The organization of collagen in cryofractured rabbit articular cartilage: a scanning electron microscopic study

Ultrastructure of adult human articular cartilage matrix after cryotechnical processing

The ultrastructure of adult human articular cartilage matrix is reexamined in tissue processed according to recently improved cryotechniques [Studer et al. (1995) J. Microsc., 179:321-332]. In truely vitrified tissue, a network of fine cross-banded filaments (10-15 nm in diameter) with a periodicity characteristic of collagen fibrils is seen throughout the extracellular substance, even within the pericellular compartment, which has hitherto been deemed free of such components. Proteoglycans fill the interstices between these entities as a homogeneously distributed granular mass; they do not manifest a morphologically identifiable reticular structure. Longitudinally sectioned collagen fibrils exhibit variations in thickness and kinking; they tend to align with their periodic banding in register and are frequently seen to split or fuse along their longitudinal course. The tendency of fibrils to form bundles is greater in deeper zones than in more superficial ones. A duality in the orientation of fibrils and fibril bundles is observed within the interterritorial matrix compartment: superimposed upon the well-characterized arcade-like structure formed by one subpopulation is another, more randomly arranged one. The classical concepts of matrix organization thus need to be modified and refined to encompass these findings.

Molecular orientations in an extruded collagenous composite, the marginal rib of the egg capsule of the dogfish Scyliorhinus canicula ; a novel lyotropic liquid crystalline arrangement and its origin in the spinnerets

The egg case of the dogfish, Scyliorhinus canicula is a composite material largely constructed from collagen fibrils. It is formed as a drawn extrusion from transverse rows of spinnerets within the lining of the nidamental gland. In the L 2 layer, which forms over 90% of the thickness of the marginal rib, each spinneret extrudes a flattened ribbon which runs the length of the rib and measures approximately 150 x 8.5 μm in cross section. The structure of these ribbons and the orientation of collagen molecules and fibrils within them has been investigated in a correlative study using: low angle x-ray diffraction; bright field microscopy of peeled preparations; transmission (TEM) and scanning (SEM) electron microscopy; confocal and quantitative polarizing microscopy. The way in which the molecular orientations are defined within the spinneret has been followed by SEM of fixed material from actively secreting nidamental glands. The extruded ribbon showed a predominantly biaxial fibril orientation in low angle x-ray diffraction patterns recorded with the beam passing horizontally through the marginal rib. This x-ray pattern is derived from a remarkably regular parabolic arrangement of fibrils superficially resembling that seen in biological twisted nematic liquid crystals. However, evidence is presented here that the arrangement in the marginal rib is novel, apparently arising from authentically curved fibres showing a splayed- or bent- rather than twisted-nematic construction. Evidence is also presented that the spinnerets are able to control molecular orientations in a nematic liquid crystal to produce this and the other arrangements seen in the egg case.

Deformation of loaded articular cartilage prepared for scanning electron microscopy with rapid freezing and freeze-substitution fixation

To investigate the effect of joint loading on collagen fibers in articular cartilage, 45 knees of adult rabbits were examined by scanning electron microscopy. The knees were loaded at the patella with a simulated "quadriceps force" of 0.5-4 times body weight for 0.5 or 25 minutes, plunge-frozen, and fixed by freeze-substitution with aldehydes. Six knees were loaded for 3 hours and then fixed conventionally. Fixed tibial plateaus were examined and then freeze-fractured through the area of tibiofemoral contact, dried, coated, and examined by scanning electron microscopy to assess the overall deformation of the tibial articular surface and matrix collagen fibers. With tissue prepared by conventional fixation used as a standard, the quality of fixation was graded by light and transmission electron microscopy of patellar cartilage taken from half of the freeze-fixed knees. In loaded specimens, an indentation was present where the femur contacted the tibial plateau. The diameter and apparent depth of the dent were proportional to the magnitude and duration of the load; no dent was seen in the controls. The thickness of the cartilage at the center of the indentation was reduced 15-80%. Meniscectomy always produced larger deformations in otherwise equivalent conditions. Icecrystal damage to cells was evident by transmission electron microscopy and scanning electron microscopy, but at magnifications as high as x 30,000 the collagen fibrils prepared by freeze-substitution and conventional aqueous methods were identical. In loaded regions, the collagen matrix of the tibial cartilage was deformed in two ways: (a) radial collagen fibers exhibited a periodic crimp, and (b) in regions where an indentation was created by the femoral condyle, the radial fibers were bent, in effect creating a tangential zone where none had existed before. The radial fibers apparently are loaded axially and buckle under normal loads.

Attachment of articular cartilage chondrocytes to the tissue form of type VI collagen

Type VI collagen is composed of a short triple helix rich in RGD sequences with globular domains at each extremity of the helix. Disulfide-bonded tetramers of the monomeric molecule associate non-covalently to form networks of microfibrils in connective tissues, including cartilage. The disulfide-bonded tetramer can be extracted with 6 M guanidine HCl and purified without pepsin digestion and is referred to here as the tissue form of type VI collagen. Type VI collagen in mature articular cartilage appears to be concentrated pericellularly. We undertook a systematic investigation using solid phase assays to establish the nature of the attachment of bovine articular cartilage chondrocytes to the intact, tissue form of bovine type VI collagen. The tissue form of type VI collagen was extracted from bovine meniscus cartilage with 6 M guanidine HCl and purified by polyethylene glycol precipitation. When equal molar quantities were coated on microwells, the tissue form of type VI collagen attached more cells than the pepsin-digested form of the molecule that lacked the globular domains. The attachment to the intact, tissue form was dose-dependent and saturable and was not inhibited by heparin or type II collagen. A linear GRGDSP peptide failed to inhibit attachment of the chondrocytes to the intact, tissue or pepsin-digested forms of type VI collagen, but totally inhibited the interaction when the intact molecule was reduced and alkylated. In contrast, a cyclic C*GRGDSPC* peptide inhibited attachment to the tissue form of type VI collagen, but not to fibronectin. The attachment had a metal ion dependence that could be satisfied by MnCl2, slightly less by MgCl2, but not at all by CaCl2. A direct interaction between the tissue form of type VI collagen and a chondrocyte cell surface receptor or receptors is a structural feature of the pericellular matrix in cartilage.

Chondrocyte deformation and local tissue strain in articular cartilage: a confocal microscopy study

It is well accepted that mechanical forces can modulate the metabolic activity of chondrocytes, although the specific mechanisms of mechanical signal transduction in articular cartilage are still unknown. One proposed pathway through which chondrocytes may perceive changes in their mechanical environment is directly through cellular deformation. An important step toward understanding the role of chondrocyte deformation in signal transduction is to determine the changes in the shape and volume of chondrocytes during applied compression of the tissue. Recently, a technique was developed for quantitative morphometry of viable chondrocytes within the extracellular matrix using three-dimensional confocal scanning laser microscopy. In the present study, this method was used to quantify changes in chondrocyte morphology and local tissue deformation in the surface, middle, and deep zones in explants of canine articular cartilage subjected to physiological levels of matrix deformation. The results indicated that at 15% surface-to-surface equilibrium strain in the tissue, a similar magnitude of local tissue strain occurs in the middle and deep zones. In the surface zone, local strains of 19% were observed, indicating that the compressive stiffness of the surface zone is significantly less than that of the middle and deep zones. With this degree of tissue deformation, significant decreases in cellular height of 26, 19, and 20% and in cell volume of 22, 16, and 17% were observed in the surface, middle, and deep zones, respectively. The deformation of chondrocytes in the surface zone was anisotropic, with significant lateral expansion occurring in the direction perpendicular to the local split-line pattern. When compression was removed, there was complete recovery of cellular morphology in all cases. These observations support the hypothesis that deformation of chondrocytes or a change in their volume may occur during in vivo joint loading and may have a role in the mechanical signal transduction pathway of articular cartilage.

Mechanical and biochemical changes in the superficial zone of articular cartilage in canine experimental osteoarthritis

The changes in the tensile mechanical properties and biochemical composition of the superficial zone of articular cartilage were examined in a canine model of early osteoarthritis generated by transection of the anterior cruciate ligament. Sixteen weeks following ligament transection, the tensile stiffness of the articular cartilage was decreased by 44% and the ion-induced stress relaxation of the tissue was increased by 57% compared with the contralateral control. Biochemical analyses indicated that the water content of the experimental tissue was increased by 13%, which was reflected as an apparent 37% decrease in the proteoglycan content and a 36% decrease in the collagen content (expressed per wet weight). The hydroxypyridinium crosslink density was decreased in the experimental tissue by 11%. A significant negative correlation was found between the ion-induced stress relaxation and the hydroxypyridinium crosslink density in both control tissue (R = -0.56) and experimental tissue (R = -0.70). No correlation was noted between the tensile stiffness and the biochemical composition of the tissue. These results suggest that, in the superficial zone of articular cartilage, the structure of the tissue may play a more important role than the composition in the determination of its mechanical properties. A major event observed in the model of early osteoarthritis appears to be the disruption and remodeling of the collagen network in the superficial zone of the articular cartilage.

Effects of temperature, concentration and articular surface removal on transient solute diffusion in articular cartilage

The diffusion of glucose, inulin and dextran into adult bovine articular cartilage was studied as a function of temperature, solute concentration and articular surface integrity. One-dimensional, transient solute diffusion experiments were performed for 5, 15 and 60 min. The diffusion and interface partition coefficients increased with increasing temperature, but exhibited no concentration dependency when the solute concentration was increased 100-fold. Relative to intact tissue, removal of the uppermost articular surface resulted in decreased solute concentrations within the tissue for all solutes and time periods tested.

Stereologic studies on collagen in bovine articular cartilage

In an ultrastructural stereologic study on bovine articular cartilage we found that collagen volume density increased with increasing distance from the joint surface and from the chondrocyte. These results not only corroborate previous biomechanical data of a vertical stiffness gradient, but they also suggest that the mechanical forces are unevenly distributed horizontally. On the other hand, although mean collagen fibril diameter showed large differences between the interterritorial compartments of the three zones, there was a population of slender fibrils in all zones and compartments. Since the coarser fibrils provide the high tensile strength (Nimni 1988), the role of the slender fibrils may be to enhance the deformability of the tissue. Moreover, in spite of substantial differences in mean fibril diameter, collagen surface densities were in the same order of magnitude in the territorial and interterritorial compartments, and only slightly lower in the pericellular compartments. The surface parameter may be important for specific molecular interactions. The collagen fibrils have different polarity, i.e. the direction of the fibrils appears to be parallel and antiparallel, about 50% running in each direction. This, together with the very high length/diameter ratio (Clark 1985), may indicate that each fibril is assembled by the concerted action of many cells. The characteristic properties of articular cartilage depend on interactions between its macromolecular components, and the present quantitative data form a basis for discussions on the specificity and regulation of such interactions.

Cartilage and diarthrodial joints as paradigms for hierarchical materials and structures

The anatomic forms of diarthrodial joints are important structural features which provide and limit the motions required for the joint. Typically, the length scale of topographic variation of anatomic forms ranges from 0.5 to 15 cm. Articular cartilage is the thin layer of hydrated soft tissue (0.5-5.0 mm thick) covering the articulating bony ends in diarthrodial joints. This tissue has a set of unique mechanical and physicochemical properties which are responsible for its load-carrying capabilities and near-frictionless qualities. The mechanical properties of articular cartilage are determined at the tissue-scale level and these properties depend on the composition of the tissue, mainly collagen and proteoglycan, and their molecular and ultrastructural organization (ultra-scale: 10(-8)-10(-6) m). Because proteoglycans possess a high density of fixed negative charges, articular cartilage exhibits a significant Donnan osmotic pressure effect. This physicochemically derived osmotic pressure is an important component of the total swelling pressure; the other component of the total swelling pressure stems from the charge-to-charge repulsive force exerted by the closely spaced (1-1.5 nm) negative charge groups along the proteoglycan molecules. Thus these interactions take place at a nano-scale level: 10(-10)-10(-9) m. Finally, cartilage biochemistry and organization are maintained by the chondrocytes which exist at a micro-scale level (10(-7)-10(-6) m). Significant mechanoelectrochemical transduction occurs within the extracellular matrix at the micro-scale level which affects and modulates cellular anabolic and catabolic activities. At present, the exact details of these transduction mechanisms are unknown. In this review, we present a summary of the hierarchical features for articular cartilage and diarthrodial joints and tables of known material properties for cartilage. Also we summarize how the multi-scale interactions in articular cartilage provide for its unique material properties and tribological characteristics.

Cell patterns in the surface of rabbit articular cartilage revealed by the backscatter mode of scanning electron microscopy

To study the distribution of cells in the surface layer of articular cartilage, rabbit hip and knee specimens were stained with silver and studied by scanning electron microscopy (SEM). The cartilage was treated en bloc using the Gomori methenamine silver technique, which stains the nuclei of exposed cells with reduced silver. The intact surface was then studied with a binocular microscope and SEM in the backscatter mode Only those cells within 30 microns of the surface stained, permitting that population to be imaged selectively. Depressions in the surface were related to groups of cells in clusters or rows bounded by collagen fibers. This study demonstrates the effectiveness of backscatter imaging in the study of chondrocytes. The relationship between surface contours and underlying cells is more complex than previously described.

Variation of collagen fiber alignment in a joint surface: a scanning electron microscope study of the tibial plateau in dog, rabbit, and man

To determine if articular cartilage collagen fiber organization differs with location on the tibial plateau, specimens from dogs, humans, and rabbits were studied by scanning electron microscopy. Joint surfaces were fixed, dehydrated, and fractured radially so that the periphery could be compared with the center on single specimens. Generally, fibers were more tightly packed in the lateral side than in the medial and the periphery as compared with the center, where the cartilage was consistently thicker and the radial zone was dominant and composed of straight vertical fibers. In the periphery, the tangential and transitional zones were better developed and contributed up to 50% of the cartilage depth in comparison to only 5% centrally. The soft, dull, malacic appearance of the center results from lack of a true surface layer of tangential collagen fibers.

Silver staining of collagen fibrils in cartilage

Direct visualization of individual collagen fibrils by light microscopy in human cartilage was achieved by applying a periodic acid-silver methenamine stain on plastic sections. Collagen fibrils, 100 nm in diameter or thicker, were delineated individually by light microscopy and were easily traced for a length beyond 100 microns. Thinner fibrils not readily visible optically were identified if arranged in compact bundles as occurring in the superficial zone of articular cartilage.

The interrelation of fiber bundles in the anterior cruciate ligament

The anterior cruciate ligaments (ACL) of dogs, humans, and rabbits were studied by light and scanning electron microscopy after fixation in situ. In all species, the ACL was composed of multiple 20 microns-wide collagen fiber bundles separated by columns of cells in fibrous capsules. These bundles were in turn grouped into fascicles of varied size. The fascicles were surrounded by thin membranous sheets that ran through the ligament forming single or multiple layers between fascicles. Splaying of the ACL at insertion was created by increased volume in the cellular intervals. Bending of the fiber bundles occurred in this region--which corresponds to the fibrocartilaginous zone. We propose that the cell layers accommodate compressive forces and the membranes allow slipping among fascicles without compromising blood supply.

The effect of shear fatigue on bovine articular cartilage

The objective of this study was to investigate the effects of mechanical fatigue in the form of cyclic shear strain on articular cartilage. Three millimeter diameter full-thickness plugs were cored from the lateral aspect of bovine tibial plateaus. Sinusoidal shear strains of +/- 5, +/- 10, and +/- 15% were applied to the specimens at 100 Hz for 3 h (a total of 108 x 10(4) cycles). The mechanical shear properties of the tissue (loss and storage moduli) were determined as a function of the number of applied strain cycles. A rapid, irreversible decrease of approximately 35% of initial modulus was found to occur in both loss and storage modulus during application of the first 90,000 cycles. Further decay in the moduli was found to occur from 90 x 10(3) to 108 x 10(4) cycles, but was of considerably smaller magnitude than the initial decrease. The moduli remained relatively constant beyond application of 108 x 10(4) cycles. No consistent change in proteoglycan content was found to be associated with the fatigue process when comparing tested specimens with fresh, untested tissue, and with experimental controls. In addition, no structural defects in the mechanically altered tissue were revealed by scanning electron microscopy.

Quantitation of structural features characterizing weight- and less-weight-bearing regions in articular cartilage: a stereological analysis of medial femoral condyles in young adult rabbits

The structural organization of articular cartilage from the medial femoral condyle of young adult rabbits has been examined after processing according to an improved fixation procedure. By using recently developed stereological methods, a quantitative analysis of chondrocyte number, surface area, volume, and matrix volume per cell was carried out in the light microscope; at the electron microscopic level, quantities of cytoplasmic components within chondrocytes (including organelles) were estimated. These measurements were made for each of the four zones from the (articular cartilage) surface down to the tidemark, and the results (for each zone) were compared between weight- and less-weight-bearing regions. In general, articular cartilage revealed considerable heterogeneity in structure throughout its depth. The number of cells per unit volume is maximal beneath the surface and decreases toward the tidemark. The size of chondrocytes, and the mean matrix volume surrounding each, increases from the surface toward the deeper zones. Comparison between weight- and less-weight-bearing regions reveals striking differences. The numerical volume density of cells in the superficial zone of regions bearing high physiological load is less than half of that in less-weight-bearing regions, chondrocyte death being principally responsible for this reduction. A comparison between the midzones of weight- and less-weight-bearing areas reveals that the former is characterized by a decrease in cell density and an approximately threefold increase in cell size in relation to the latter. The increase in cell volume is attributable principally to an accumulation of intermediate filaments and glycogen particles, and represents an adaptation to increased functional requirements. Near the tidemark, numerical volume densities of chondrocytes in both weight- and less-weight-bearing locations are similar, but the larger cell size in the former still persists.

Localization of type IX collagen in chondrons isolated from porcine articular cartilage and rat chondrosarcoma

Chondrocytes, each with their pericellular matrix bounded by a fibrous capsule, can be extracted singly or in groups from both mature pig articular cartilage and chondrosarcoma tissue. These structures, termed chondrons, are thought to anchor the chondrocytes in the matrix and protect them from the compressive forces experienced when articular cartilage is under load. The capsule of these chondrons contains both type II and type IX collagens and is composed of fine fibrillar material, unlike the large banded fibres of type II collagen found in the rest of the matrix. This suggests a role for type IX collagen in regulating the diameter of type II fibres to produce the fine fibrillar structure of the chondron capsules.

Chondrons extracted from canine tibial cartilage: preliminary report on their isolation and structure

We report on the morphology and structure of single and multiple chondrons isolated from homogenized samples of fresh and fixed canine tibial cartilage. Phase contrast, Nomarski, and scanning electron microscopy observations show each chondron to be composed of a chondrocyte and its pericellular matrix enclosed within a "felt-like" pericellular capsule. The extraction of intact chondrons from cartilage homogenates confirms the structural validity of the chondron concept and emphasizes the intrinsic mechanical strength of the capsule. Frayed collagen fibers radiate from multiple chondron columns suggesting a shear-resistant, structural interrelationship between capsular components and type II collagen fibers. Future development of chondron extraction procedures could provide a unique model with which to study the structure, biochemistry, and function of articular cartilage chondrocytes and their pericellular microenvironment.

Chondrons in cartilage: ultrastructural analysis of the pericellular microenvironment in adult human articular cartilages

A combination of scanning and transmission electron microscopy was used to investigate the morphology and ultrastructure of normal human articular cartilage sampled from adult amputation specimens. This study confirms our previous observations on canine articular cartilage, which showed middle and deep layer chondrocytes surrounded by a pericellular matrix and enclosed within a pericellular capsule composed of filamentous and fine fibrillar materials. Pores in the "felt-like" organization of the capsular weave progressively decreased in size from the inner to the outer border of the capsule. Matrix vesicles were found embedded within the capsular weave and distributed throughout the territorial matrix. It is suggested that the chondrocyte, its pericellular matrix, and capsule together constitute the "chondron," a primary functional and metabolic unit of cartilage that acts hydrodynamically to protect the integrity of the chondrocyte and its pericellular microenvironment during compressive loading.