Differentiation of mononuclear precursors into osteoclasts on the surface of Si-substituted hydroxyapatite C.M. Botelho, 1,2 R.A. Brooks, 3 G. Spence, 3 I. McFarlane, 4 M.A. Lopes, 1,2 S.M. Best, 5 J.D. Santos, 1,2 N. Rushton, 3 W. Bonfield 5 1 INEB - Instituto de Engenharia Biome ´dica, Laborato ´rio de Biomateriais, Rua do Campo Alegre, 823, 4150 –180 Porto, Portugal 2 FEUP - Faculdade de Engenharia da Universidade do Porto, DEMM, Rua Dr. Roberto Frias, 4200 – 465 Porto, Portugal 3 Orthopaedic Research Unit, Box 180, Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2QQ, United Kingdom 4 Clinical Biochemistry, Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2QQ, United Kingdom 5 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom Received 26 August 2005; revised 28 December 2005; accepted 10 January 2006 Published online 31 May 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.30726 Abstract: In healthy bone, resorption and synthesis are in perfect coordination. In previous studies we demonstrated that the incorporation of silicon into the hydroxyapatite (HA) lattice enhances the proliferation and differentiation of human osteoblasts. Therefore, the aim of this study was to demonstrate the effect of silicon-substituted HA (0.8 and 1.5 wt % Si-HA) on the differentiation of mononuclear cells into osteoclasts, using two different starting cultures, peripheral blood mononuclear cells (PBMC) and monocytes expressing the CD14 antigen (CD14 + ). Through this study, it was pos- sible to demonstrate that Si-HA allows the differentiation of mononuclear cells into mature osteoclasts, independent of the starting culture, PBMC or CD14 + . Most of the cells on the surface of the materials expressed osteoclastic markers: actin rings, several nuclei, positivity for tartrate-resistant acid phosphatase (TRAP), and vitronectin receptor. In the presence of osteoclasts, a higher release of calcium and phosphate into the medium from the 1.5 wt % Si-HA sub- strate was detected when compared to the HA substrate; therefore, these results indicate higher osteoclastic resorp- tive activity on the 1.5 wt % Si-HA surface. Si-HA can be resorbed by cellular mechanisms and have a stimulatory effect on osteoclasts, although the underlying mechanism is still poorly understood. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res 78A: 709 –720, 2006 Key words: peripheral blood mononuclear cells; monocytes CD14 positive; differentiation; osteoclasts; silicon-substi- tuted hydroxyapatite INTRODUCTION The ideal bone graft for many applications would be a material that can both be resorbed and induce bone formation, and thereby being completely replaced by new bone. Bone is a dynamic organ because of its constant remodeling. This process can be divided into two main steps: resorption, where the “old” bone is resorbed by the osteoclasts, and synthesis, where the osteoblasts lay down new layers of bone. 1 In healthy bone, resorption and synthesis are in perfect coordination. Disruption to the balance be- tween resorption and formation can lead to osteopo- rosis, which is characterized by the loss of bone mass, 2 and osteopetrosis, resulting from a failure of oste- oclasts to resorb bone. 3 The osteoclast is the only cell capable of resorbing mature bone. It is a tissue-specific macrophage polykaryon created by the differentiation and fusion of monocyte/macrophage precursor cells. 4 The resorption takes place under the ruffled border, where protons and proteases are secreted, leading to the formation of resorption lacunae. 4 In the last decade, there has been a breakthrough in the understanding of osteoclastogenesis, because of the discovery of the receptor activator of nuclear factor ligand (RANKL) in 1998. It was identified as an osteoblast-producing ligand, which promotes oste- oclast differentiation. 4,5 RANK is a type I transmem- brane receptor of the TNF receptor superfamily that was identified in a dendritic cell cDNA library. 6 Sev- eral studies showed that it is possible to generate Correspondence to: R.A. Brooks; email: rb10003@cam.ac.uk Contract grant sponsor: Fundac ¸a ˜o para a Cie ˆncia e Tecno- logia (FCT); contract grant numbers: SFRH/BD/6173 and POCTI/CTM/49238/2002 © 2006 Wiley Periodicals, Inc.