Weight loading young chicks inhibits bone elongation and promotes growth plate ossification and vascularization

The mechanical stimuli resulting from weight loading play an important role in mature bone remodeling. However, the effect of weight loading on the developmental process in young bones is less well understood. In this work, chicks were loaded with bags weighing 10% of their body weight during their rapid growth phase. The increased load reduced the length and diameter of the long bones. The average width of the bag-loaded group's growth plates was 75 +/- 4% that of the controls, and the plates showed increased mineralization. Northern blot analysis, in situ hybridization, and longitudinal cell counting of mechanically loaded growth plates showed narrowed expression zones of collagen types II and X compared with controls, with no differences between the relative proportions of those areas. An increase in osteopontin (OPN) expression with loading was most pronounced at the bone-cartilage interface. This extended expression overlapped with tartarate-resistant acid phosphatase staining and with the front of the mineralized matrix in the chondro-osseous junction. Moreover, weight loading enhanced the penetration of blood vessels into the growth plates and enhanced the gene expression of the matrix metalloproteinases MMP9 and MMP13 in those growth plates. On the basis of these results, we speculate that the mechanical strain on the chondrocytes in the growth plate causes overexpression of OPN, MMP9, and MMP13. The MMPs enable penetration of the blood vessels, which carry osteoclasts and osteoblasts. OPN recruits the osteoclasts to the cartilage-bone border, thus accelerating cartilage resorption in this zone and subsequent ossification which, in turn, contributes to the observed phenotype of narrower growth plate and shorter bones.