Femurs and ulnae were dehydrated in graded alcohols, cleared in xylene, and embedded in MMA following standard protocols.39 (link) For femurs, thick-cut sections were taken ~3 mm proximal to the tibiofibular junction and manually ground down to ~30 μm. For ulnae, thick-cut sections were taken ~2 mm proximal and ~2 mm distal to the ulna midshaft and manually ground down to ~30 μm. A single unstained section from each bone was imaged digitally on a fluorescent microscope using filter sets that provided excitation and emission for the calcein and alizarin wavelengths. Digital images were imported into Image-Pro Express (Media Cybernetics, Inc.), and the following histomorphometric measurements were recorded for the periosteal surface: total perimeter, single label perimeter (sL.Pm), double label area (dL.Ar) and perimeter (dL.Pm), total bone, and marrow area. The following results were calculated: mineral apposition rate (MAR = dL.Ar/dL.Pm/11 d), mineralizing surface (MS/BS = (0.5 × sL.Pm + dL.Pm) / total perimeter × 100), and bone formation rate (BFR/BS = MAR × MS / BS × 3.65). Relative formation parameters were calculated for each mouse by subtracting the nonloaded (left ulna) values form the loaded (right ulna) values. Histology sections, staining, and analyses were conducted by the Histology Core Facilities within the Indiana Center for Musculoskeletal Health.