Expression of fibroblast growth factor receptor-3 (FGFR3), signal transducer and activator of transcription-1, and cyclin-dependent kinase inhibitor p21 during endochondral ossification: differential role of FGFR3 in skeletal development and fracture repair

Cell Proliferation, Chondrogenic Differentiation, and Cartilaginous Tissue Formation in Recombinant Silk Fibroin with Basic Fibroblast Growth Factor Binding Peptide

Regeneration of articular cartilage remains a challenge for patients who have undergone cartilage injury, osteochondritis dissecans and osteoarthritis. Here, we describe a new recombinant silk fibroin with basic fibroblast growth factor (bFGF) binding peptide, which has a genetically introduced sequence PLLQATLGGGS, named P7. In this study, we cultured a human mesenchymal cell line derived from bone marrow, UE6E7-16, in wild-type fibroin sponge (FS) and recombinant silk fibroin sponge with P7 peptide (P7 FS). We compared cell proliferation, chondrogenic differentiation and cartilaginous tissue formation between the two types of sponge. After stimulation with bFGF at 3 ng/mL, P7 FS showed significantly higher cell growth (1.2-fold) and higher cellular DNA content (5.6-fold) than did wild-type FS. To promote chondrogenic differentiation, cells were cultured in the presence of TGF-β at 10 ng/mL for 28 days. Immunostaining of P7 FS showed SOX9-positive cells comparable to wild-type FS. Alcian-Blue staining of P7 FS also showed cartilaginous tissue formation equivalent to wild-type FS. A significant increase in cell proliferation in P7 FS implies future clinical application of this transgenic fibroin for regeneration of articular cartilage. To produce cartilaginous tissue efficiently, transgenic fibroin sponges and culture conditions must be improved. Such changes should include the selection of growth factors involved in chondrogenic differentiation and cartilage formation.

Clinical-genetic analysis of selected genes involved in the development of the human skeleton in 128 Czech patients with suspected congenital skeletal abnormalities

BACKGROUND

Congenital skeletal abnormalities are a heterogeneous group of diseases most commonly associated with small or disproportionate growth, cranial and facial dysmorphisms, delayed bone maturation, etc. Nonetheless, no detailed genotype-phenotype correlation in patients with specific genetic variants is readily available. Ergo, this study focuses on the analysis of patient phenotypes with candidate variants in genes involved in bone growth as detected by molecular genetic analysis.

METHODS

In this study we used molecular genetic methods to analyse the ACAN, COL2A1, FGFR3, IGFALS, IGF1, IGF1R, GHR, NPR2, STAT5B and SHOX genes in 128 Czech children with suspected congenital skeletal abnormalities. Pathogenic variants and variants of unclear clinical significance were identified and we compared their frequency in this study cohort to the European non-Finnish population. Furthermore, a prediction tool was utilised to determine their possible impact on the final protein. All clinical patient data was obtained during pre-test genetic counselling.

RESULTS

Pathogenic variants were identified in the FGFR3, GHR, COL2A1 and SHOX genes in a total of six patients. Furthermore, we identified 23 variants with unclear clinical significance and high allelic frequency in this cohort of patients with skeletal abnormalities. Five of them have not yet been reported in the scientific literature.

CONCLUSION

Congenital skeletal abnormalities may lead to a number of musculoskeletal, neurological, cardiovascular problems. Knowledge of specific pathogenic variants may help us in therapeutic procedures.

Giant Cells of Various Lesions Are Characterised by Different Expression Patterns of HLA-Molecules and Molecules Involved in the Cell Cycle, Bone Metabolism, and Lineage Affiliation: An Immunohistochemical Study with a Review of the Literature

Giant cells (GCs) are thought to originate from the fusion of monocytic lineage cells and arise amid multiple backgrounds. To compare GCs of different origins, we immunohistochemically characterised the GCs of reactive and neoplastic lesions (n = 47). We studied the expression of 15 molecules including HLA class II molecules those relevant to the cell cycle, bone metabolism and lineage affiliation. HLA-DR was detectable in the GCs of sarcoidosis, sarcoid-like lesions, tuberculosis, and foreign body granuloma. Cyclin D1 was expressed by the GCs of neoplastic lesions as well as the GCs of bony callus, fibroid epulis, and brown tumours. While cyclin E was detected in the GCs of all lesions, p16 and p21 showed a heterogeneous expression pattern. RANK was expressed by the GCs of all lesions except sarcoid-like lesions and xanthogranuloma. All GCs were RANK-L-negative, and the GCs of all lesions were osteoprotegerin-positive. Osteonectin was limited to the GCs of chondroblastoma. Osteopontin and TRAP were detected in the GCs of all lesions except xanthogranuloma. RUNX2 was heterogeneously expressed in the reactive and neoplastic cohort. The GCs of all lesions except foreign body granuloma expressed CD68, and all GCs were CD163- and langerin-negative. This profiling points to a functional diversity of GCs despite their similar morphology.

Lineage-specific differences and regulatory networks governing human chondrocyte development

To address large gaps in our understanding of the molecular regulation of articular and growth plate cartilage development in humans, we used our directed differentiation approach to generate these distinct cartilage tissues from human embryonic stem cells. The resulting transcriptomic profiles of hESC-derived articular and growth plate chondrocytes were similar to fetal epiphyseal and growth plate chondrocytes, with respect to genes both known and previously unknown to cartilage biology. With the goal to characterize the regulatory landscapes accompanying these respective transcriptomes, we mapped chromatin accessibility in hESC-derived chondrocyte lineages, and mouse embryonic chondrocytes, using ATAC-sequencing. Integration of the expression dataset with the differentially accessible genomic regions revealed lineage-specific gene regulatory networks. We validated functional interactions of two transcription factors (TFs) (RUNX2 in growth plate chondrocytes and RELA in articular chondrocytes) with their predicted genomic targets. The maps we provide thus represent a framework for probing regulatory interactions governing chondrocyte differentiation. This work constitutes a substantial step towards comprehensive and comparative molecular characterizations of distinct chondrogenic lineages and sheds new light on human cartilage development and biology.

P21 deficiency exhibits delayed endochondral ossification during fracture healing

INTRODUCTION

Endochondral ossification is a complex biological phenomenon involving a variety of factors and cells. Cyclin-dependent kinase inhibitor 1 (p21) inhibits cell cycle progression and is affected by external stress. We recently reported that embryonic endochondral ossification is unaffected by endogenous p21 deficiency. In this study, we evaluated whether p21 expression affects endochondral ossification during fracture healing.

METHODS

Tibial fractures were introduced into p21 knockout (p21^-/-) (n = 24) and wild-type C57BL/6 (p21^+/+) (n = 24) mice at age 10 weeks. Fracture healing was evaluated using radiological, histological, and immunohistochemical (IHC) analyses. The effect of p21 small interfering RNA (siRNA) on ATDC5 cells was assessed in vitro.

RESULTS

The Allen score for fracture healing was lower in p21^-/- mice than in p21^+/+ mice. In addition, p21^-/- mice exhibited larger calluses and lower bone mineral density. IHC analyses showed that p21^-/- mice exhibited delayed endochondral ossification via the Ihh-Runx2-Osterix pathway in vivo. Down-regulation of p21 expression in ATDC5 cells delayed endochondral ossification in vitro.

CONCLUSIONS

p21 deficiency leads to delayed endochondral ossification by attenuating the Ihh-Runx2-Osterix pathway in vivo, and p21 deficiency in hypertrophic chondrocytes causes delayed differentiation of hypertrophic chondrocytes in vitro. p21 plays a role in endochondral ossification during fracture healing.

Identification and Tissue-Specific Characterization of Novel SHOX-Regulated Genes in Zebrafish Highlights SOX Family Members Among Other Genes

SHOX deficiency causes a spectrum of clinical phenotypes related to skeletal dysplasia and short stature, including Léri-Weill dyschondrosteosis, Langer mesomelic dysplasia, Turner syndrome, and idiopathic short stature. SHOX controls chondrocyte proliferation and differentiation, bone maturation, and cellular growth arrest and apoptosis via transcriptional regulation of its direct target genes NPPB, FGFR3, and CTGF. However, our understanding of SHOX-related pathways is still incomplete. To elucidate the underlying molecular mechanisms and to better understand the broad phenotypic spectrum of SHOX deficiency, we aimed to identify novel SHOX targets. We analyzed differentially expressed genes in SHOX-overexpressing human fibroblasts (NHDF), and confirmed the known SHOX target genes NPPB and FGFR among the most strongly regulated genes, together with 143 novel candidates. Altogether, 23 genes were selected for further validation, first by whole-body characterization in developing shox-deficient zebrafish embryos, followed by tissue-specific expression analysis in three shox-expressing zebrafish tissues: head (including brain, pharyngeal arches, eye, and olfactory epithelium), heart, and pectoral fins. Most genes were physiologically relevant in the pectoral fins, while only few genes were also significantly regulated in head and heart tissue. Interestingly, multiple sox family members (sox5, sox6, sox8, and sox18) were significantly dysregulated in shox-deficient pectoral fins together with other genes (nppa, nppc, cdkn1a, cdkn1ca, cyp26b1, and cy26c1), highlighting an important role for these genes in shox-related growth disorders. Network-based analysis integrating data from the Ingenuity pathways revealed that most of these genes act in a common network. Our results provide novel insights into the genetic pathways and molecular events leading to the clinical manifestation of SHOX deficiency.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

Activated FGFR3 promotes bone formation via accelerating endochondral ossification in mouse model of distraction osteogenesis

Achondroplasia (ACH) is one of the most common short-limbed skeletal dysplasias caused by gain-of-function mutations in the fibroblast growth factor receptors 3 (FGFR3) gene. Distraction osteogenesis (DO) is a treatment option for short stature in ACH in some countries. Although the patients with ACH usually show faster healing in DO, details of the newly formed bone have not been examined. We have developed a mouse model of DO and analyzed new bone regenerates of the transgenic mice with ACH (Fgfr3^ach mice) histologically and morphologically. We established two kinds of DO protocols, the short-DO consisted of 5days of latency period followed by 5days of distraction with a rate of 0.4mm per 24h, and the long-DO consisted of the same latency period followed by 7days of distraction with a rate of 0.3mm per 12h. The callus formation was evaluated radiologically by bone fill score and quantified by micro-CT scan in both protocols. The histomorphometric analysis was performed in the short-DO protocol by various stainings, including Villanueva Goldner, Safranin-O/Fast green, tartrate-resistant acid phosphatase, and type X collagen. Bone fill scores were significantly higher in Fgfr3^ach mice than in wild-type mice in both protocols. The individual bone parameters, including bone volume and bone volume/tissue volume, were also significantly higher in Fgfr3^ach mice than in wild-type mice in both protocols. The numbers of osteoblasts, as well as osteoclasts, around the trabecular bone were increased in Fgfr3^ach mice. Cartilaginous tissues of the distraction region rapidly disappeared in Fgfr3^ach mice compared to wild-type mice during the consolidation phase. Similarly, type X collagen-positive cells were markedly decreased in Fgfr3^ach mice during the same period. Fgfr3^ach mice exhibited accelerated bone regeneration after DO. Accelerated endochondral ossification could contribute to faster healing in Fgfr3^ach mice.

FGFR3 deficient mice have accelerated fracture repair

Bone fracture healing is processed through multiple biological stages that partly recapitulates the skeletal development process. FGFR3 is a negative regulator of chondrogenesis during embryonic stage and plays an important role in both chondrogenesis and osteogenesis. We have investigated the role of FGFR3 in fracture healing using unstabilized fracture model and found that gain-of-function mutation of FGFR3 inhibits the initiation of chondrogenesis during cartilage callus formation. Here, we created closed, stabilized proximal tibia fractures with an intramedullary pin in Fgfr3^-/-mice and their littermate wild-type mice. Fracture healing was evaluated by radiography, micro-CT, histology, and real-time polymerase chain reaction (RT-PCR) analysis. The fractured Fgfr3^-/- mice had increased formation of cartilaginous callus, more fracture callus, and more rapid endochondral ossification in fracture sites with up-regulated expressions of chondrogenesis related gene. The fractures of Fgfr3^-/- mice healed faster with accelerated fracture callus mineralization and up-regulated expression of osteoblastogenic genes. The healing of fractures in Fgfr3^-/- mice was accelerated in the stage of formation of cartilage and endochondral ossification. Downregulation of FGFR3 activity can be considered as a potential bio-therapeutic strategy for fracture treatment.

MicroRNA-455-3p modulates cartilage development and degeneration through modification of histone H3 acetylation

Histone acetylation regulated by class I histone deacetylases (HDACs) plays a pivotal role in matrix-specific gene transcription and cartilage development. While we previously demonstrated that microRNA (miR)-455-3p is upregulated during chondrogenesis and can enhance early chondrogenesis, the mechanism underlying this process remains largely unclear. In this study, we characterized the effect of miR-455-3p on histone H3 acetylation and its role during cartilage development and degeneration. We observed that miR-455-3p was highly expressed in proliferating and pre-hypertrophic chondrocytes, while HDAC2 and HDAC8 were primarily expressed in hypertrophic chondrocytes. Meanwhile, miR-455-3p suppressed the activity of reporter constructs containing the 3'-untranslated regions of HDAC2/8, inhibited HDAC2/8 expression and promoted histone H3 acetylation at the collagen 2 (COL2A1) promoter in human SW1353 chondrocyte-like cells. Treatment with the HDAC inhibitor trichostatin A (TSA) resulted in increased expression of cartilage-specific genes and promoted glycosaminoglycan deposition. Moreover, TSA inhibited matrix metalloproteinase 13 (Mmp13) expression and promoted nuclear translocation of SOX9 in interleukin-1-treated primary mouse chondrocytes. Lastly, knockdown of HDAC2/3/8 increased SRY (sex-determining region Y)-box 9 (SOX9) and decreased Runt-related transcription factor 2 (RUNX2) expression. Taken together, these findings suggest that miR-455-3p plays a critical role during chondrogenesis by directly targeting HDAC2/8 and promoting histone H3 acetylation, which raises possibilities of using miR-455-3p to influence chondrogenesis and cartilage degeneration.

A new hypothesis for the cancer mechanism

Several observations have led us to a new hypothesis for cancer mechanism. First, that cancer appears only on those multicellular organisms with complicated wound-healing capacities. Second, that wounds considered as risk factors can be identified in all cancers in clinics. And finally, that oncogene activation appears not only in cancer, but also in normal physiology and noncancer pathology processes. Our proposed hypothesis is that cancer is a natural wound healing-related process, which includes oncogene activations, cytokine secretions, stem cell recruitment differentiation, and tissue remodeling. Wounds activate oncogenes of some cells and the latter secrete cytokines to recruit stem cells to heal the wounds. However, if the cause of the wound or if the wound persists, such as under the persistent UV and carcinogen exposures, the continuous wound healing process will lead to a clinical cancer mass. There is no system in nature to stop or reverse the wound healing process in the middle stage when the wound exists. The outcome of the cancer mechanism is either healing the wound or exhausting the whole system (death). The logic of this cancer mechanism is consistent with the rationales of the other physiological metabolisms in the body-for survival. This hypothesis helps to understand many cancer mysteries derived from the mutation theory, such as why cancer only exists in a small proportion of multicellular organisms, although they are all under potential mutation risks during DNA replications. The hypothesis can be used to interpret and guide cancer prevention, recurrence, metastasis, in vitro and in vivo studies, and personalized treatments.

Reactive oxygen species generated by NADPH oxidase 2 and 4 are required for chondrogenic differentiation

Although generation of reactive oxygen species (ROS) by NADPH oxidases (Nox) is thought to be important for signal transduction in nonphagocytic cells, little is known of the role ROS plays in chondrogenesis. We therefore examined the possible contribution of ROS generation to chondrogenesis using both ATDC5 cells and primary chondrocytes derived from mouse embryos. The intracellular level of ROS was increased during the differentiation process, which was then blocked by treatment with the ROS scavenger N-acetylcysteine. Expression of Nox1 and Nox2 was increased upon differentiation of ATDC5 cells and primary mouse chondrocytes, whereas that of Nox4, which was relatively high initially, was decreased gradually during chondrogenesis. In developing limb, Nox1 and Nox2 were highly expressed in prehypertrophic and hypertrophic chondrocytes. However, Nox4 was highly expressed in proliferating chondrocytes and prehypertrophic chondrocytes. Depletion of Nox2 or Nox4 expression by RNA interference blocked both ROS generation and differentiation of ATDC5 cells, whereas depletion of Nox1 had no such effect. We also found that ATDC5 cells depleted of Nox2 or Nox4 underwent apoptosis. Further, inhibition of Akt phosphorylation along with subsequent activation of ERK was observed in the cells. Finally, depletion of Nox2 or Nox4 inhibited the accumulation of proteoglycan in primary chondrocytes. Taken together, our data suggest that ROS generated by Nox2 or Nox4 are essential for survival and differentiation in the early stage of chondrogenesis.

Healing of non-displaced fractures produced by fatigue loading of the mouse ulna

We developed a fatigue loading protocol in mice to produce a non-displaced ulnar fracture in vivo, and characterized the early healing response. Using adult (5 month) C57Bl/6 mice, we first determined that cyclic compression of the forelimb under load-control leads to increasing applied displacement and, eventually, complete fracture. We then subjected the right forelimbs of 80 mice to cyclic loading (2 Hz; peak force approximately 4N) and limited the displacement increase to 0.75 mm (60% of the average displacement increase at complete fracture). This fatigue protocol created a partial, non-displaced fracture through the medial cortex near the ulnar mid-shaft, and reduced ulnar strength and stiffness by >50%. Within 1 day, there was significant upregulation of genes related to hypoxia (Hif1a) and osteogenesis (Bmp2, Bsp) in loaded ulnae compared to non-loaded, contralateral controls. The gene expression response peaked in magnitude near day 7 (e.g., Osx upregulated 8-fold), and included upregulation of FGF-family genes (e.g., Fgfr3 up 6-fold). Histologically, a localized periosteal response was seen at the site of the fracture; by day 7 there was abundant periosteal woven bone surrounding a region of cartilage. From days 7 to 14, the woven bone became denser but did not increase in area. By day 14, the woven-bone response resulted in complete recovery of ulnar strength and stiffness, restoring mechanical properties to normal levels. In the future, the fatigue loading approach can be used create non-displaced bone fractures in transgenic and knockout mice to study the mechanisms by which the skeleton rapidly repairs damage.

Fibroblast growth factor expression during skeletal fracture healing in mice

Fibroblast growth factors (FGFs) are important signaling molecules that regulate many stages of endochondral bone development. During the healing of a skeletal fracture, several features of endochondral bone development are reactivated. To better understand the role of FGFs in skeletal fracture healing, we quantitatively evaluated the temporal expression patterns of Fgfs, Fgf receptors (Fgfrs), and molecular markers of bone development over a 14-day period following long bone fracture in a mouse model. These studies identify distinct groups of FGFs that are differentially expressed and suggest active stage-specific roles for FGF signaling during the fracture repair process.

Gain-of-function mutation of FGFR3 results in impaired fracture healing due to inhibition of chondrocyte differentiation

Fracture healing is a complicated regeneration process which to some extent recapitulates bone development. Fibroblast growth factor receptor 3 (FGFR3) has a negative regulatory effect on endochondral ossification, and FGFR3 is also expressed in prehypertrophic and hypertrophic chondrocytes during fracture healing. However, the actual role of FGFR3 during bone regeneration is not fully understood. Therefore we investigated the role of FGFR3 in fracture repair using a non-stabilized fracture model. Fracture repair in gain-of-function mutation of FGFR3 (Fgfr3(G369C/+)) mice was delayed, with more cartilage callus on day 14 and residue of cartilage in the callus on day 21. Histologic, in-situ hybridization and qRT-PCR analysis showed that differentiation of mesenchymal cells into chondrocytes and hypertrophic differentiation was delayed in Fgfr3(G369C/+) mice during fracture healing. These results indicated that activating mutation of FGFR3 could lead to impaired bone repair due to inhibition of chondrocyte differentiation.

Comparative toxicogenomic analysis of the hepatotoxic effects of TCDD in Sprague Dawley rats and C57BL/6 mice

In an effort to further characterize conserved and species-specific mechanisms of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated toxicity, comparative temporal and dose-response microarray analyses were performed on hepatic tissue from immature, ovariectomized Sprague Dawley rats and C57BL/6 mice. For temporal studies, rats and mice were gavaged with 10 or 30 microg/kg of TCDD, respectively, and sacrificed after 2, 4, 8, 12, 18, 24, 72, or 168 h while dose-response studies were performed at 24 h. Hepatic gene expression profiles were monitored using custom cDNA microarrays containing 8567 (rat) or 13,361 (mouse) cDNA clones. Affymetrix data from male rats treated with 40 microg/kg TCDD were also included to expand the species comparison. In total, 3087 orthologous genes were represented in the cross-species comparison. Comparative analysis identified 33 orthologous genes that were commonly regulated by TCDD as well as 185 rat-specific and 225 mouse-specific responses. Functional annotation using Gene Ontology identified conserved gene responses associated with xenobiotic/chemical stress and amino acid and lipid metabolism. Rat-specific gene expression responses were associated with cellular growth and lipid metabolism while mouse-specific responses were associated with lipid uptake/metabolism and immune responses. The common and species-specific gene expression responses were also consistent with complementary histopathology, clinical chemistry, hepatic lipid analyses, and reports in the literature. These data expand our understanding of TCDD-mediated gene expression responses and indicate that species-specific toxicity may be mediated by differences in gene expression which may help explain the wide range of species sensitivities and will have important implications in risk assessment strategies.

Effects of low-dose, intermittent treatment with recombinant human parathyroid hormone (1-34) on chondrogenesis in a model of experimental fracture healing

Recent studies have demonstrated that intermittent administration of parathyroid hormone (PTH) enhances osteogenesis (hard callus formation) and increases mechanical strength in experimental fracture healing. Thus far, however, effects of PTH on chondrogenesis (soft callus formation) during fracture healing have not been fully elucidated. In the present study, we analyzed the underlying molecular mechanism by which exogenous PTH would affect chondrogenesis in a model of experimental fracture healing. Unilateral femoral fractures were produced in 2-month-old Sprague-Dawley rats. Daily subcutaneous injections of 10 microg/kg of recombinant human PTH(1-34) [rhPTH(1-34)] were administered over a 28-day period of fracture healing. Control animals were injected with vehicle solution (normal saline) alone. The results showed that, on day 14 after fracture, cartilage area in the PTH-treated group was significantly increased (1.4-fold) compared with the controls, but this increase was not observed at days 21 and 28. In the early stage of chondrogenesis (days 4-7), cell proliferation, expressed as the rate of proliferating cell nuclear antigen-positive cells, was increased in mesenchymal (chondroprogenitor) cells but not chondrocytes in the PTH-treated group compared with controls. In addition, gene expression of SOX-9 was up-regulated in the PTH-treated group on day 4 (1.4-fold), and this was accompanied by enhanced expression of pro-alpha1 (II) collagen (1.8-fold). After 14 days, there were no significant differences between groups in either cell proliferation or the expression levels of cartilage differentiation-related genes (SOX-9, pro-alpha1 (II) collagen, pro-alpha1 (X) collagen and osteopontin). These results suggest that intermittent treatment with low-dose rhPTH(1-34) induces a larger cartilaginous callus but does not delay chondrocyte differentiation during fracture healing.

Intact fibula improves fracture healing in a rat tibia osteotomy model

Rat tibia fractures are often used in fracture healing studies. Usually the fracture is stabilized with an intramedullary pin, which provides bending stiffness, but little torsional stiffness. The objective of this research was to determine the in vitro torsional rigidity of an osteotomized tibia with and without the fibula, and to determine if this difference influences the healing process in vivo. In vitro eleven rat tibias received an osteotomy, were stabilized with an intramedullary pin, and were tested in internal rotation to determine the torsional rigidity. The fibula was then manually broken and the torsional rigidity measured again. In vivo 18 rats received a tibial osteotomy, eight of which had an additional fractured fibula. After three weeks, the rats were sacrificed and the tibias were analyzed. Bone density in the fracture callus was measured with qCT. Bending rigidity and maximum breaking moment were determined in three-point bending. In vitro testing demonstrated that the torsional rigidity with an intact fibula was nearly two times higher than when the fibula was fractured. Though the torsional rigidity was still small in comparison with an intact bone, it resulted in a significantly different healing process in vivo. Rats with intact fibulas had significantly higher bone mineral density, bending rigidity, and maximum breaking moment compared to rats with a fractured fibula. These results indicate that torsional stability considerably affects the healing process. In a fracture model, it is critical to characterize the mechanical environment of the fracture.

Histochemical evidence of the initial chondrogenesis and osteogenesis in the periosteum of a rib fractured model: implications of osteocyte involvement in periosteal chondrogenesis

We have examined cellular events at the early stages of periosteal chondrogenesis and osteogenesis induced by bone fracture, using a well-standardized rib fracture model of the mouse. The initial cellular event was recognized as considerable proliferation in the deeper layer referred to as the "cambium layer" of the periosteum, as evidenced by numerous proliferating cell nuclear antigen-positive cells. The periosteal cartilage and bone were then regenerated directly from the region of the most-differentiated cell, i.e., mature osteoblasts of the cambium layer both close to and distant from the fracture site. Therefore, periosteal osteoblasts appeared to have the potential to differentiate into chondrogenic and osteoblastic lineages. CD31-positive blood vessels were uniformly localized along the periosteum that was regenerating cartilage and bone, being therefore indicative of less influence on the initiation of osteochondrogenesis. In contrast, however, the regenerated periosteal cartilage or bone extended from the cortical bones included dead or living osteocytes, respectively. Empty lacunae and lacunae embedded with amorphous materials were found close to the regenerated cartilage, while intact osteocytes persisted adjacent to the regenerated bone. The embedded lacunae with amorphous materials would render the tissue fluid, nutrients, oxygen, and several secretory factors such as dentin matrix protein-1 impossible to be delivered to the periosteal osteoblasts that interconnect osteocytes via gap junctions. Our study thus provides two major clues on initial cellular events in response to bone fracture: the potentiality of periosteal osteoblastic differentiation into a chondrogenic lineage, and a putative involvement of osteocytes in periosteal cartilage and bone regeneration.