Replacement of Bone Marrow by Bone in Rat Femurs: The Bone Bioreactor

Abstract

During development and repair of bone, two distinct yet complementary mechanisms, intramembranous and endochondral, mediate new bone formation via osteoblasts. Because mechanical bone marrow ablation leads to the rapid and transient formation of new bone in the marrow cavity, we postulated that parathyroid hormone (PTH), which is a bone anabolic hormone, enhances the formation of new bone that forms after marrow ablation. We subjected the left femur of rats to mechanical marrow ablation, or sham operation, and injected the animals daily with PTH or vehicle for 1, 2, or 3 weeks in a first experiment, then with PTH, parathyroid hormone-related peptide (PTHrP), or vehicle for 3 weeks in a second experiment. We subjected both femurs from each rat to soft X-ray, peripheral quantitative computed tomography, computed tomography on a microscale, and histological analysis, and determined the concentration of serum osteocalcin. In addition, in the second experiment, we determined the serum concentration of calcium, tartrate-resistant acid phosphatase (TRAP), and receptor activator of NF-kappaB ligand (RANKL) at 3 weeks, and subjected femurs to biomechanical testing. Following treatment with PTH or PTHrP for 3 weeks, bone filled the marrow cavity of the shafts whose marrow had been ablated. PTH increased trabecular density in the right femur, but failed to induce bone formation in the medullary region of the right unoperated femoral shafts. The newly formed bone endowed left femoral shafts with improved biomechanical properties when compared to those of right femurs and left femurs from control, sham-operated, and vehicle-treated rats. PTHrP, like PTH, increased serum osteocalcin, but neither increased serum calcium, TRAP, or RANKL at 3 weeks. Our results reveal that the newly formed bone that follows marrow ablation is responsive to PTH, expand the role of PTH in bone, and might open new avenues of investigations to the field of regenerative medicine and tissue engineering. Local bone marrow removal in conjunction with pharmacologic intervention with an anabolic agent might provide a technique for rapid preferential site-directed bone growth in areas of high bone loss.

Endogenous parathyroid hormone–related protein compensates for the absence of parathyroid hormone in promoting bone accrual in vivo in a model of bone marrow ablation

“…Mechanical BMX provokes a predictable sequence of events, including the formation of a blood clot with inflammation and capillary invasion, mesenchymal cell migration, osteoblast proliferation, and formation of new cancellous medullary bone, followed by osteoclastic bone resorption and bone marrow reconstitution . Previous studies have employed this model to investigate the effect of exogenous PTH on the formation of new bone that forms after marrow ablation . Those results showed that the abundance of bone formed in response to PTH1‐34 in the marrow cavity after marrow ablation is far more than that formed in response to either the hormone alone without marrow ablation, or to marrow ablation alone .…”

Rapid cell culture and pre-clinical screening of a transforming growth factor-β (TGF-β) inhibitor for orthopaedics

“…New intramedullary bone was induced by femoral reaming as previously described in rats [28] and mice [29]. An incision was made on the right hind leg lateral to the knee and a 25G needle pushed through the cortex of the distal femur between the intercondyles and unilaterally up the medullary canal to loosen the marrow.…”

PTH and PTHrP signaling in osteoblasts

“…These data suggested that PTHrP is required for normal turnover of bone cells, and activation of apoptosis in mature osteoblasts could represent a potential mechanism whereby cells no longer forming matrix are cleared to provide access for new cells. Besides PTH’s clinical use for the treatment of osteoporosis, its potential for the application in fracture healing, tissue engineering and implant integration are being explored [65–68]. …”