P2. Progenitor cells from iliac crest autograft survive transplantation and contribute to spinal fusion

Background Context

Posterior lumbar fusion (PLF) is frequently augmented with iliac crest autograft. Although autograft is the gold standard supplement to promote fusion, the molecular mechanism by which it induces bone formation has not been well elucidated. Specifically, it is unknown if progenitor cells within the autograft contribute to the biology of the fusion mass.

Purpose

Using an innovative lineage tracing technology in a murine model of PLF, we tested the hypothesis that progenitor cells from iliac crest autograft survive transplantation and contribute to spinal fusion.

Study Design/Setting

In vivo study.

Patient Sample

Animals: 8-week-old male or female lineage tracing animals, Sox9-CreERT; Ai14(progenitor cells), Collagen1a1-CreERT;Ai9(osteoblast), and Aggrecan-CreERT;Ai9(chondrocytes) were examined.

Outcome Measures

Posterolateral bone formation was quantified by microcomputed tomography, from which 2D and 3D reconstruction were produced. To evaluate the presence of tdTomato+ cells within the area of spinal fusion that survive from the donor graft, IVIS imaging and detailed frozen histologic assessment were performed. Frozen sections were likewise stained with hematoxylin and eosin(H&E) and Safranin O and evaluated under light microscopy.

Methods

Autograft harvested from the iliac crest of Sox9-CreERT;tdTomato or Aggrecan-Cre ERT;Ai9 or Collagen1a1 Cre ERT were transplanted into the posterolateral gutters between L3 and L5 of non-Cre containing littermates. Following transplantation, recipient mice received tamoxifen (100mg/kg, i.p. twice weekly) to induce recombination and expression of tdTomato(Cy3) until sacrifice. In order to provide comparative analysis, we conducted autograft transplantation harvested from wild-type mouse into each Cre mouse.

Results

Mice receiving bone autograft from either transgenic lineage were found to have markedly greater new bone formation and bony bridging of adjacent pedicles compared to sham controls by 6 weeks post implantation. When evaluated at 2 weeks post implantation, histologic analysis demonstrated safranin O staining within the fusion mass, indicating the presence of chondrocyte. By 6 weeks post implantation, positive tdTomato signal from every lineage reporter (SOX), Aggrecan, and COL1) were visible within the fusion mass and found to localize newly formed bone.

Conclusions

Using innovative lineage tracing technology, these data indicate that progenitor cells (SOX9) from iliac crest autograft not only survive transplantation but become osteoblasts (COL1) via within the fusion mass. In addition, the presence of a chondrocyte intermediate suggests that the fusion mass is formed through endochondral ossification. Moreover, our data suggests that the periosteum plays a crucial role as a primary source of stem cells.

FDA Device/Drug Status

This abstract does not discuss or include any applicable devices or drugs.