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Bone is removed or replaced in defined locations by targeting osteoclasts and osteoblasts in response to its local history of mechanical loading. There is increasing evidence that osteocytes modulate this targeting by their apoptosis, which is associated with locally increased bone resorption. To investigate the role of osteocytes in the control of loading-related modeling or remodeling, we studied the effects on osteocyte viability of short periods of mechanical loading applied to the ulnae of rats. Loading, which produced peak compressive strains of -0.003 or -0.004, was associated with a 78% reduction in the resorption surface at the midshaft. The same loading regimen resulted in a 40% relative reduction in osteocyte apoptosis at the same site 3 days after loading compared with the contralateral side (P = 0.01). The proportion of osteocytes that were apoptotic was inversely related to the estimated local strain (P < 0.02). In contrast, a single short period of loading resulting in strains of -0.008 engendered both tissue microdamage and subsequent bone remodeling and was associated with an eightfold increase in the proportion of apoptotic osteocytes (P = 0.02) at 7 days. This increase in osteocyte apoptosis was transient and preceded both intracortical remodeling and death of half of the osteocytes (P < 0.01). The data suggest that osteocytes might use their U-shaped survival response to strain as a mechanism to influence bone remodeling. We hypothesize that this relationship reflects a causal mechanism by which osteocyte apoptosis regulates bone's structural architecture. in vivo; rat ulnae; osteocytes; cell death

Caspase-directed apoptosis usually fragments cells, releasing nonfunctional, prothrombogenic, membrane-bound apoptotic bodies marked for rapid engulfment by macrophages. Blood platelets are functional anucleate cells generated by specialized fragmentation of their progenitors, megakaryocytes (MKs), but committed to a constitutive caspase-independent death. Constitutive formation of the proplatelet-bearing MK was recently reported to be caspase-dependent, apparently involving mitochondrial release of cytochrome c, a known pro-apoptogenic factor. We extend those studies and report that activation of caspases in MKs, either constitutively or after Fas ligation, yields platelets that are functionally responsive and evade immediate phagocytic clearance, and retain mitochondrial transmembrane potential until constitutive platelet death ensues. Furthermore, the exclusion from the platelet progeny of caspase-9 present in the progenitor accounts for failure of mitochondrial release of cytochrome c to activate caspase-3 during platelet death. Thus, progenitor cell death by apoptosis can result in birth of multiple functional anucleate daughter cells.

Cultures were established from neonatal rat muscle cells, satellite cells, and L6 myoblasts and changes in protein metabolism were determined as development proceeded. For all three cell types, culture protein content increased with increasing myotube content. The beta-adrenergic agonist clenbuterol (added to a final concentration of 10 to the -7 M) significantly stimulated fusion (as indicated by creatine kinase activity) in neonatal muscle cultures and also increased culture protein content. This was associated with a stimulation in both the fractional (ks, percentage/day, +13%, P < .05) and absolute (As, micrograms/day, +19%, P < .05) rates of protein synthesis within 24 h after drug administration. At 48 h, As was increased by 42% above that of controls (P < .01). In contrast, in satellite cell cultures, clenbuterol had no consistent effects on either protein accretion, creatine kinase activity, or protein synthesis (ks and As). Similarly, the drug had no stimulatory effect on protein synthesis and protein accretion in L6 myoblast or L6 myotube cultures (and no effect in neonatally derived fibroblast cultures). It is concluded that the fusion response to clenbuterol and, therefore, changes in protein metabolism and protein accretion are greatly dependent on the origin and genetic integrity of muscle cells.

The goal of this study was to assess the effect of the addition of TGF β3, alone or in combination with loading, on the survival of osteocytes in 3D human explant cancellous bone during long-term culture in an ex vivo loading bioreactor. Human cancellous bone explants were cultured for up to 14 days with or without TGF β3 (15 ng ml−1) and with or without loading (300 cycles, at 1 Hz, producing 4000 microstrain). Bone core response was visualized using undecalcified histology with morphological methods after embedding with Technovit 9100 New® resin. Histological examination revealed normal gross level bone structure with or without the application of load or the addition of TGF β3. The viability of the osteocytes within the bone was assessed by lactate dehydrogenase (LDH) activity. We demonstrate that this ex vivo loading bioreactor is able to maintain a high percentage (over 50%) of viable osteocytes throughout the bone explants after 14 days in ex vivo culture. Further to this, the combination of daily loading and TGF β3 administration produced superior osteocyte survival at the core centres when compared to loading or TGF β alone. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]