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Osteocyte: the unrecognized side of bone tissue

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Abstract

Introduction

Osteocytes represent 95% of all bone cells. These cells are old osteoblasts that occupy the lacunar space and are surrounded by the bone matrix. They possess cytoplasmic dendrites that form a canalicular network for communication between osteocytes and the bone surface. They express some biomarkers (osteopontin, β3 integrin, CD44, dentin matrix protein 1, sclerostin, phosphate-regulating gene with homologies to endopeptidases on the X chromosome, matrix extracellular phosphoglycoprotein, or E11/gp38) and have a mechano-sensing role that is dependent upon the frequency, intensity, and duration of strain.

Discussion

The mechanical information transmitted into the cytoplasm also triggers a biological cascade, starting with NO and PGE2 and followed by Wnt/β catenin signaling. This information is transmitted to the bone surface through the canalicular network, particularly to the lining cells, and is able to trigger bone remodeling by directing the osteoblast activity and the osteoclastic resorption. Furthermore, the osteocyte death seems to play also an important role. The outcome of micro-cracks in the vicinity of osteocytes may interrupt the canalicular network and trigger cell apoptosis in the immediate surrounding environment. This apoptosis appears to transmit a message to the bone surface and activate remodeling. The osteocyte network also plays a recognized endocrine role, particularly concerning phosphate regulation and vitamin D metabolism. Both the suppression of estrogen following menopause and chronic use of systemic glucocorticoids induce osteocyte apoptosis. On the other hand, physical activity has a positive impact in the reduction of apoptosis. In addition, some osteocyte molecular elements like sclerostin, connexin 43, E11/gp38, and DKK1 are emerging as promising targets for the treatment of various osteo-articular pathologies.

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Acknowledgments

The authors want to thank Eric Dolléans for technical help during sample preparation and cutting. Confocal microscopy was performed in collaboration with David Gosset and Chantal Pichon from the Centre de Biophysique Moléculaire, CNRS UPR4301, University of Orléans and Inserm, Orléans, France. Transmission electron microscopy was achieved in collaboration with Rustem Uzbekov, Pierre-Yves Sizaret, and Brigitte Arbeille from the Laboratoire de Biologie Cellulaire, Faculté de Médecine, Tours, France.

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Correspondence to G. Y. Rochefort.

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Rochefort and Pallu contributed equally to this paper.

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Rochefort, G.Y., Pallu, S. & Benhamou, C.L. Osteocyte: the unrecognized side of bone tissue. Osteoporos Int 21, 1457–1469 (2010). https://doi.org/10.1007/s00198-010-1194-5

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