gms | German Medical Science

Objectives: Non-healing bone defects are still a clinical challenge that cell-based tissue engineering approaches aim to overcome. Adipose tissue-derived stromal cells (ASC) are a highly attractive cell source for treatment because they can be isolated non-invasively and in larger quantities than bone marrow-derived stromal cells (BMSC). However, unlike BMSC, ASC do not form ectopic bone together with β-TCP demonstrating their inferior osteogenic capacity. Nevertheless, this deficit may be irrelevant in an orthotopic bone environment rich in pro-osteogenic growth factors. Therefore, the objective of this study was to compare the bone healing capacity of human ASC and BMSC in combination with calcium phosphate nanoparticles in a calvarial defect model.

Methods: In vitro cytotoxicity of β-TCP nanoparticles (< 200 nm) was tested with BMSC and ASC by an MTT assay. To evaluate their potential for bone defect healing β-TCP nanoparticles were immobilized on a collagen carrier seeded with expanded ASC or BMSC and implanted into 4 mm calvarial bone defects in immunodeficient mice. Cell-free β-TCP constructs served as control. Defect filling was analyzed by µCT 4 and 8 weeks after implantation and assessed by histology (8 weeks).

Results and conclusion: β-TCP nanoparticles were not cytotoxic for BMSC and ASC in vitro. In calvarial bone defects new mineralized tissue had formed in BMSC-treated and to a lesser extent in ASC-treated and cell-free control defects after 4 weeks. Remarkably, ASC-treated defects contained significantly less mineralized tissue than the cell-free controls, indicating inhibitory effects of ASC on bone healing. Until 8 weeks, defect filling progressed highly significantly in the BMSC group and finally resulted in considerably more bone healing than in cell-free controls. The ASC group still showed notably less defect filling than any other group. Histology revealed considerable new bone formation in 6/6 BMSC defects, while only 3/6 cell-free controls and 3/6 ASC-treated defects contained some new bone tissue. In situ hybridization with species-specific genomic DNA probes demonstrated an engraftment of human cells in BMSC-treated defects including osteocytes and osteoblasts whereas ASC-treated defects contained less new bone and little human cells.

BMSC engrafted, underwent osteogenesis and stimulated bone healing beyond the results obtained with the novel biocompatible β-TCP nanoparticle collagen carrier alone. In contrast, the orthotopic environment was insufficient to overcome the inferior bone forming capacity of ASC, which rather inhibited the host-derived healing response.