Borate Glass Supports the In Vitro Osteogenic Differentiation of Human Mesenchymal Stem Cells

Bioactive ceramics have the ability to bond to surrounding bone and potentially enhance bone in-growth. Silicate based bioactive glasses and glass-ceramics, such as 45S5 bioactive glass, have been widely investigated for bone repair or as scaffolds for cell-based bone tissue engineering. Recent data have demonstrated that silica-free borate glasses also exhibit bioactive behavior and have been shown to convert to calcium phosphate at a remarkably rapid rate. Due to its relative novelty in biological applications, the cytocompatibity of borate glass is largely unknown. The objectives of this study were to investigate the cytocompatibility of borate glass by in vitro cell culture with human mesenchymal stem cells (hMSCs) and hMSC-derived osteoblasts (hMSC-Obs). The choice of hMSCs and hMSC-Obs is based on the rationale that both cell types are preferred cell lineages attached to bone implants. Borate glass particles with diameters of 212-355 μ m were loosely compacted into porous disks (porosity ≈ 40%) followed by sintering at 600°C. Partial or nearly complete conversion of the borate glass to calcium phosphate (Ca-P) material was achieved by soaking the disks for 1 day or 7 days in a 0.25 molar K ₂ HPO ₄ solution at 37°C and at pH of 9.0. Bone marrow derived hMSCs and hMSC-Obs adhered to the pores of borate glass disks. Upon 2 week incubation of cell-seeded borate glass disks, hMSC-Obs markedly synthesized alkaline phosphatase, an early osteogenic marker. These data provide preliminary evidence of cytocompatibility of borate glass and its role in supporting the osteogenic differentiation of mesenchymal stem cells.