Enhancing the osteoinductive properties of hydroxyapatite by the addition of human mesenchymal stem cells, and recombinant human osteogenic protein-1 (BMP-7) in vitro § Eleftherios Tsiridis a,b, * , Amit Bhalla a,b , Zubier Ali a,b , Neelam Gurav a,b , Manolis Heliotis a , Sanjukta Deb a , Lucy DiSilvio a,b a Department of Biomaterial and Biomimetics, Guy’s, King’s and St. Thomas’ Dental Institute, University of London, London SE1 1UL, UK b Institute of Orthopaedics, Royal Free and University College London Medical School, Royal National Orthopaedic Hospital Trust, Stanmore, Middlesex, HA7 4LP, UK. Introduction Over 800,000 grafting procedures are performed annually in the USA. 32 Limitations of allograft and autograft have increased the need for synthetic bone graft substitute and bone tissue engineering. 30 An optimal bone graft substitute should consist of Injury, Int. J. Care Injured (2006) 37S, S25—S32 www.elsevier.com/locate/injury KEYWORDS Hydroxyapatite; OP-1; BMP-7; Mesenchymal stem cells; Osteoinduction; Bone graft Summary Hydroxyapatite (HA) has been widely used as a bone graft substitute. In this study, we investigated whether the addition of osteogenic protein-1 (OP-1) further enhanced the weak osteoinductive properties of hydroxyapatite when loaded with human mesenchymal stem cells (h-MSCs). Over a 14 day period, cell proliferation in both groups was assessed qualitatively using SEM and quantitatively using alamar blue assay. Cell differentiation was also evaluated by measurement of ALP activity, which was expressed against total DNA. HA/MSC loaded with OP-1 demonstrated a statistically significant increase ( p < 0.001) in cell proliferation at all time points in comparison to unloaded samples. ALP activity per DNA was also significantly enhanced ( p < 0.001) in loaded samples when compared to unloaded controls. SEM demon- strated increased cellular attachment and proliferation into HA pores at all time points in the loaded samples. Our study suggests that the osteoinductive potential of HA can be improved in vitro by the combined incorporation of MSCs and OP-1. # 2006 Elsevier Ltd. All rights reserved. § No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study. * Corresponding author at: Academic Department of Trauma & Orthopaedics St. James’s University Hospital, Beckett Street, Leeds, LS9 7TF, UK. Tel.: +44 113 20 66460; fax: +44 113 20 65156. E-mail address: etsiridis@doctors.org.uk (E. Tsiridis). 0020–1383/$ — see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.injury.2006.08.021