The capacity of bone tissue to alter its mass and structure in response to mechanical demands has long been recognized but the cellular mechanisms involved remained poorly understood. Over the last several years significant progress has been made in this field, which we will try to summarize. These studies emphasize the role of osteocytes as the professional mechanosensory cells of bone, and the lacuno-canalicular porosity as the structure that mediates mechanosensing. Strain-derived flow of interstitial fluid through this porosity seems to mechanically activate the osteocytes, as well as ensuring transport of cell signaling molecules and nutrients and waste products. This concept allows an explanation of local bone gain and loss, as well as remodeling in response to fatigue damage, as processes supervised by mechanosensitive osteocytes.

It has been known for more than a century that bone tissue adapts to functional stress by changes in structure and mass. However, the mechanism by which stress is translated into cellular activities of bone formation and resorption is unknown. We studied the response of isolated osteocytes derived from embryonic chicken calvariae to intermittent hydrostatic compression as well as pulsating fluid flow, and compared their response to osteoblasts and periosteal fibroblasts. Osteocytes, but not osteoblasts or periosteal fibroblasts, reacted to 1 h pulsating fluid flow with a sustained release of prostaglandin E2. Intermittent hydrostatic compression stimulated prostaglandin production to a lesser extent: after 6 and 24 h in osteocytes and after 6 h in osteoblasts. These data provide evidence that osteocytes are the most mechanosensitive cells in bone involved in the transduction of mechanical stress into a biological response. The results support the hypothesis that stress on bone causes fluid flow in the lacunar-canalicular system, which stimulates the osteocytes to produce factors that regulate bone metabolism.

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Although the structural design of cellular bone (i.e., bone containing osteocytes that are regularly spaced throughout the bone matrix) dates back to the first occurrence of bone as a tissue in evolution, and although osteocytes represent the most abundant cell type of bone, we know as yet little about the role of the osteocyte in bone metabolism. Osteocytes descend from osteoblasts. They are formed by the incorporation of osteoblasts into the bone matrix. Osteocytes remain in contact with each other and with cells on the bone surface via gap junction-coupled cell processes passing through the matrix via small channels, the canaliculi, that connect the cell body-containing lacunae with each other and with the outside world. During differentiation from osteoblasts to mature osteocyte the cells lose a large part of their cell organelles. Their cell processes are packed with microfilaments. In this review we discuss the various theories on osteocyte function that have taken in consideration these special features of osteocytes. These are 1) osteocytes are actively involved in bone turnover; 2) the osteocyte network is through its large cell-matrix contact surface involved in ion exchange; and 3) osteocytes are the mechanosensory cells of bone and play a pivotal role in functional adaptation of bone. In our opinion, especially the last theory offers an exciting concept for which some biomechanical, biochemical, and cell biological evidence is already available and which fully warrants further investigations.

Recently, the first clinical reports on bone regeneration by two recombinant human bone morphogenetic proteins (rhBMPs), BMP-2 and BMP-7 (also named osteogenic protein-1, OP-1) have been published (1-4) . Although both BMPs were able to support bone regeneration, a significant variation in individual response was observed with both proteins. Animal studies and laboratory experiments reveal a number of conditions that influence the osteoinductivity of BMP, such as BMP concentration, carrier properties and influence of local and systemic growth factors and hormones. In this paper, these studies and the clinical reports are reviewed, and the conditions that modulate the BMP-dependent osteoinduction are discussed. The information may provide clues as to how the performance of recombinant human BMP as bone-graft substitute in humans can be improved. European Journal of Endocrinology 142 9-21 BMP and demineralized bone matrixThe history of bone morphogenetic proteins (BMPs) began with the observation that demineralized bone matrix (DBM) is able to induce ectopic bone formation in subcutaneous and intramuscular pockets in rodents (5, 6). This bone induction process has been studied extensively (7-13). Histological and biochemical analyses showed that cartilage appears 5-10 days after implantation of active DBM (8). This cartilage mineralizes by day 7-14 and is subsequently replaced by bone (7-9). After 21 days, haematopoietic bone marrow formation can be observed (12). These cellular events observed after DBM implantation mimic embryonic bone development and normal fracture repair (11). As DBM-related bone formation was observed to occur at ectopic sites, it was assumed that pluripotent mesenchymal cells are attracted to the site of implantation. Isolation of the bone-inducing substance revealed that certain proteins were responsible, which were termed bone morphogenetic proteins (BMPs) or osteogenetic proteins (OPs).The use of DBM in treating bone defects has proven beneficial for bone regeneration both in animals and in humans (14-17). DBM has become widely accepted as a bone-graft substitute in clinical practice (18-22), but its bone inductive capacity has been questioned (23, 24). In several studies, including our own, histology revealed that new bone was generated by osteoconduction rather than osteoinduction (25-27) -that is, bone regeneration occurred by growth of existing host bone on the DBM granules, which acted as a scaffold, rather than by de novo differentiation of bone, independent of pre-existing bone. Lack of inductive properties of DBM may be related to the procedures of production of commercially available DBM, as preservation of osteoinductive activity can be affected by the processing (28-30) or sterilization procedures (31). It is also possible that DBM of human origin, which is preferred for use in clinical practice, is less osteoinductive than DBM derived from animals, which is commonly used in animal studies. Several studies show that DBM from long-lived species such as baboon and human...