Fig 1.
Mechanical loading increases trabecular bone volume and thickness in proximal tibial bone in Bmpr1a cKO bone.
Trabecular architecture was measured in the following groups: Bmpr1a cKO mice undergoing exercise or no-exercise regimes compared to control mice also undergoing either an exercise or no exercise regime (n = 6–11 per group). (A) Representative 3D μCT images of the proximal metaphyseal regions of the tibia. (B) Measurement of bone volume per tissue volume (BV/TV). (C) Measurement of bone mineral density (BMD). (D) Measurement of trabecular thickness (TbTh). Mean±SEM, *, p<0.05, **, p<0.01, ***, p<0.001, ns, not significant.
Fig 2.
Mechanical loading decreased cortical porosity in Bmpr1a cKO bones.
(A) Representative 2D μCT images of the cortical compartment of the tibial diaphysis. Several small pores were seen in the non-exercised cKO bone. Bones from Bmpr1a cKO mice undergoing exercise or no-exercise regimes were compared to bones from control mice also undergoing either an exercise or no exercise regime. (B) Measurement of cortical porosity. (C) Measurement of cortical bone volume. Mean±SEM, *, p<0.05, ns, not significant.
Fig 3.
Mechanical loading increased trabecular bone area via decreased osteoclastogenesis in Bmpr1a cKO, whereas control bone was not affected.
(A) Masson Trichrome staining showed collagen (blue) in bone and marrow cells (red) in the distal metaphyseal regions of femoral bone. An increase in collagen area was seen in exercised cKO bone. Bar = 200 μm. (B) TRAP staining on sections of femora. Osteoclast numbers, osteoclast surface, and total bone surface were measured using ImageJ. Nuclei were stained with hematoxylin (blue). Yellow lines represent 200 μm boundaries from the growth plate in the distal metaphysis. Bar = 100 μm. (C) Measurement of the trabecular bone area per total area of femoral metaphysis (BA/TA). (D) Osteoclast number per total bone surface (OC/BS) (E) PCR results showing levels of Sost. (F) Real-time PCR results showing levels of osteoblast markers (Sp7, Runx2, Ocn). (G) Real-time PCR results showing levels of osteoclast markers (Trap, Mmp9, Rankl). mRNA was harvested from tibial bone after 6 weeks of exercise or at the equivalent time in no-exercise mice. Mean±SEM, *, p<0.05, **, p<0.01, ***, p<0.001, ns, not significant.
Fig 4.
Mechanical loading altered diameters of collagen fibrils in area-dependent manner.
Nonmineralized-bone collagen fibrils were randomly photographed at 46000-fold magnification using transmission electron microscopy (TEM), and diameters of collagen fibrils were measured in 4 groups (n≥3 samples, n≥500 collagen fibrils per group) Measurement of fibril diameters in cortical areas showed the same impact of exercise on the diameters in both groups.
Fig 5.
Mechanical loading restored toughness of cKO tibiae.
Mechanical properties of the tibiae in each of the 4 genotype/exercise groups were measured by 4-point-bending. (A-C) Tissue-level properties, (A) Stress (yield stress (open bar), ultimate stress (solid bar)); cKO bones had significantly lower ultimate stress (a1) than control bones. The control mice had decreased ultimate stress with exercise (b1). (B) Strain (yield strain (open bar), post-yield strain (solid bar)); cKO bones had significantly lower yield strain (a2) than controls. The control mice had decreased yield strain with exercise (b2). Post-yield strain in cKO was increased by exercise (c1). (C) Toughness (pre-yield toughness (open bar), post-yield toughness (solid bar)); cKO bones had significantly lower pre-yield toughness (a3). Exercise significantly increased post-yield toughness (c2) in cKO bones, but pre-yield toughness in control bones declined with exercise (b3). (D-F) Whole bone properties; similar effects of genotype and exercise as at the tissue-level were found. (D) Force (yield force (open bar), ultimate force (solid bar)). (E) Deformation (yield deformation (open bar), post yield deformation (solid bar)), (F) Work (pre-yield work (open bar), post-yield work (solid bar)). Nex, non-exercised, Exe, exercised. Annotations next to bars; a, comparisons between Control Nex and cKO Nex. b, comparisons between control Nex and control Exe, c, comparisons between cKO Nex and cKO Exe. Mean±SEM; *, p<0.05, **, p<0.01.