Mikko Lammi

Articular cartilage covers the ends of synovial joints and its function is to distribute loads between opposing bones and provide almost nonfrictional sliding properties for joints. It is comprised of cells (chondrocytes) embedded in an extracellular matrix (ECM) containing mainly collagen, proteoglycans (PGs) and interstitial fluid. Chondrocytes are responsible for the production of ECM macromolecules and thus, for maintaining the integrity of the ECM. Alterations in cell biomechanics might lead to changes in cell biosynthesis and viability.

     Degeneration of cartilage in osteoarthritis (OA) results in dysfunction and severe pain in joints. Although the onset and progression of OA have been studied extensively, it is still not known whether the structural changes related to cartilage degeneration occur concurrently with changes in chondrocyte biomechanics. More specifically, it is unclear how the integrity of the cartilage matrix, the alterations in the amount of cartilage macromolecules, and the changes related to cartilage structure and composition in early OA, are interconnected to changes in chondrocyte behavior. Also the detailed explanations behind the observed phenomena remain unknown.

     In this thesis, the effects of the structure and composition of the ECM and pericellular matrices (PCM) on chondrocyte biomechanics were studied. Three-dimensional fluorescence microscopy was used in order to analyze changes occurring in chondrocyte volume and morphology in bovine and rabbit cartilages before and after osmotic and mechanical loading. The effects of sample integrity, collagen degradation, and degeneration of cartilage in early OA on chondrocyte behavior were examined in detail. Collagen content, collagen orientation angle, and PG content were analyzed with Fourier transform infrared microspectroscopy, polarized light microscopy and digital densitometry, respectively. Computational modeling was utilized in order to identify the underlying mechanisms behind the observed phenomena.

     Chondrocyte volume and morphology following osmotic and mechanical loading were found to be dependent on the structure and integrity of the ECM and PCM. Cells in both intact and explant cartilages swelled in response to hypotonic loading, but volume recovery was only present in cartilage explants where sample integrity was compromised. Chondrocytes in enzymatically degraded cartilage, where the amount of collagen was reduced and the collagen network was severely fibrillated, were also found to recover back to the original, unchallenged volume. In a rabbit model of early OA, chondrocytes under mechanical loading increased in volume, whereas cell volumes in the contralateral joint cartilage decreased. Detailed structural and finite element analyses revealed that the decrease in the amount of fixed charge density (FCD) of the tissue and PCM, and the orientation and stiffness of the collagen fibers in the ECM explained the changed cell volumetric behavior in early experimentally-induced OA.

     To conclude, biomechanical responses of chondrocytes in osmotically and mechanically loaded cartilage were greatly affected by their mechanical environment. Tissue integrity, decrease in collagen content, and collagen network fibrillation altered cell volumetric behavior following osmotic loading. The loss of FCD in the PCM, and fibrillation of the superficial ECM in the very early stage of experimentally-induced OA explained the cell volume increase following mechanical loading of cartilage. Changes in cell biomechanics have earlier been found to be related to altered cell biosynthesis and impaired cell viability; all of which may lead to further changes in cartilage

structure and accelerate the cartilage degeneration. Thus, it is of great importance to understand chondrocyte behavior in situ/in vivo in order to devise strategies to slow down, stop or even reverse the development of OA.