Aging, Osteocytes, and Mechanotransduction

status: publishe

Incorporation of anterior iliac crest or calvarial bone grafts in reconstructed atrophied maxillae:A randomized clinical trial with histomorphometric and micro-CT analyses

BACKGROUND: Autologous bone grafts have been applied successfully to severely atrophied maxilla via a preimplant procedure. Differences in graft incorporation at the microscopic level can be the decisive factor in the choice between anterior iliac crest and calvarial bone. PURPOSE: To compare conversion of anterior iliac crest bone and calvarial bone 4 months after grafting of the edentulous maxilla. MATERIALS AND METHODS: Twenty consecutive patients were randomly assigned to either anterior iliac crest (n = 10) or calvarial (n = 10) bone harvesting to reconstruct their atrophied maxillae. Biopsies were taken from both fresh bone grafts and reconstructed maxillae after 4 months healing, at time of implant placement. Micro-CT, histomorphometric and histological analyses were performed. RESULTS: Micro-CT analysis revealed that both the anterior iliac crest and calvarial bone grafts retained their volume and bone mass after being incorporated in the maxilla, but with a favor for calvarial bone grafts: calvarial bone grafts had a higher mineral density before and after incorporation. Both bone grafts types were well incorporated after 4 months of healing with preservation of bone volume and mineral density. Although the fresh bone biopsies were similar histomorphometrically, after 4 months of graft incorporation, the osteoid percentage and osteocyte count remained higher in the anterior iliac crest bone whereas the percentage of bone was higher in the calvarial bone grafts compared to the anterior iliac crest bone grafts. CONCLUSIONS: Both donor sites, that is, anterior iliac crest and calvarial bone, are well suited to provide a reliable and stable basis for implant placement 4 months after grafting with mineral density, porosity, and resorption rate in favor of calvarial bone grafts

Histomorphometric and micro-CT analyses of calvarial bone grafts used to reconstruct the extremely atrophied maxilla

Background Calvarial bone grafts are successful in the reconstruction of the severely atrophied maxilla as a pre-implant procedure. However, not much is known about graft incorporation at the microscopic level. Purpose This study aimed to assess calvarial bone conversion 4 months after being grafted in the edentulous maxillary bone. Materials and methods In 13 patients (age:65.3 +/- 8.7 years) the atrophic maxilla was reconstructed with autologous calvarial bone. Biopsies were taken from fresh calvarial bone grafts and from the reconstructed maxillae after 4 months of healing. Micro-CT, histomorphometric, and histological analysis were performed. From three patients biopsies were obtained after 9, 11, or 45 months. Results The micro-CT analysis revealed that in the maxilla the calvarial bone was well preserved even after 45 months. Histology showed progressive incorporation of grafted bone within a maxillary bone. Osteoid and osteocytes were present in all biopsies indicating new bone formation and vital bone. Histomorphometrically, the percentage of grafted bone volume over total volume decreased from 79.8% (IQR78.7-83.3) in fresh calvarial grafts to 59.3% (IQR44.8-64.6) in healed grafts. The biopsies were taken after 9, 11, and 45 months showed similar values. Conclusions Calvarial bone grafts result in stable and viable bone, good incorporation into native maxillary bone, and a minor decrease in bone volume after healing. Consequently, they provide a solid base for implant placement in severely atrophied edentulous maxillary bone

Enhanced Osteogenic and Vasculogenic Differentiation Potential of Human Adipose Stem Cells on Biphasic Calcium Phosphate Scaffolds in Fibrin Gels

For bone tissue engineering synthetic biphasic calcium phosphate (BCP) with a hydroxyapatite/ -tricalcium phosphate (HA/ -TCP) ratio of 60/40 (BCP60/40) is successfully clinically applied, but the high percentage of HA may hamper efficient scaffold remodelling. Whether BCP with a lower HA/ -TCP ratio (BCP20/80) is more desirable is still unclear. Vascular development is needed before osteogenesis can occur. We aimed to test the osteogenic and/or vasculogenic differentiation potential as well as degradation of composites consisting of human adipose stem cells (ASCs) seeded on BCP60/40 or BCP20/80 incorporated in fibrin gels that trigger neovascularization for bone regeneration. ASC attachment to BCP60/40 and BCP20/80 within 30 min was similar (>93%). After 11 days of culture BCP20/80-based composites showed increased alkaline phosphatase activity and DMP1 gene expression, but not RUNX2 and osteonectin expression, compared to BCP60/40-based composites. BCP20/80-based composites also showed enhanced expression of the vasculogenic markers CD31 and VEGF189, but not VEGF165 and endothelin-1. Collagen-1 and collagen-3 expression was similar in both composites. Fibrin degradation was increased in BCP20/80-based composites at day 7. In conclusion, BCP20/80-based composites showed enhanced osteogenic and vasculogenic differentiation potential compared to BCP60/40-based composites in vitro, suggesting that BCP20/80-based composites might be more promising for in vivo bone augmentation than BCP60/40-based composites

Expression of muscle anabolic and metabolic factors in mechanically loaded MLO-Y4 osteocytes

Lack of physical activity results in muscle atrophy and bone loss, which can be counteracted by mechanical loading. Similar molecular signaling pathways are involved in the adaptation of muscle and bone mass to mechanical loading. Whether anabolic and metabolic factors regulating muscle mass, i.e., insulin-like growth factor-I isoforms (IGF-I Ea), mechano growth factor (MGF), myostatin, vascular endothelial growth factor (VEGF), or hepatocyte growth factor (HGF), are also produced by osteocytes in bone in response to mechanical loading is largely unknown. Therefore, we investigated whether mechanical loading by pulsating fluid flow (PFF) modulates the mRNA and/or protein levels of muscle anabolic and metabolic factors in MLO-Y4 osteocytes. Unloaded MLO-Y4 osteocytes expressed mRNA of VEGF, HGF, IGF-I Ea, and MGF, but not myostatin. PFF increased mRNA levels of IGF-I Ea (2.1-fold) and MGF (2.0-fold) at a peak shear stress rate of 44Pa/s, but not at 22Pa/s. PFF at 22 Pa/s increased VEGF mRNA levels (1.8- to 2.5-fold) and VEGF protein release (2.0- to 2.9-fold). Inhibition of nitric oxide production decreased (2.0-fold) PFF-induced VEGF protein release. PFF at 22 Pa/s decreased HGF mRNA levels (1.5-fold) but increased HGF protein release (2.3-fold). PFF-induced HGF protein release was nitric oxide dependent. Our data show that mechanically loaded MLO-Y4 osteocytes differentially express anabolic and metabolic factors involved in the adaptive response of muscle to mechanical loading (i.e., IGF-I Ea, MGF, VEGF, and HGF). Similarly to muscle fibers, mechanical loading enhanced expression levels of these growth factors in MLO-Y4 osteocytes. Although in MLO-Y4 osteocytes expression levels of IGF-I Ea and MGF of myostatin were very low or absent, it is known that the activity of osteoblasts and osteoclasts is strongly affected by them. The abundant expression levels of these factors in muscle cells, in combination with low expression in MLO-Y4 osteocytes, provide a possibility that growth factors expressed in muscle could affect signaling in bone cells

Inhibition of Osteoclastogenesis by Mechanically Loaded Osteocytes: Involvement of MEPE

In regions of high bone loading, the mechanoresponsive osteocytes inhibit osteoclastic bone resorption by producing signaling molecules. One possible candidate is matrix extracellular phosphoglycoprotein (MEPE) because acidic serine- and aspartate-rich MEPE-associated motif peptides upregulate osteoprotegerin (OPG) gene expression, a negative regulator of osteoclastogenesis. These peptides are cleaved from MEPE when relatively more MEPE than PHEX (phosphate-regulating gene with homology to endopeptidases on the X chromosome) is present. We investigated whether mechanical loading of osteocytes affects osteocyte-stimulated osteoclastogenesis by involvement of MEPE. MLO-Y4 osteocytes were mechanically loaded by 1-h pulsating fluid flow (PFF; 0.7 ± 0.3 Pa, 5 Hz) or kept under static control conditions. Recombinant MEPE (0.05, 0.5, or 5 μg/ml) was added to some static cultures. Mouse bone marrow cells were seeded on top of the osteocytes to determine osteoclastogenesis. Gene expression of MEPE, PHEX, receptor activator of nuclear factor kappa-B ligand (RANKL), and OPG by osteocytes was determined after PFF. Osteocytes supported osteoclast formation under static control conditions. Both PFF and recombinant MEPE inhibited osteocyte-stimulated osteoclastogenesis. PFF upregulated MEPE gene expression by 2.5-fold, but not PHEX expression. PFF decreased the RANKL/OPG ratio at 1-h PFF treatment. Our data suggest that mechanical loading induces changes in gene expression by osteocytes, which likely contributes to the inhibition of osteoclastogenesis after mechanical loading of bone. Because mechanical loading upregulated gene expression of MEPE but not PHEX, possibly resulting in the upregulation of OPG gene expression, we speculate that MEPE is a soluble factor involved in the inhibition of osteoclastogenesis by osteocytes

Fluoride inhibits the response of bone cells to mechanical loading

The response of bone cells to mechanical loading is mediated by the cytoskeleton. Since the bone anabolic agent fluoride disrupts the cytoskeleton, we investigated whether fluoride affects the response of bone cells to mechanical loading, and whether this is cytoskeleton mediated. The mechano-response of osteoblasts was assessed in vitro by measuring pulsating fluid flow-induced nitric oxide (NO) production. Osteocyte shape was determined in hamster mandibles in vivo as parameter of osteocyte mechanosensitivity. Pulsating fluid flow (0.7 ± 0.3 Pa, 5 Hz) stimulated NO production by 8-fold within 5 min. NaF (10-50 μM) inhibited pulsating fluid flow-stimulated NO production after 10 min, and decreased F-actin content by ~3-fold. Fluid flow-induced NO response was also inhibited after F-actin disruption by cytochalasin B. NaF treatment resulted in more elongated, smaller osteocytes in interdental bone in vivo. Our results suggest that fluoride inhibits the mechano-response of bone cells, which might occur via cytoskeletal changes. Since decreased mechanosensitivity reduces bone mass, the reported anabolic effect of fluoride on bone mass in vivo is likely mediated by other factors than changed bone cell mechanosensitivity. © 2011 The Society of The Nippon Dental University