Intermittent compressive load stimulates osteogenesis and improves osteocyte viability in bones cultured “in vitro”

Abstract

The effect of mechanical stresses on osteogenesis, the viability of osteocytes and their metabolic activity in organ culture of bones intermittently loaded "in vitro" are reported. Metatarsal bones, isolated from 12-day-old rats, were cultured in BGJb medium (with 10% foetal calf serum, 75 micrograms/ml of ascorbic acid, 100 U/ml of penicillin and 100 micrograms/ml of streptomycin), in humidified air enriched by 5% CO2 and 30% O2, and loaded in our original device for 1/2 an hour at 1 Hz. homotypic isolated and unloaded bones, cultured in the same medium, were taken as controls. The ALP (alkaline phophatase activity) increases in the media of loaded bones in comparison with the control bones. The percentage of viable osteocytes is significantly greater in loaded than in control bones. TEM observations demonstrate that in both loaded and control unloaded bones, osteocytes show well developed organelle machinery and several gap junctions with adjacent cellular processes. In the cells of loaded bones, however, a higher number of cytoplasmic organelles and gap junctions were found. In particular, RER increases twice, gap junctions three times. The induced osteogenesis and the TEM observations demonstrate the suitability of this experimental model and support the recent advanced hypothesis according to which the mechanical loading may exert a trophic function on osteocytes, stimulating both the proteic synthesis in the above-mentioned cells and the cell-to-cell communication. Furthermore, the loading is likely to exert a biological stimulus on osteoblasts via signalling molecules produced by osteocytes.

Identification of Osteoblast/Osteocyte Factor 45 (OF45), a Bone-specific cDNA Encoding an RGD-containing Protein That Is Highly Expressed in Osteoblasts and Osteocytes

“…Osteocytes are terminally differentiated osteoblasts, which become encased in the mineralized matrix during bone formation (46). The functional role of the osteocyte remains unclear; however, they can communicate through dendritic processes and have been theorized to be the primary sensors of mechanical strain in bone (47)(48)(49). We confirmed the expression within osteocytes at the protein level by immunohistochemistry.…”

Mechanically stimulated osteocytes regulate osteoblastic activity via gap junctions

“…We (1) and others (5-7) demonstrated that fluid flow increases gap junction expression and function in osteocytic mouse long bone osteocyte (MLO-Y4) cells; however, Thi et al (41) demonstrated that fluid flow decreases gap junction function and expression in MLO-Y4, emphasizing the need for additional examination of this issue. In vivo, Lozupone et al (30) demonstrated that mechanical loading of rat metatarsal bones increased the incidence of osteocytic gap junctions, and we demonstrated that expression of Cx43 by osteocytes is increased in areas of bone exposed to tension relative to areas exposed to compression or to control bone (39).…”

Osteocyte-Specific Monoclonal Antibody MAb OB7.3 Is Directed Against Phex Protein

“…However, this strain-derived flow, for which experimental evidence is reported by Knothe Tate et al (42) and Knothe Tate and Knothe, (43) may not only act as the mechano-signal-multiplying system of bone but also may improve osteocyte viability by facilitating exchange of nutrients and waste products. (44,45) Similarly, this mechanism may be crucial in the transport of the hypothetical phosphate-regulating factor to the osteocytes, where it is to be activated/deactivated by Phex. Therefore, it would be interesting to study whether immobility leading to decreased canalicular fluid flow also leads to disturbed phosphate metabolism and hypophosphatemia.…”