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48 Volume 1 | Issue 1/2015 Journal of Oral Science & Rehabilitation Os teopon tin expres s io n i n ano rgani c bovi ne bo ne tion, given that OPN expression in the cement lines may indicate bone formation or destruction as a function of the cells attached to the particle surface. However, this dual behavior of OPN re- mainsunclear. OPN can be expressed in numerous tissues52 andbymultiplecelltypes,14 includingosteoblasts and osteoclasts.53 Kunii et al. confirmed the ex- pression ofOPN mRNAand protein in human os- teocytes32 using different antibodies.52 Osteo- cyteshavemultiplefunctionsandregulatediffer- ent processes in bone homeostasis.54 After six months ofhealing, osteocytes present inthe new vital bone in our biopsies consistently expressed OPN.OPNexpressioninosteocyteswasinversely correlated with the count of osteoblasts (CD56- positive cells) per mm2 (rho coefficient = -0.828, p = 0.042, Spearman test). Araújo et al. detected OPN expression on ABB particles in dogs during the early stages of healing.13 According to their study, after the migration of polymorphonuclear cellstotheABBsurface,thesecellsarereplacedby TRAP-positive cells (osteoclasts), which will re- movematerialfromthesurfaceofthexenograft.13 Subsequently, osteoblasts will attach to the re- modeled surface and bone apposition will begin. However, OPN behavior is likely different during the late stage of healing. In the present samples, osteoblastswererarelyobservedintheABBparti- cles, and a significant inverse relationship was found between OPN expression and CD56 ex- pression (rho coefficient = -828, p = 0.04, Spear- man test). This higher expression with fewer osteoblasts and the positive correlation between OPN and CD68 expression (rho coeffi- cient = 0.583, p = 0.009; rho coefficient = 0.938 p = 0.006; Spearman test, respectively) indicate that OPN expression on ABB particles promotes resorption at six months of healing. In support of thishypothesis,itisknownthatOPNcanbothac- tivate osteoclasts for bone matrix resorption and induce their migration in an αvβ3 integrin-depen- dent manner.55, 56 Nakamura et al. demonstrated that αvβ3 integrin plays a key role in osteoclast migration, which is essential for efficient osteo- clastic bone resorption.57 Perrotti et al. reported that multinucleated cells generated onABBwere positive for the αvβ3 subunit of vitronectin,58 the main integrin-mediating osteoclast cell attach- menttothebonematrix;59 hence,thismechanism may be functional on ABB particles. OPN is a lig- and for the αvβ3 integrin through an RGD se- quence.60 The osteoclast cell membrane must be sealed to the substrate by means of cell surface receptors and proteins of the integrin family before beginning its proteolytic activity. The binding of OPN to the αvβ3 integrin in the seal- ing zone orpodosomes appears essentialtothe reorganization ofthe actin cytoskeleton for os- teoclast motility.61 Additionally, osteoclast ad- hesion and osteoclast migration are mediated by phosphorylated OPN,62 and this biological event is regulated by endogenous TRAP.63 In- terestingly, TRAP and OPN expressions showed similar patterns on ABB particle sur- faces at six months of healing in our samples, indicating the close relationship between OPN expression and the osteoclastic resorption of ABB. Questions have been raised about this re- sorption of the particles,64, 13 but this proposi- tion is supported byourhistologicalfindings on these particles of bone-remodeling units with multinucleated cells in different stages of dif- ferentiation (CD68 positive and TRAP positive) that promote these phenomena. In our view, these results may in part explain OPN-medi- ated ABB resorption during the late stage of graft healing. Ourfindingssupporttheproposalthatosteo- clastsarethesourceofOPNinbonecementlines during remodeling.32 The detection of OPN ex- pression within lacunae represents clear evi- denceofthesecretionofOPNbyosteoclasts,be- cause exogenously added OPN has no access to thesesites.23 Conclusion Immunohistochemical expression of OPN is re- latedtoboneremodelingandmaturationchanges inmaxillarysinusliftwithABBxenograft. Acknowledgments The authors are grateful to María Dolores Ro- dríguez Martínez and Jorge A. Payá for technical assistance and to Richard Davies for assistance withtheEnglishtranslation. Competinginterests The authors declare that theyhave no competing interests.Thisinvestigationwassupportedinpart byResearchGroups#CTS-138andCTS-583(Re- gionalGovernmentofAndalusia,Spain). Pablo Galindo Moreno,* Pedro Hernández Cortés,† Miguel Padial Molina,* María Luisa Vizoso,‡ Vicente Crespo Lora,‡ Francisco O’Valle‡ * Oral surgery and implant dentistry department, School of Dentistry, University of Granada, Spain † Department of orthopedic surgery, San Cecilio univer- sity hospital, Granada, Spain ‡ Department of pathology, Faculty of Medicine, and Institute of Bio-pathology and Regenerative Medicine, University of Granada, Spain Corresponding author: Prof. Pablo Galindo Moreno Calle Recogidas, 39, 5º Izq 18005 Granada Spain T +34 958 52 0658 F +34 958 52 0658 pgalindo@ugr.es T +34958520658 F +34958520658