Dramatic increase in cortical thickness induced by femoral marrow ablation followed by a 3-month treatment with PTH in rats

Abstract

We previously reported that following mechanical ablation of the marrow from the midshaft of rat femurs, there is a rapid and abundant but transient growth of bone, and this growth is enhanced and maintained over a 3-week period by the bone anabolic hormone parathyroid hormone (PTH). Here, we asked whether further treatment with PTH or bisphosphonates can extend the halflife of the new bone formed in lieu of marrow. We subjected the left femur of rats to mechanical marrow ablation and treated the animals 5 days a week with PTH for 3 weeks (or with vehicle as a control) to replace the marrow by bone. Some rats were euthanized and used as positive controls or treated with vehicle, PTH, or the bisphosphonate alendronate for a further 9 weeks. We subjected both femurs from each rat to soft X-ray, peripheral quantitative computed tomography (pQCT), micro-computed tomography (mCT), dynamic histomorphometry analysis, and biomechanical testing. We also determined the concentrations of serum osteocalcin to confirm the efficacy of PTH. Treatment with PTH for 3 months dramatically enhanced endosteal and periosteal bone formation, leading to a 30% increase in cortical thickness. In contrast, alendronate protected the bone that had formed in the femoral marrow cavity after marrow ablation and 3 weeks of treatment with PTH but failed to promote endosteal bone growth or to improve the biomechanical properties of ablated femurs. We further asked whether calcium-phosphate cements could potentiate the formation of bone after marrow ablation. Marrow cavities from ablated femurs were filled with one of two calcium-phosphate cements, and rats were treated with PTH or PBS for 84 days. Both cements helped to protect the new bone formed after ablation. To some extent, they promoted the formation of bone after ablation, even in the absence of any anabolic hormone. Our data therefore expand the role of PTH in bone engineering and open new avenues of investigation to the field of regenerative medicine and tissue engineering. Local bone marrow aspiration in conjunction with an anabolic agent, a bisphosphonate, or a calcium-phosphate cement might provide a new platform for rapid preferential site-directed bone growth in areas of high bone loss. ß

Additive Effects of Mechanical Marrow Ablation and PTH Treatment on de Novo Bone Formation in Mature Adult Rats

“…When subjected to x-ray analysis, ablated femurs from rats treated with PBS revealed mildly enhanced radiopacity compared with control rats, similar to previous results in younger animals [2,3]. In contrast, ablated femurs from rats treated with PTH revealed a highly intense radiopacity ( Figure 1).…”

“…This suggested that the new bone formed had, for the most part, been resorbed by osteoclasts. In contrast, the new bone formed in ablated femurs from rats treated with PTH was extremely abundant, to an extent that appeared similar to that reported previously in younger rats (Figure 4) [2,3]. Higher magnification analysis of that new bone revealed its lining by active osteoblasts, and the presence of small osteocytic lacunae, indicating that the new bone was being actively formed ( Figure 5).…”

“…This new bone forms without a cartilage enlagen and is therefore intramembranous. We next reported that extending the daily PTH treatment for three months after marrow ablation leads to a dramatic increase in cortical thickness that persists even after the intramedullary bone is resorbed [3]. Together, these findings supported the possible use of this method for rapid, site-specific bone growth, with specific application for supporting bone grown in anatomical areas particularly prone to osteoporosis-induced fracture in high-risk individuals.…”

“…Bone marrow ablation was performed as previously described [2,3]. In brief, rats were anesthetized with a combination of ketamine (50 mg/kg) and xylazine (10 mg/kg) intraperitoneally.…”

Cutting Edge: Parathyroid Hormone Facilitates Macrophage Efferocytosis in Bone Marrow via Proresolving Mediators Resolvin D1 and Resolvin D2

“…During wound healing macrophages phagocytose debris and apoptotic cells. PTH has robust anabolic actions during traumatic injuries associated with increased apoptosis such as bone marrow ablation (20). Hence, PTH could act in the marrow to recruit macrophages and promote the release of specialized pro-resolving mediators that facilitate efferocytosis.…”