Bone and Aletered Mechanical Loading: Basic Science Approaches
Deficits in the rate and amount of new bone formation significantly contribute to the bone loss that occurs during periods of skeletal unloading. This study was designed to investigate the mechanisms underlying these deficits with the specific goal of investigating 1) if the rate/quantity of bone formation can be artificially stimulated in unloaded bones, and 2) if declines in formation are caused by reductions in osteoblast function or an impaired differentiation of osteoblast progenitor cells. Skeletally-mature male mice (C3H) were divided into five groups, three hindlimb unloaded (HU) groups, one control (C) group, and baseline control (BC). One HU group (HU_AB) and the C group (C_AB) underwent marrow ablation (AB), a procedure that stimulates bone formation, on day 0; one HU group was unloaded for 10d prior to AB and then returned to HU (HU10_AB); the final HU group served as an unloaded control. Animals were sacrificed post-AB at 1–3d (to assess changes in gene expression) or 10–14d (to assess bone formation). Cancellous bone analyses included peripheral quantitative computed tomography to assess bone mineral density (BMD) and fluorescent histomorphometry to assess the rate (MAR) and quantity (MS/BS) of bone formation. After 10d, cancellous BMD was significantly increased in HU_AB (+28%), C_AB (+42%) and HU10_AB (+43%) groups vs both BC and HU groups. Preliminary results (n = 3–5/gp) show that at 14d post-AB, MAR was increased in C_AB (+60%) vs BC, confirming the stimulatory effect of AB on bone formation. HU_AB animals had non-significantly higher MAR vs BC and HU groups, while HU10_AB animals had a higher MAR vs BC (+52%) and vs HU (+113%). All three HU groups, independent of AB, tended to have lower MS/BS compared to BC. These preliminary results suggest that bone formation rate can be stimulated in unloaded bones, yet numbers of active osteoblasts may be limited. Ongoing analyses will assess osteoblast lined bone surfaces and osteoblast-specific gene expression patterns, both which will provide key insight into the mechanisms responsible for these altered responses to marrow ablation in unloaded bone.