The Effect of Training Exercise on Cartilage Zonal Composition and Structure at Two Differently Loaded Sites in the Juvenile Equine Metacarpophalangeal Joint

Background: The aim of this study was to investigate the depth related spatial distribution of biochemical and structural components in articular cartilage at different topographical sites experiencing different loading within the juvenile equine metacarpophalangeal joint in two groups of foals subjected to different exercise conditions. Methods: Sixteen Warmblood foals were included in the study and were randomly assigned into 1 of 2 exercise groups (box rest and box rest plus training) during the first 5 months of life and subsequently euthanized. Osteochondral plugs were harvested from two sites which have previously been shown to experience different load levels and sectioned into 100 μm thick slices. Cartilage slices were analysed for glycosaminoglycan, DNA, and total collagen content, as well as content of hydroxylysine residues (HYL) in collagen and the collagen cross-link Hydroxylysylpyridinoline (HP). Collagen fibril architecture (Orientation Angle and Parallelism Index) was determined using quantitative polarized light microscopy. Results: The constantly loaded site 2 exhibited higher glycosaminoglycan (GAG) and HP content with HYL higher in the superficial and middle layers of the trained group, while the intermittently peak loaded site 1 showed greater collagen accumulation in deeper layers but lower cross-link density. Structural analyses (Oa and PI) confirmed that collagen fibril orientation and alignment adapt regionally, with training enhancing fibril organization in superficial zones. Conclusion: This study illustrates that mechanical loading is a key driver of functional adaptation, resulting in distinct zonal and topographical differences in the biochemical and structural composition of equine cartilage at 5 months.