Osteoblast precursors and inflammatory cells arrive simultaneously to sites of a trabecular-bone injury

Abstract

Background and purpose — Fracture healing in the shaft is usually described as a sequence of events, starting with inflammation, which triggers mesenchymal tissue formation in successive steps. Most clinical fractures engage cancellous bone. We here describe fracture healing in cancellous bone, focusing on the timing of inflammatory and mesenchymal cell type appearance at the site of injuryMaterial and methods — Rats received a proximal tibial drill hole. A subgroup received clodronate-containing liposomes before or after surgery. The tibiae were analyzed with micro-CT and immunohistochemistry 1 to 7 days after injury.Results — Granulocytes (myeloperoxidase) appeared in moderate numbers within the hole at day 1 and then gradually disappeared. Macrophage expression (CD68) was seen on day 1, increased until day 3, and then decreased. Mesenchymal cells (vimentin) had already accumulated in the periphery of the hole on day 1. Mesenchymal cells dominated in the entire lesion on day 3, now producing extracellular matrix. A modest number of preosteoblasts (RUNX2) were seen on day 1 and peaked on day 4. Osteoid was seen on day 4 in the traumatized region, with a distinct border to the uninjured surrounding marrow. Clodronate liposomes given before the injury reduced the volume of bone formation at day 7, but no reduction in macrophage numbers could be detected.Interpretation — The typical sequence of events in shaft fractures was not seen. Mesenchymal cells appeared simultaneously with granulocyte and macrophage arrival. Clodronate liposomes, known to reduce macrophage numbers, seemed to be associated with the delineation of the volume of tissue to be replaced by bone. Most fracture healing studies in animal models concern cortical bone in shafts. However, most fractures in patients occur in cancellous bone in the metaphysis, such as the distal radius or in the vertebrae. A growing body of evidence suggests that there are important differences between the healing processes in cortical and cancellous bone.

Depletion of cytotoxic (CD8+) T cells impairs implant fixation in rat cancellous bone

“…A possible explanation to this might be the inherent biological differences between cortical and cancellous bone; cortical bone in the shaft region is relatively poor in local mesenchymal cells, compared to cancellous bone which is rich in mesenchymal cells . Furthermore, it seems that the mesenchymal cells (MSC's) in cancellous bone can respond to a trauma independently from inflammatory cells as they can initiate a healing response almost instantly . Cortical bone on the other hand rely on the recruiting function of the initial inflammation .…”

“…14 Furthermore, it seems that the mesenchymal cells (MSC's) in cancellous bone can respond to a trauma independently from inflammatory cells as they can initiate a healing response almost instantly. 15 Cortical bone on the other hand rely on the recruiting function of the initial inflammation. [16][17][18] Although cancellous bone does not seem to need the recruiting function of the initial inflammation, the depletion and hence the impaired healing in this experiment, still shows that the mesenchymal cells in the cancellous bone need the influence from inflammatory cells for proper tissue regeneration.…”

Different effects of Wnt/β‐catenin activation and parathyroid hormone on diaphyseal and metaphyseal in the early phase of femur bone healing of mice

“…Perforations disrupted the cortical, periosteal, and endocortical surfaces and extended into the marrow but did not disrupt the opposite cortical wall. Then, a defect of the same depth and diameter (1 mm) was made in the metaphysis of the femur . During recovery from anaesthesia, all mice were kept under heat lamps to maintain constant body temperature.…”

“…Quantitative morphometric data were based on the region of interest as follows: a diameter of 0.8 mm was taken at the defect centre with a depth of 0.4 mm. The percentage of bone volume to total volume (BV/TV) was calculated for the bone injury sites by micro‐CT …”

Different effects of Wnt/β-catenin activation and PTH activation in adult and aged male mice metaphyseal fracture healing

“…The X-ray tube potential was 45 kVp, and the voxel size was 10 μm 3 . The obtained image was analyzed and three-dimensionally reconstructed using EVS Beam software (1400 Hounsfield unit was set to bone) [27]. The area centered on the defect center with a diameter of 0.8 mm and a depth of 0.4 mm was set as the region of interest, and the percentage of bone volume to total volume (BV/TV) was calculated.…”