Preferential orientation of collagen/biological apatite in growing rat ulna under an artificial loading condition

Mechanical loading plays a key role in altering macroscopic and microscopic bone structure through functional adaptation; however, the anisotropic micro-organization of collagen and biological apatite (BAp) in adaptively created bone tissue is not well understood in spite of its importance in the mechanical function of bone. In this study, we artificially applied axial compressive loading (15N at 2 Hz) to a growing rat ulna 10 min/day for 3 weeks to induce new bone under increased mechanical stimulus. Artificial loading induced marked increases in the structural properties of the loaded ulna; the cortical bone area of the mid-shaft was 43.3% larger in the loaded ulna than the contralateral control ulna. The newly formed bone was located mainly on the medial periosteal surface of the ulnar mid-shaft, which experienced the highest compressive strain. The present study firstly clarified that new bone induced by an artificial load showed preferential orientation of collagen and BAp c-axis along the ulnar long axis, which is similar to the pre-existing bone, although the degree of orientation and bone mineral density (BMD) were still impaired after loading for 3 weeks. This anisotropic organization of collagen and BAp crystals corresponded to that of osteocytes, implying that osteocytes are involved in the formation of anisotropic bone micro-organization which is important aspect of bone mechanical function regarding material properties.