Passive osmotic properties of in situ human articular chondrocytes within non‐degenerate and degenerate cartilage

Osteoarthritis is characterized by many factors, including proteoglycan loss, decreased collagen stiffness, and increased cartilage hydration. Chondrocyte swelling also occurs, and correlates with the degree of osteoarthritis, however, the cause is unknown but might be related to alterations to their passive osmotic properties. We have used two-photon confocal laser scanning microscopy to measure the passive osmotic characteristics of in situ chondrocytes within relatively non-degenerate and degenerate human tibial plateau cartilage, and in chondrocytes isolated from relatively non-degenerate cartilage. Explants with bone attached were taken from a total of 42 patients undergoing arthroplasty and graded macroscopically and microscopically into two groups, grade 0 + 1 and grade 2 + 3. There was a significant increase in cartilage hydration between these two groups (P < 0.05), however, there was no change when medium osmolarity was varied over approximately 0-480 mOsm. The passive osmotic behavior of in situ chondrocytes (at 4 degrees C) was identical over a range of culture medium osmolarities ( approximately 0-515 mOsm), however, the maximum swelling of cells within degenerate cartilage and isolated chondrocytes was greater compared to those in non-degenerate cartilage. The swelling in the majority of in situ chondrocytes was accounted for by the reduced interstitial osmolarity occurring with cartilage degeneration. There was, however, a small population of in situ chondrocytes whose volume was in excess (>/=2,500 microm(3)) of that predicted from the decreased interstitial osmotic pressure. These results show that for the majority of cells studied, the differences in passive chondrocyte volume between relatively non-degenerate, degenerate, and isolated cells were entirely accounted for by changes to the extracellular osmolarity (180-515 mOsm).