Osteocyte function, osteocyte death and bone fracture resistance

The function of the most numerous cell in bone, the osteocyte, has until re cently been mysterious and at times controversial. There is now an emerging consensus that osteocytes modulate signals arising from mechanical loading and so direct the appearance and disappearance of bone tissue at the micro scopic level, which allows bone as an organ both to grow and to adapt effic iently to the body's mechanical needs for strength with lightness. Osteocyt es appear to use some molecular signalling pathways that are familiar from other tissues, such as the generation of nitric oxide and prostaglandins as well as directing cell-cell communication via gap junctions. They may also direct the removal of damaged or redundant bone through mechanisms linked to their own apoptosis or via the secretion of specialised cellular attachm ent proteins such as osteopontin. Osteocytes possess receptors for parathyr oid hormone/parathyroid hormone related peptide and both oestrogen receptor s alpha and beta. They also express molecules which in nerve cells are invo lved with glutamate neuro-transmission. At least some of these receptors an d their ligands may regulate osteocyte apoptosis and modulate osteocyte sig nalling. (C) 2000 Published by Elsevier Science Ireland Ltd. All rights res erved.