Site-specific cell-tissue interactions in rabbit knee joint articular cartilage

Relationships between cartilage structure and superficial in situ chondrocyte deformations were investigated from 6 different knee joint locations (n=10 knees). Depth dependent cartilage structure and composition were quantified with microscopic/microspectroscopic methods. Medial tibial cartilages had the lowest superficial collagen content, highest collagen orientation angle, and highest proteoglycan content in the pericellular matrix relative to that in the extracellular matrix, coupled with the largest chondrocyte deformations. In contrast, femoral groove and lateral tibial cartilages had the highest superficial collagen contents, lowest collagen orientation angles, and low normalized proteoglycan contents in the pericellular matrix, coupled with the smallest chondrocyte deformations. To study cell-tissue interactions further, observations (n=57) from all locations were pooled and a multivariable linear regression was performed. Cell width deformations (R²=0.57) correlated with collagen orientation angle (standardized regression coefficient β=0.398) and collagen content (β=-0.402). Cell height deformations (R²=0.52) also correlated with collagen orientation (β=-0.248) and collagen content (β=0.455). Cell volume change upon cartilage compression (R²=0.41) correlated with collagen content (β=0.435) and proteoglycan content (β=0.279). In conclusion, higher collagen and proteoglycan contents combined with lower collagen orientation angle in the extracellular matrix were related to reductions in superficial chondrocyte deformations. Also, a steep gradient of proteoglycan content from the extracellular to the pericellular matrix was associated with increased cell deformation, particularly in the medial tibial plateau cartilage.