CW Archer

Defects of articular cartilage that do not penetrate to the subchondral bone fail to heal spontaneously. Defects that penetrate to the subchondral bone elicit an intrinsic repair response that yields a fibrocartilaginous repair tissue which is a poor substitute for hyaline articular cartilage. Many arthroscopic repair strategies employed utilise this intrinsic repair response to induce the formation of a repair tissue within the defect. The goal, however, is to produce a repair tissue that has the same functional and mechanical properties of hyaline articular cartilage. To this end, autologous osteochondral transfer can provide symptomatic relief. This technique involves the excision of healthy cartilage plugs from 'non-load bearing' regions of the joint for implantation into the defect. Cell based transplantation methods currently involve the transplantation of expanded autologous chondrocytes to the defects to form a repair tissue. This technique again involves the excision of healthy cartilage from the joint for expansion. Current research is exploring the potential use of mesenchymal stem cells as a source for tissue engineering, as well as the combination of cells with biodegradable scaffolds. Although current repair strategies improve joint function, further research is required to prevent future degeneration of repair tissue.

The three-dimensional architecture of bovine articular cartilage collagen and its relationship to split lines has been studied with scanning electron microscopy. In the middle and superficial zones, collagen was organised in a layered or leaf-like manner. The orientation was vertical in the intermediate zone, curving to become horizontal and parallel to the articular surface in the superficial zone. Each leaf consisted of a fine network of collagen fibrils. Adjacent leaves merged or were closely linked by bridging fibrils and were arranged according to the split-line pattern. The surface layer (lamina splendens) was morphologically distinct. Although ordered, the overall collagen structure was different in each plane (anisotropic) a property described in previous morphological and biophysical studies. As all components of the articular cartilage matrix interact closely, the three-dimensional organisation of collagen is important when considering cartilage function and the processes of cartilage growth, injury and repair.

Adult mouse articular cartilage (AC) has not been thoroughly described using high resolution imaging techniques, despite the fact that the availability of knockout mice with specific extracellular matrix (ECM) mutations have renewed interest in using the mouse as a model for a variety of different human conditions. With osteoarthritis affecting millions of people worldwide, investigations into the structure and, therefore, the ability of AC to act as a load-bearing tissue, are crucial for developing treatments and prevention techniques to limit the degree of severity in this condition. Cryofixation and formaldehyde fixation as well as chemical digestion of the uncalcified regions of AC were used in combination with bright field light, polarised light and scanning electron microscopy to image the structure of adult mouse AC. Chemical digestion of the tissue revealed unique insights into the structure of mouse AC and the high cellular density of the tissue. Tightly packed sheets of collagen fibrils formed the territorial matrix (TM) of the deep zone. These were observed closely surrounding the chondrons, after applying both chemical and cryofixation techniques. The interterritorial matrix (IM), in contrast, was more isotropically arranged. The results of the study have implications for the interpretation of biomechanical functionality of mouse AC with probable applications to other species.

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.