Background: In many pre-clinical studies of cartilage tissue, it has been generally assumed that the major difference of the tissue between the species is the tissue thickness, which is related to the size of the animal itself. At present, there appear to be lack of studies demonstrating the relationship between chondrocyte densities, protein content, gene expressions and cartilage thickness in the various animal models that are commonly used. The present study was conducted to determine whether or not chondrocyte density, proteoglycan/protein content and selective chondrocyte gene expression are merely related to the cartilage thickness (thus animal size), and not the intrinsic nature of the species being investigated. Mature animals (rabbit, rats and goats) were sacrificed for their hind knee cartilages. Image analyses were performed on five consecutive histological sections, sampled from three pre-defined locations at the lateral and medial femoral condyles. Cartilage thickness, chondrocyte density, Glycosaminoglycan (GAGs)/protein content and gene expression levels for collagen II and SOX-9 were compared across the groups. Correlation analysis was done between cartilage thickness and the other variables.
Results: The mean cartilage thickness of rats, rabbits and goats were 166.5 ± 10.9, 356.2 ± 25.0 907.5 ± 114.6 μm, respectively. The mean cartilage cell densities were 3.3 ± 0.4×10(-3) for rats, 2.6 ± 0.3×10(-3) for rabbits and 1.3 ± 0.2×10(-3) cells/μm2 for goats. The mean μg GAG/mg protein content were 23.8 ± 8.6 in rats, 20.5 ± 5.3 in rabbits and 328.7 ± 64.5 in goats; collagen II gene expressions were increased by 0.5 ± 0.1 folds in rats; 0.6 ± 0.1 folds in rabbits, and 0.1 ± 0.1 folds in goats, whilst the fold increase of SOX-9 gene expression was 0.5 ± 0.1 in rats, 0.7 ± 0.1 in rabbits and 0.1 ± 0.0 in goats. Cartilage thickness correlated positively with animals' weight (R2 =0.9856, p = 0.001) and GAG/protein content (R2 =0.6163, p = <0.001). Whereas, it correlates negatively with cell density (R2 = 0.7981, p < 0.001) and cartilage gene expression levels (R2 = 0.6395, p < 0.001).
Conclusion: There are differences in the composition of the articular cartilage in diverse species, which are not directly dependent on the cartilage thickness of these animals but rather the unique characteristics of that species. Therefore, the species-specific nature of the cartilage tissue should be considered during any data interpretation.