Cartilage swelling is an early event in osteoarthritis (OA). However, the response of chondrocytes to increased tissue hydration is unknown. This work studied the volume and morphology of living in situ human chondrocytes as a function of cartilage degeneration.
The tibial plateaus from knee joints of 40 patients were obtained following above-knee amputations or knee arthroplasty, and degree of cartilage degeneration from 0 (non-eroded) to 3 (extensive fibrillations) was assessed using several criteria. In situ chondrocytes were labeled with fluorescent indicators (calcein for living cells, propidium iodide for dead cells) permitting the quantification of volume and visualisation of morphology of cells within the cartilage zones by confocal scanning laser microscopy (CLSM).
Chondrocyte volume within superficial and mid-zones, but not of deep zone cells, increased significantly (P<0.05 and P<0.02, respectively; one-way analysis of variance), with degree of cartilage hydration and degeneration. The volume increase ( approximately 90% for mid-zone chondrocytes, grade 3 cartilage) was greater than that which might occur following loss/excision of sub-chondral bone (<15% swelling). The CLSM technique utilised here revealed that approximately 40% of chondrocytes within all cartilage grades exhibited at least one cytoplasmic processes of <8 microm. The presence of these processes did not indicate a cell body of larger volume than cells without processes, and did not contribute to cell volume.
The volume of in situ chondrocytes within the superficial and mid-zones increased with cartilage degeneration. Cell swelling was greater than that expected from the increased hydration in OA, suggesting that an increase in chondrocyte volume might play a role in the changes to matrix metabolism occurring in OA.