Biomaterials 28 (2007) 2155–2162 The in vitro osteoclastic degradation of nacre D. Duplat a,Ã , A. Chabadel b , M. Gallet c , S. Berland a , L. Be´douet a , M. Rousseau a , S. Kamel c , C. Milet a , P. Jurdic b , M. Brazier c , E. Lopez a a De´partement Milieux et Peuplements Aquatiques USM 401, UMR/CNRS 5178 BOME, Muse´um National d’Histoire Naturelle, 43, rue Cuvier, 75231 Paris cedex 05, France b Laboratoire de Biologie Mole´culaire de la Cellule, ENS/CNRS 5161, IFR 128 Biosciences Lyon-Gerland, 46, alle´e d’Italie, 69007 Lyon, France c UMRO, Laboratoire de Pharmacie Clinique, Faculte´de Pharmacie, 1, rue des Louvels, 80000 Amiens, France Received 17 October 2006; accepted 4 January 2007 Available online 10 January 2007 Abstract Osteoclast activity was studied on nacre, the mother of pearl (MOP) in order to assess the plasticity of bone resorbing cells and their capacity to adapt to a biomineralized material with a different organic and mineral composition from that of its natural substrate, bone. Pure MOP, a natural biomineralized CaCO 3 material, was obtained from Pinctada oyster shell. When implanted in the living system, nacre has proven to be a sustainable bone grafting material although a limited surface degradation process. Osteoclast stem cells and mature osteoclasts were cultured on MOP substrate and osteoclast precursor cells were shown to differentiate into osteoclasts capable of resorbing nacre substrate. However, analysis of the organization of the cytoskeleton showed that both a sealing zone and a podosome structure were observed on the nacre substrate. Moreover, MOP resorption efficiency was consistently found to be lower than that of bone and appeared to be a limited process. r 2007 Elsevier Ltd. All rights reserved. Keywords: Nacre; Bone graft; Osteoclast; Adhesion; Biodegradation 1. Introduction Bone is a biomineralized structure that provides an internal support system in all higher vertebrates and participates in maintaining calcium homeostasis in the body. It is composed of bone cells and an organic matrix, mainly type I collagen, strengthened by calcium and phosphate deposition in the form of hydroxyapatite. The two major bone cells are osteoblasts, the bone forming cells [1] and osteoclasts, the bone resorbing cells [2]. Osteoclasts are multinucleated, giant cells derived by differentiation and fusion of hematopoietic cells of the monocyte macrophage lineage. Their bone degradation function depends on the organization of the actin cytoskeleton forming a cavity where protons and proteases degrade mineral and organic matrix, respectively. The activity of osteoclasts and osteoblasts determines bone remodeling [3], the biological process for the renewal of the bone tissue and maintenance of mechanical integrity, metabolic function and calcium homeostasis in healthy adults. An imbalance in bone formation and bone resorption may be involved in bone rescue after bone loss due to injury or disease. However, bone has a limited capacity to recover and the missing bone must often be supplied as a biomaterial [4]. Interactions between the bone material and bone cells obviously determine the success or failure of the repair process [5]. Mother of pearl (MOP) or nacre, a natural biominer- alized CaCO 3 crystal synthesized in bivalve mollusk shells as aragonite, is a biocompatible and bioactive material, in particular because of signal molecules entrapped in its organic framework. In vitro studies have shown MOP bioactivity on osteoblast cell lines [6–9]. Osteoclast activity has been widely studied on different calcium phosphate and carbon apatite materials [10] and this has demon- strated the importance of the crystallinity and structure of the material in contact with the osteoclasts. In vivo ARTICLE IN PRESS www.elsevier.com/locate/biomaterials 0142-9612/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2007.01.015 Ã Corresponding author. Tel.: +33 140 793 702; fax: +33 140 795 620. E-mail address: duplat@mnhn.fr (D. Duplat).