis and cartilage-like molecules (e.g. aggrecan and type II collagen) in the ECM contribute to its ability to withstand compression. Pathological changes are common and are known as enthesopathies.

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.

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.

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.

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.

Immunolocalisation of collagens in the developing rat molar tooth

This study identifies different types of collagens during tooth development, maturation and ageing. Tissues from the rat first molar (from animals ranging in age from E14 to 104 wk postnatally) were immunostained using a panel of mono- and polyclonal antibodies against types I, II, III, IV, VI, IX and X collagen, fibronectin and laminin. During tooth development, types I and III collagens were expressed in the dental papilla at all stages but were also unexpectedly observed in the stellate reticulum of the enamel organ. Transient expression of type II collagen was also observed in the stellate reticulum during the late bell stage. Types IV and VI collagens, with laminin and fibronectin, were located within the basement membranes of the tooth germ. Collagen types I and III were observed within the developing follicle/periodontal ligament, type III predominating where collagen fibres were inserting into the alveolar bone and cementum. The pattern of types I and III collagen labelling within the periodontal ligament and the dental pulp did not change with age. Thus, some unusual collagen localisations were observed in the tooth germ, particularly within the stellate reticulum.

Changes in the cytoskeleton of cells within the periodontal ligament and dental pulp of the rat first molar tooth during ageing

The periodontal ligament and dental pulp connective tissues are foetal-like, and their process of ageing may therefore differ from other tissues of mesenchymal origin. Several studies have already highlighted the lack of age changes in the extracellular matrix (ECM), but more needs to be known about cellular changes. For this study, tissues from the first molar teeth of Wistar rats aged 12 wk and 104 wk were compared by immunolocalisation of cytoskeletal components. Tissues from the first molar were immunostained, employing a panel of 16 monoclonal antibodies against cytokeratins, vimentin, F-actin and tubulin. Within the pulp, labelling for vimentin in both odontoblasts and pulpal fibroblasts and F-actin in the cell processes of odontoblasts was detected at both ages but with marked reduction in labelling in the older tissue. Within the periodontal ligament, vimentin labelling was weaker in the aged tissue, especially nearer the cementum. More significantly, the fibroblasts of the aged periodontal ligament expressed cytokeratin 19. In contrast to reports of little age change to the ECM, the cells of the pulp, and particularly the periodontal ligament, show marked changes to their cytoskeletal components.

Tendons and ligaments--an overview

The structure, range of functions, blood supply, nerve supply, biochemical composition and development of tendons and ligaments are reviewed. The importance of their cells is often overlooked because of the obvious role of the extracellular matrix (ECM) in determining the physical properties of tendons and ligaments. However, it is emphasised that tendon and ligament cells have elaborate cell processes that form a three dimensional network extending throughout the extracellular matrix. The cells communicate with each other via gap junctions that could form the basis of an important load sensing system allowing the tendon to modify its ECM. Tendons and ligaments have three specialised regions along their length-the myotendinous junction, the region where tendons change direction by wrapping around bony pulleys and the enthesis (bony insertion site). The myotendinous junction is a common site of muscle strains and pulls, the wrap-around region is frequently fibrocartilaginous and a common site for degenerative change, and the enthesis may be fibrous or fibrocartilaginous according to location, and is a common site for degenerative changes or 'enthesopathies'. Enthesis fibrocartilage is just one of a series of protective devices reducing wear and tear at insertion sites. Consideration is also given to the structure and function of tendon sheaths and to the dramatic effects of exercise and deprivation on tendons and ligaments-exercise strengthens, but even relatively short periods of immobilisation can dramatically weaken tendons and ligaments.

Tendon cells in vivo form a three dimensional network of cell processes linked by gap junctions

Tendons respond to mechanical load by modifying their extracellular matrix. The cells therefore sense mechanical load and coordinate an appropriate response to it. We show that tendon cells have the potential to communicate with one another via cell processes and gap junctions and thus could use direct cell/cell communication to detect and/or coordinate their load responses. Unfixed cryosections of adult rat digital flexor tendons were stained with the fluorescent membrane dye DiI to demonstrate cell shape. Similar sections were immunolabelled with monoclonal antibodies to rat connexin 32 or connexin 43 to demonstrate gap junctions and counterstained with propidium iodide to show nuclei, or the membrane stain DiOC7 to show cell membranes. Sections were examined with a laser scanning confocal microscope and 3-dimensional reconstructions were prepared from optical section series to demonstrate cell shape and the position of connexin immunolabel. Cells had a complex interconnected morphology with gap junctions at points of contact with other cells. Cell bodies contained the nucleus and extended broad flat lateral cell processes that enclosed collagen bundles and interacted with similar processes from adjacent cells. They also had long thin longitudinal processes interacting with the cell process network further along the tendon. Connexin 43 occurred where cell processes met and between cell bodies, whereas connexin 32 was only found between cell bodies. The results indicate the presence of a 3-dimensional communicating network of cell processes within tendons. The intimate relationship between cell processes and collagen fibril bundles suggests that the cell process network could be involved in load sensing and coordination of response to load. The presence of 2 different types of connexins suggests that there could be at least 2 distinct communicating networks.

An immunohistochemical study of enthesis development in the medial collateral ligament of the rat knee joint

The changing distributions of collagens and glycosaminoglycans have been studied at the attachments of the medial collateral ligament during postnatal development. The ligament is of particular interest because it has a fibrocartilaginous attachment to the femoral epiphysis, but a fibrous one to the tibial metaphysis. Ligaments were examined in rats killed at birth and at 2, 4, 6, 8, 10, 20, 30, 45, 60, 90 and 120 days after birth. Cryosections were immunolabelled with monoclonal and polyclonal antibodies against types I and II collagen, chondroitin 4 and 6 sulfate, dermatan and keratan sulfate. Although the ligament is attached at both ends to bones that develop from cartilage, there was a striking difference in collagen labelling. Type II collagen was only found in spicules of calcified cartilage in bone beneath the tibial enthesis after ossification had commenced, but there was a continuous band of labelling at all stages of development at the femoral enthesis. Initially, the cartilage at the femoral attachment lacked type I collagen, but by 45 days labelling was continuous from ligament to bone. Continuity of labelling was seen much earlier at the tibial enthesis, as soon as bone had formed. There were also marked changes in glycosaminoglycan distribution. Keratan sulfate was present at both entheses up to 45 days, but only at the femoral enthesis thereafter. Both attachments labelled throughout life for dermatan sulfate, but chondroitin 4 and 6 sulfate were only found at the femoral end. The results suggest that enthesial cartilage at the femoral attachment was initially derived from the cartilaginous bone rudiment but was quickly eroded on its deep surface by endochondral ossification as bone formed at the attachment site. It was replaced by fibrocartilage developing in the ligament. This mechanism allows enthesis cartilage/fibrocartilage to contribute to the growth of a bone at a secondary centre of ossification in addition to dissipating stress at the ligament-bone junction.

Tendons in health and disease

SUMMARY. This article reviews the structure, functions and mechanical properties of normal tendons and surveys the range and causes of tendon diseases. Specialized regions of tendons are considered, including the myotendinous junction, enthesis and regions where tendons rub against bone. Particular emphasis is placed on recent literature and on aspects of tendons that are relevant to manual therapists. Copyright 1996 Harcourt Publishers Ltd.

Ultrastructure of fibrocartilages at the insertion of the rat Achilles tendon

The ultrastructure of 3 fibrocartilages is described at the insertion of the adult rat Achilles tendon. Enthesial fibrocartilage lies at the tendon-bone junction, sesamoid fibrocartilage in the deep surface of the tendon where it presses on the calcaneus, and periosteal fibrocartilage covers the opposing surface of the bone. All had some features that could be interpreted as typical of fibrous tissues and other features that were more characteristic of cartilage. The general extracellular matrix was largely fibrous and had relatively little visible proteoglycan. In contrast, the cells had features more common in cartilage--dilated rough endoplasmic reticulum, glycogen and lipid, and pericellular matrix rich in proteoglycans and fine collagen fibrils. The periosteal fibrocartilage was the most 'cartilaginous' in character, probably because it develops rapidly after birth as a secondary cartilage from the calcaneal periosteum whereas enthesial and sesamoid fibrocartilages develop by metaplasia of tendon fibroblasts. A major difference between the 3 fibrocartilages was the arrangement of their collagen fibrils. There were parallel bundles in enthesial fibrocartilage but interweaving networks in the sesamoid and periosteal fibrocartilages. This probably reflects different functional demands of the tissues. The sesamoid and periosteal fibrocartilages directly formed the boundaries of the retrocalcaneal bursa and were not covered by synovium. Both were lined by an electron dense, articular surface lamina that prevented cell contact with the bursa. It significance is unclear, but it could protect against wear and tear, represent material being shed into the bursal cavity or control nutrient exchange with the synovial fluid.

High resolution magnetic resonance imaging of the proximal interphalangeal joints. Correlation with histology and production of a three-dimensional data set

The magnetic resonance imaging appearance of the proximal interphalangeal joints of cadavers was correlated with histology of the same specimen allowing many small-scale features to be identified that might otherwise have been misinterpreted. It enabled the magnetic resonance signal to be understood at a tissue and cellular level, allowing identification of synovial folds extending from the extensor tendon and volar plate, the entheses of the proper collateral ligament, the epitenon of the flexor tendons and the presence of osteophytes and sites of cartilage erosion. The main difficulties in matching two-dimensional magnetic resonance images with histology were the differing section thicknesses of the two methods and shrinkage of histological specimens. There are many advantages in producing high resolution three-dimensional datasets-the magnetic resonance section thickness is reduced and the individual components of the joint can be viewed simultaneously in two or more planes. A unique magnetic resonance atlas of three dimensional joint structure is presented.

Changing expression of intermediate filaments in fibroblasts and cementoblasts of the developing periodontal ligament of the rat molar tooth

The distributing of vimentin and cytokeratin intermediate filaments within the cells of the dental follicle and developing periodontal ligament is described during eruption of the rat 1st molar tooth. Alcohol-fixed tissues from animals ranging from neonates to 12 wk old were cryosectioned, immunolabelled with monoclonal antibodies against vimentin and a range of cytokeratins and examined by indirect immunofluorescence. Vimentin was observed in follicular and periodontal ligament fibroblasts in all animals and at all stages of eruption. It was also observed in cementoblasts after disruption of the epithelial root sheath (of Hertwig) which is responsible for determining the shape of the developing root. Prior to eruption, cytokeratins were restricted to epithelial components of the developing tooth, including the root sheath. However, they were seen in cementoblasts on disruption of the root sheath at 2 wk and in periodontal ligament fibroblasts at 3 wk after birth, when the tooth was erupting but had not reached occlusion. On occlusion (at 4 wk), fibroblasts no longer labelled for cytokeratins but cementoblasts associated with acellular cementum formation continued to express them. These results demonstrate temporal and spatial changes within the cells of the developing periodontal connective tissues and suggest that the appearance of cytokeratins in periodontal fibroblasts and cementoblasts may be related to mechanical changes during tooth eruption. Further, the results suggest different origins for cementoblasts associated with cellular and acellular cementum formation.

Fibrocartilage associated with human tendons and their pulleys

The presence of fibrocartilage in tendons that wrap around bony or fibrous pulleys is well known. It is an adaptation to resisting compression or shear, but the extent to which the structure of most human tendons is modified where they contact pulleys is less clear, for there has been no single comprehensive survey of a large number of sites. Less is known of the structure of the corresponding pulleys. In the present study, 38 regions of tendons that wrap around bony pulleys or pass beneath fibrous retinacula have been studied in routine histology sections taken from each of 2 or 3 elderly dissecting room cadavers. Most of the corresponding pulleys have also been examined. Fibrocartilage was present in 22 of the 38 tendon sites and it was most conspicuous where the tendons pressed predominantly against bone rather than retinacula and where they showed a large change in direction. Fibrocartilage was more characteristic of tendons at the ankle than the wrist, probably because the long axis of the foot is at right angles to that of the leg. There was considerable variation in the structure of tendon fibrocartilage. The most fibrocartilaginous tendons had oval or round cells embedded in a highly metachromatic matrix with interwoven or spiralling collagen fibres. At other sites, fibrocartilage cells were arranged in rows between parallel collagen fibres. The differences probably relate to differences in development. A single tendon could be modified at successive points along its length and fibrocartilage could be present in the endotenon and epitenon as well as in the tendon itself. Pathological changes seen in 'wrap around' tendons were fragmentation and partial delamination of the compressed surface, chondrocyte clustering, fatty infiltration and bone formation. Three types of pulleys were described for tendons--bony prominences and grooves, fibrous retinacula and synovial joints. The extent of cartilaginous differentiation on the periosteum of bony pulleys frequently mirrored that in the corresponding tendon. The cartilage or fibrocartilage prevents the tendon from 'sawing' through the bone. Some of the best known retinacula were largely fibrous, though the inferior peroneal retinaculum and the trochlea for the superior oblique were cartilaginous. The results underline the considerable regional heterogeneity in different tendons and their pulleys. They show that one tendon is not like another and that tendons may need to be carefully selected for particular surgical transfers or joint reconstructions.

The development of fibrocartilage in the rat intervertebral disc

The development of fibrocartilage in rat lumbar intervertebral discs has been correlated with an immunohistochemical analysis of the changing distribution of extracellular matrix components. Disc anlagen were first recognised by embryonic day 14 as segmental cell condensations. By E16, the notochord formed a series of bulges, each representing a future nucleus pulposus, and the annulus fibrosus had differentiated in the disc anlagen. The inner part of the annulus was composed of cartilage which linked that of adjacent vertebral bodies. The outer part was fibroblastic, with layers of parallel fibroblasts. The long axes of the cells in successive layers lay at an angle of approximately 90 degrees to each other. This criss-cross orientation of cells preceded the oriented deposition of collagen fibres to form the lamellae. Disc anlagen were immunolabelled weakly for types I and III collagen, chondroitin 6-sulphate and dermatan sulphate. Later tissue differentiation was marked by the appearance of type II collagen, chondroitin 4-sulphate and keratan sulphate in the inner annulus. These components also appeared in the outer annulus, but only in adult animals, and indicated metaplastic change in the lamellar fibroblasts. Fibrocartilage in the nucleus pulposus was only seen in old animals, and the origin of the tissue was less clear. However, the fibrocartilage cells appeared to be derived from the cartilage end plate and/or from the inner annulus. We conclude that fibrocartilage in the intervertebral disc is derived from several sources and that the radial distribution patterns of extracellular matrix components in the adult disc are explained by the embryonic origins of its parts.

Structure and histopathology of the insertional region of the human Achilles tendon

The Achilles tendon inserts onto the calcaneus, and the retrocalcaneal bursa intervenes between it and the bone immediately proximal to the enthesis. The enthesis, the bursa, and the bursal walls form a complex insertional region protecting against wear and tear. We examined the structure and histopathology of the insertional region in 50 tendons from cadavers (age at time of death, 57-96 years). The enthesis contained fibrocartilage typical of attachment sites. In specimens with a prominent superior tuberosity (the majority), the walls of the bursa also were fibrocartilaginous. On the anterior wall, fibrocartilage replaced the calcaneal periosteum; on the posterior wall, there was a sesamoid fibrocartilage in the deep part of the tendon. When the tuberosity was not prominent, the bursal fibrocartilages were absent. Histopathological features were observed in 31 entheses. Bone spurs extended from the calcaneus into the tendon and probably formed by endochondral ossification of enthesial fibrocartilage. Longitudinal fissures were splits in the fibrocartilage along the lines of the endotenon, and small transverse tears occurred at the tendon-bone junction. Longitudinal fissures showed evidence of repair; they were filled with amorphous material and surrounded by clusters of cells. In the bursal walls, calcification or degradation, or both, were observed in 37 specimens and usually involved both sesamoid and periosteal fibrocartilages. These fibrocartilages could therefore be implicated in retrocalcaneal bursitis.

Cytoskeleton of the mesenchymal cells of the rat dental papilla and dental pulp

This study describes the immunolocalization of actin, cytokeratins and vimentin during differentiation of the dental papilla in the rat. Incisors and first molars were sectioned from mandibles of Wistar rats from embryonic day (E)-14 to (E)-21 and weeks 1, 2, 3, 12 and 104 after birth, fixed in 90% alcohol, decalcified in EDTA, infiltrated with 5% sucrose, frozen in dry ice, and cryosectioned at 10 microns. The sections were immunolabelled using indirect immunofluorescence with a panel of monoclonal antibodies and FITC-phalloidin for F-actin localization. F-actin was present in follicular mesenchyme and odontoblast processes. Vimentin labelled dental papilla fibroblasts, differentiating, functional (secretory) and aged odontoblasts. Vimentin was uniformly localized in the cytoplasm of pre-odontoblasts but was redistributed to the apical pole of these cells during polarization. Of the cytokeratins, only cytokeratin 19 was found in differentiating odontoblasts. It was not present in dental papilla fibroblasts, functional or aged odontoblasts. These results suggest that actin and the redistribution of vimentin may be involved in odontoblast differentiation and odontoblast process formation/support and that these events may be preceded by the expression of cytokeratin 19.

Cytoskeleton of cartilage cells

The cytoskeleton of chondrocytes consists of microfilaments made of actin, microtubules made of tubulin, and intermediate filaments made of a variety of subunits. Actin filaments are not prominent in vivo but may form in vitro. In culture, changes in filament polymerisation are important in determining cell shape, initiating chondrogenesis, and maintaining the chondrogenic phenotype. Microtubules, besides their role in cell division, organise the distribution of organelles and are involved in secretory transport mechanisms in collagen and proteoglycan synthesis. A variety of intermediate filaments may be present, frequently forming large whorled aggregates. The filaments include vimentin, cytokeratins, and glial fibrillary acidic protein. These may occur at different depths in articular cartilage. Vimentin accumulates during development of some fibrocartilages with increased mechanical loading. Together with other elements of the cytoskeleton, intermediate filaments could form part of a mechanotransduction system by which cells respond to external forces and sense changes in their external environment.

The joint capsule: structure, composition, ageing and disease

The joint capsule is vital to the function of synovial joints. It seals the joint space, provides passive stability by limiting movements, provides active stability via its proprioceptive nerve endings and may form articular surfaces for the joint. It is a dense fibrous connective tissue that is attached to the bones via specialised attachment zones and forms a sleeve around the joint. It varies in thickness according to the stresses to which it is subject, is locally thickened to form capsular ligaments, and may also incorporate tendons. The capsule is often injured, leading to laxity, constriction and/or adhesion to surrounding structures. It is also important in rheumatic disease, including rheumatoid arthritis and osteoarthritis, crystal deposition disorders, bony spur formation and ankylosing spondylitis. This article concentrates on the specialised structures of the capsule--where capsular tissues attach to bone or form part of the articulation of the joint. It focuses on 2 joints: the rat knee and the proximal interphalangeal (PIP) joint of the human finger. The attachments to bone contain fibrocartilage, derived from the cartilage of the embryonic bone rudiment and rich in type II collagen and glycosaminoglycans. The attachment changes with age, when type II collagen spreads into the capsular ligament or tendon, or pathology--type II collagen is lost from PIP capsular attachments in rheumatoid arthritis. Parts of the capsule that are compressed during movement adapt by becoming fibrocartilaginous. Such regions accumulate cartilage-like glycosaminoglycans and may contain type II collagen, especially in aged material.(ABSTRACT TRUNCATED AT 250 WORDS)

Capsular tissues of the proximal interphalangeal joint: normal composition and effects of Dupuytren's disease and rheumatoid arthritis

Three fibrocartilages associated with the proximal interphalangeal joint are described--at the attachment of the central slip to bone, within the slip where it passes over the joint, and the volar plate. Material was obtained at surgery following trauma, Dupuytren's disease and rheumatoid arthritis. The fibrocartilages were structurally distinct and immunolabelled differently with monoclonal antibodies to extracellular matrix components. All fibrocartilages from normal and Dupuytren's fingers contained chondroitin and keratan sulphate. Type II collagen was present in all attachment zones, although there was little in rheumatoid fingers. It was also present in the dorsal hood of some normal fingers, but not in pathological specimens or the volar plate. The results show that the fibrocartilages are dynamic tissues whose composition varies according to function and use, and changes in disease.

Loss of the fibrocartilaginous lining of the intertubercular sulcus associated with rupture of the tendon of the long head of biceps brachii

Fibrocartilage lines the intertubercular sulcus of the humerus and protects both the bone and the tendon of the long head of biceps brachii where the tendon passes through the sulcus. It provides a smooth, resilient, lubricated gliding surface on the bone. The fibrocartilage is highly metachromatic and organised into distinct superficial and deep zones. In the superficial zone, the cells are small and the fibres run parallel to the articular surface. In the deep zone, the cells are large and rounded and the coarse bundles of fibres are interwoven. In 6 of the 26 dissecting room cadavers examined the tendons were completely ruptured. In these, the fibrocartilage was replaced by loose connective tissue that resembled the synovium of the tendon sheath. The results suggest that bone fibrocartilage exhibits dynamic behaviour in response to changes in its environment, in the same manner as tendon fibrocartilage.

Development and ageing of phenotypically distinct fibrocartilages associated with the rat Achilles tendon

We describe by routine histology and by immunohistochemistry three phenotypically and developmentally distinct fibrocartilages associated with the Achilles tendon of the rat. All the fibrocartilages develop after birth and show significant age-related changes in the composition of their extracellular matrix. Attachment-zone fibrocartilage occurs at the insertion of the tendon on the calcaneus. It derives from the cartilage rudiment of the calcaneus and from the region where the tendon merges with the perichondrium. The extracellular matrix contain type II collagen and chondroitin sulphate. Compressive tendon fibrocartilage occurs in the deep part of the tendon where it presses against the calcaneus, and is derived by metaplasia of tendon cells. The cells label strongly for the intermediate filament vimentin, and the extracellular matrix contains chondroitin and keratan sulphates, but type II collagen only in very old animals (> 2 years). Calcaneal fibrocartilage covered the posterior surface of the calcaneus where it was in contact with the Achilles tendon. It labelled intensely for type II collagen and contained chondroitin and keratan sulphates. The cells were rich in vimentin. This fibrocartilage was derived from the calcaneal perichondrium.

Cartilage and related tissues in the trunk and fins of teleosts

The structure and distribution of cartilage and related tissues in the dorsal fin, caudal fin and vertebrae of teleosts were studied in 11 species. With the exception of Zellknorpel, all the tissues previously described in teleost heads were present in the trunk and fins, although they were found in smaller quantities. The distribution of the supporting tissues indicates that they serve different functions. Hyaline cartilage was restricted to vertebral and fin bones undergoing endochondral ossification, fibro/cell-rich cartilage acted as an articular tissue, and hyaline-cell cartilage and its subtypes formed flexible and resilient supports in the caudal fin. Mucous connective tissue was packed as a space-filler around neurovascular bundles in fin rays, and chondroid bone was found beneath articular surfaces. The differences between cranial, and trunk and fin supporting tissues may reflect developmental as well as functional differences between the cranial and postcranial skeleton.

Chondrogenesis and myogenesis in micromass cultures of mesenchyme from mouse facial primordia

The face develops from small buds of tissue positioned around the primitive mouth. The chondrogenic and myogenic cell populations contained within these facial primordia in mouse embryos have been investigated in short-term micromass culture. Chondrogenesis occurred in frontonasal mass mesenchyme from E11-E13 embryos, in maxillary mesenchyme from E12.5 embryos and was absent in mandibular mesenchyme. Myogenesis was greatest in mandibular mesenchyme, moderate in maxillary mesenchyme and low in the frontonasal mass. When compared with chick embryos the mouse facial primordia have lower chondrogenic potential, which in the case of the frontonasal mass may be related to the relative outgrowth of the primordia in the two species. Chondrogenesis in the mouse mandibular mesenchyme may be affected by the presence of a large population of odontogenic mesenchyme. The behavior of myogenic cell populations is related to the pattern of the musculature of the face, as the mandible contains the most muscle, the maxilla some, and the frontonasal mass none. However, the presence of myoblasts in early mesenchyme of all primordia may indicate that, as with chick, facial primordia are initially seeded with muscle cells and that the size of the cell population is subsequently controlled according to the development of the musculature within the primordia.

The structure of the insertions of the tendons of biceps brachii, triceps and brachialis in elderly dissecting room cadavers

The terminal portions of the tendon of brachialis, and the distal tendons of biceps brachii and triceps, were compared by routine histology. All tendons came from elderly dissecting room cadavers. There were pronounced quantitative differences between the 3 tendons in (1) the thickness of the attachment-zone fibrocartilage, (2) the thickness of cortical calcified tissue, and (3) the percentage of bone to marrow. There was significantly more uncalcified fibrocartilage at the attachment of biceps than at the other sites, reflecting greater range of movement of the tendon at this site. The thickness of cortical calcified tissue and the percentage of bone to marrow were significantly greater at the attachment of brachialis than either biceps or triceps. The large quantities of bone at the attachment of brachialis may be related more to the importance of the coronoid process in buttressing the elbow joint than to any special requirement for large amounts of calcified tissue at the tendon attachment. Near its attachment zone, the biceps tendon splits into superficial and deep laminae that are distinct from the macroscopic subdivision of this tendon. It is suggested that the lamination may facilitate the movements of pronation and supination. In support of this, the deep portion of the superficial lamina contained fibrocartilage where it rubbed against the attachment-zone of the deep lamina. In one body, the fibrocartilage of the biceps attachment-zone was subject to degenerative changes, including cell clumping and matrix fissuring.

Development of functionally distinct fibrocartilages at two sites in the quadriceps tendon of the rat: the suprapatella and the attachment to the patella

This paper describes the post-natal development of two fibrocartilages in the quadriceps tendon of the rat. The compression-resisting fibrocartilage of the suprapatella was derived from a cell population present in neonates and positioned on the deep surface of the tendon of vastus intermedius. The cells secreted a metachromatic, coarsely fibrous extracellular matrix that was rich in chondroitin sulphate but lacked keratan sulphate or type II collagen. The cells themselves accumulated large quantities of vimentin. The adult form of the suprapatella was attained 8 weeks after birth. The fibrocartilage of the attachment zone of the quadriceps tendon to the patella was formed in a different manner. In animals up to 4 weeks of age, the quadriceps tendon inserted directly into the cartilage model of the patella. When later this was resorbed, and replaced by bone, the cartilage at the attachment zone remained, along with that of the articular surface of the patella. Attachment-zone fibrocartilage was therefore rich in type II collagen, unlike that of the suprapatella. Thus two functionally different fibrocartilages have been shown to have different origins, even when separated by only a short distance within the same tendon. The compositional differences between attachment-zone and compressive region fibrocartilages are also due to their different origins.

Age-related changes in tendon fibrocartilage

Age-related changes are reported in the rat suprapatella: a fibrocartilage that resists compression of the quadriceps tendon against the femur in the flexed knee. The suprapatella was studied by histology, immunohistochemistry and electron microscopy in rats aged 11-14 weeks, and 12, 15, 18 and 24 months. Type II collagen was absent in the matrix of animals 11-14 weeks old, but appeared by 12 months; immunolabelling increased further with age. Chondroitin sulphate was present in all animals, although immunolabelling decreased with age. Keratan sulphate appeared transiently at 12 months. The structure of the suprapatellar cells also changed with age. In some respects the suprapatellar cells of aged rats are similar to those of younger animals; they contain relatively few organelles and their cytoplasm is packed with intermediate filaments which contain vimentin. However, lipid droplets and glycogen are more prominent in older animals, and the nuclei become elaborately infolded and multilobed. Type II collagen was present in rats aged 11-14 weeks in fibrocartilage of the attachment of quadriceps femoris to the patella, but with increasing age it spread proximally, further into the tendon.