Differential expression of fibrillar collagen genes during callus formation

Comparison of healing process in open osteotomy model and open fracture model: Delayed healing of osteotomies after intramedullary screw fixation

Murine osteotomy and fracture models have become the standard to study molecular mechanisms of bone healing. Because there is little information whether the healing of osteotomies differs from that of fractures, we herein studied in mice the healing of femur osteotomies compared to femur fractures. Twenty CD-1 mice underwent a standardized open femur osteotomy. Another 20 mice received a standardized open femur fracture. Stabilization was performed by an intramedullary screw. Bone healing was studied by micro-CT, biomechanical, histomorphometric and protein expression analyses. Osteotomies revealed a significantly lower biomechanical stiffness compared to fractures. Micro-CT showed a reduced bone/tissue volume within the callus of the osteotomies. Histomorphometric analyses demonstrated also a significantly lower amount of osseous tissue in the callus of osteotomies (26% and 88% after 2 and 5 weeks) compared to fractures (50% and 100%). This was associated with a delayed remodeling. Western blot analyses demonstrated comparable BMP-2 and BMP-4 expression, but higher levels of collagen-2, CYR61 and VEGF after osteotomy. Therefore, we conclude that open femur osteotomies in mice show a markedly delayed healing when stabilized less rigidly with an intramedullary screw. This should be considered when choosing a model for studying the mechanisms of bone healing in mice.

Changes of procollagen type III N-terminal propeptide (PIIINP) concentrations during healing of mandible fractures treated with biodegradable and titanium fixations

PURPOSE

The aims of the study were to evaluate procollagen type III N-terminal propeptide (PIIINP) concentrations in blood serum of males in the course of normal healing of mandible fractures, and to determine the correlations between kinetic changes of PIIINP, stages of fracture healing and the applied treatment.

MATERIAL AND METHODS

We collected blood serum samples from 43 male patients aged between 20 and 30 years, treated for mandible fractures. The patients were divided into 2 groups depending on the type of osteosynthesis used for immobilization of the fragments. Group I (n=10) consisted of patients whose reduced bone fragments were fixed with biodegradable fixations, while group II (n=33) included patients with titanium osteosynthesis devices. The control group (n=25) consisted of healthy males at the same age. PIIINP concentrations were determined with the radioimmunological method (RIA).

RESULTS

We found a significant increase in PIIINP concentrations in both study groups (I and II) at defined stages of mandible fracture healing. Differences were noticed in the dynamics of the increase depending on the type of applied osteosynthesis devices.

CONCLUSION

The results point to the fact that the injury and/or mandible fracture increase the collagen type III metabolism and its dynamics depends on the type of the used bone fixation.

Recent advances in the use of serological bone formation markers to monitor callus development and fracture healing

The failure of an osseous fracture to heal, or the development of a nonunion, is common; however, current diagnostic measures lack the capability of early and reliable detection of such events. Analyses of radiographic imaging and clinical examination, in combination, remain the gold standard for diagnosis; however, these methods are not reliable for early detection. Delayed diagnosis of a nonunion is costly from both the patient and treatment standpoints. In response, repeated efforts have been made to identify bone metabolic markers as diagnostic or prognostic tools for monitoring bone healing. Thus far, the evidence regarding a correlation between the kinetics of most bone metabolic markers and nonunion is very limited. With the aim of classifying the role of biological pathways of bone metabolism and of understanding bone conditions in the development of osteoporosis, advances have been made in our knowledge of the molecular basis of bone remodeling, fracture healing, and its failure. Procollagen type I amino-terminal propeptide has been shown to be a reliable bone formation marker in osteoporosis therapy and its kinetics during fracture healing has been recently described. In this article, we suggest that procollagen type I amino-terminal propeptide presents a good opportunity for early detection of nonunion. We also review the role and potential of serum PINP, as well as other markers, as indications of fracture healing.

Are bone turnover markers capable of predicting callus consolidation during bone healing?

The aim of this study was to determine the ability of the following bone turnover markers to monitor the course of callus consolidation during bone healing: Carboxy-terminal propeptide of procollagen type I (PICP), skeletal alkaline phosphatase (sALP), and amino-terminal propeptide of type III procollagen (PIlINP). Since interfragmentary movements have been proven to possess the ability to document the progression of bone healing in experimental studies, correlations between bone turnover markers and interfragmentary movements in vivo were investigated. Therefore, two different types of osteosyntheses representing different mechanical situations at the fracture site were compared in an ovine osteotomy model. Blood samples were taken preoperatively and postoperatively in weekly intervals over a nine-week healing period. At the same intervals, interfragmentary movements were measured in all sheep. After nine weeks, animals were sacrificed and the tibiae were evaluated both mechanically and histologically. Wide interindividual ranges were observed for all bone turnover markers. The systemic PICP level did not increase with callus consolidation. The bone-healing model seemed to influence the systemic level of PIIINP and sALP but no general correlation between bone turnover markers and interfragmentary movements could be detected. No differences between the different types of osteosyntheses and thus the different mechanical situations were observed. All analyzed markers failed as general predictors for the course of callus consolidation during bone healing.

Noninvasive in situ evaluation of osteogenic differentiation by time-resolved laser-induced fluorescence spectroscopy

The clinical implantation of bioengineered tissues requires an in situ nondestructive evaluation of the quality of tissue constructs developed in vitro before transplantation. Time-resolved laser-induced fluorescence spectroscopy (TR-LIFS) is demonstrated here to noninvasively monitor the formation of osteogenic extracellular matrix (ECM) produced by putative stem cells (PLA cells) derived from human adipose tissue. We show that this optical spectroscopy technique can assess the relative expression of collagens (types I, III, IV, and V) within newly forming osteogenic ECM. The results are consistent with those obtained by conventional histochemical techniques (immunofluorescence and Western blot) and demonstrate that TR-LIFS is a potential tool for monitoring the expression of distinct collagen types and the formation of collagen cross-links in intact tissue constructs.

The molecular biology of distraction osteogenesis

Distraction osteogenesis has become a mainstay in bone tissue engineering and has significantly improved our armamentarium for reconstructive craniomaxillofacial procedures. However, although the biomechanical, histological, and ultrastructural changes associated with distraction osteogenesis have been widely described, the molecular mechanisms governing the formation of new bone in the interfragmental gap of gradually distracted bone segments remain largely unclear. Recently, a rat model of mandibular distraction was described that provides an excellent environment for deciphering the molecular mechanisms that mediate distraction osteogenesis. This article presents the hypotheses and current research that have furthered knowledge of the molecular mechanisms that govern distraction osteogenesis. Recent studies have implicated a growing number of cytokines that are intimately involved in the regulation of bone synthesis and turnover. The gene regulation of numerous cytokines (transforming growth factor-beta1, -beta2, -beta3, bone morphogenetic proteins, insulin-like growth factor-1, fibroblast growth factor-2) and extracellular matrix proteins (osteonectin, osteopontin) during distraction osteogenesis have been best characterized and are discussed in this article. It is believed that understanding the biomolecular mechanisms that mediate membranous distraction osteogenesis may guide the development of targeted strategies designed to improve distraction osteogenesis and accelerate bone healing.

Hypoxia regulates osteoblast gene expression

Vascular disruption secondary to fracture creates a hypoxic gradient of injury wherein the oxygen tension at the center of the wound is very low. In vivo this hypoxic microenvironment stimulates the expression of a variety of cytokines from inflammatory cells, fibroblasts, endothelial cells, and osteoblasts. In order to begin to dissect this complex system, we have examined the effects of hypoxia on isolated osteoblast gene expression in vitro. Understanding gene expression in this system may facilitate the development of targeted therapeutic modalities designed to accelerate fracture repair and reduce complications. Using an established model of in vitro hypoxia, we have analyzed the expression of genes involved in bone matrix production and turnover. Subconfluent neonatal rat calvarial osteoblasts were exposed to hypoxia (pO(2) = 35-40 mm Hg) and total cellular RNA was collected at 0, 3, 6, 24, and 48 h. Northern analysis was used to analyze the expression patterns of (1) transforming growth factors (TGFs)-beta1, -beta2, and -beta3 and their type I receptor; (2) collagens I and III; and (3) tissue inhibitor of metalloproteinase-1. We have demonstrated a marked elevation of TGF-beta1 gene expression within 3 h of hypoxia. Although neither TGF-beta2 nor TGF-beta3 expression was affected by hypoxia, the TGF-beta type I receptor was substantially upregulated within 6 h. In addition, extracellular matrix scaffolding molecules (collagens I and III) were markedly, but differentially, upregulated. Finally, we have demonstrated that the expression of an inhibitor of extracellular matrix turnover, the tissue inhibitor of metalloproteinase-1, was strikingly decreased in response to hypoxia. These results imply that hypoxia can affect osseous healing by altering the expression of cytokines, bone-specific extracellular matrix molecules, and their regulators.

Rat mandibular distraction osteogenesis: part III. Gradual distraction versus acute lengthening

Distraction osteogenesis is a well-established method of endogenous tissue engineering. This technique has significantly augmented our armamentarium of reconstructive craniofacial procedures. Although the histologic and ultrastructural changes associated with distraction osteogenesis have been extensively described, the molecular mechanisms governing successful membranous distraction remain unknown. Using an established rat model, the molecular differences between successful (i.e., osseous union with gradual distraction) and ineffective (i.e., fibrous union with acute lengthening) membranous bone lengthening was analyzed. Herein, the first insight into the molecular mechanisms of successful membranous bone distraction is provided. In addition, these data provide the foundation for future targeted therapeutic manipulations designed to improve osseous regeneration. Vertical mandibular osteotomies were created in 52 adult male Sprague-Dawley rats, and the animals were fitted with customized distraction devices. Twenty-six animals underwent immediate acute lengthening (3 mm; a length previously shown to result in fibrous union) and 26 animals were gradually distracted (after a 3-day latency period, animals were distracted 0.25 mm twice daily for 6 days; total = 3 mm). Four mandibular regenerates were harvested from each group for RNA analysis on 5, 7, 9, 23, and 37 days postoperatively (n = 40). Two mandibular regenerates were also harvested from each group and prepared for immunohistochemistry on postoperative days 5, 7, and 37 (n = 12). In addition to the 52 experimental animals, 4 control rats underwent sham operations (skin incision only) and mandibular RNA was immediately collected. Control and experimental specimens were analyzed for collagen I, osteocalcin, tissue inhibitor of metalloproteinase-1, and vascular endothelial growth factor mRNA and protein expression. In this study, marked elevation of critical extracellular matrix molecules (osteocalcin and collagen I) during the consolidation phase of gradual distraction compared with acute lengthening is demonstrated. In addition, the expression of an inhibitor of extracellular matrix turnover, tissue inhibitor of metalloproteinase-1, remained strikingly elevated in gradually distracted animals. Finally, this study demonstrated that neither gradual distraction nor acute lengthening appreciably alters vascular endothelial growth factor expression. These results suggest that gradual distraction osteogenesis promotes successful osseous bone repair by regulating the expression of bone-specific extracellular matrix molecules. In contrast, decreased production or increased turnover of bone scaffolding proteins (i.e., collagen) or regulators of mineralization (i.e., osteocalcin) may lead to fibrous union during acute lengthening.

Collagen structure regulates fibril mineralization in osteogenesis as revealed by cross-link patterns in calcifying callus

Although >80% of the mineral in mammalian bone is present in the collagen fibrils, limited information is available about factors that determine a proper deposition of mineral. This study investigates whether a specific collagen matrix is required for fibril mineralization. Calcifying callus from dog tibias was obtained at various times (3-21 weeks) after fracturing. At 3 weeks, hydroxylysine (Hyl) levels were almost twice as high as in control bone, gradually reaching normal levels at 21 weeks. The decrease in Hyl levels can only be the result of the formation of a new collagen network at the expense of the old one. The sum of the cross-links hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP) in callus matched that of bone at all stages of maturation. However, the ratio HP/LP was 2.5-4.5 times higher in callus at 3-7 weeks than in normal bone and was normalized at 21 weeks. Some 40% of the collagen was nonmineralized at the early stages of healing, reaching control bone values (approximately 10%) at 21 weeks. In contrast, only a small increase in callus mineral content from 20.0 to 22.6 (% of dry tissue weight) from week 3 to 21 was seen, indicating that initially a large proportion of the mineral was deposited between, and not within, the fibrils. A strong relationship (r = 0.80) was found between the ratio HP/LP and fibril mineralization; the lower the HP/LP ratio, the more mineralized the fibrils were. Because the HP/LP ratio is believed to be the result of a specific packing of intrafibrillar collagen molecules, this study implies that mineralization of fibrils is facilitated by a specific orientation of collagen molecules in the fibrils.

Gene expression of TGF-beta, TGF-beta receptor, and extracellular matrix proteins during membranous bone healing in rats

Poorly healing mandibular fractures and osteotomies can be troublesome complications of craniomaxillofacial trauma and reconstructive surgery. Gene therapy may offer ways of enhancing bone formation by altering the expression of desired growth factors and extracellular matrix molecules. The elucidation of suitable candidate genes for therapeutic intervention necessitates investigation of the endogenously expressed patterns of growth factors during normal (i.e., successful) fracture repair. Transforming growth factor beta1 (TGF-beta1), its receptor (Tbeta-RII), and the extracellular matrix proteins osteocalcin and type I collagen are thought to be important in long-bone (endochondral) formation, fracture healing, and osteoblast proliferation. However, the spatial and temporal expression patterns of these molecules during membranous bone repair remain unknown. In this study, 24 adult rats underwent mandibular osteotomy with rigid external fixation. In addition, four identically treated rats that underwent sham operation (i.e., no osteotomy) were used as controls. Four experimental animals were then killed at each time point (3, 5, 7, 9, 23, and 37 days after the procedure) to examine gene expression of TGF-beta1 and Tbeta-RII, osteocalcin, and type I collagen. Northern blot analysis was used to compare gene expression of these molecules in experimental animals with that in control animals (i.e., nonosteotomized; n = 4). In addition, TGF-beta1 and T-RII proteins were immunolocalized in an additional group of nine animals killed on postoperative days 3, 7, and 37. The results of Northern blot analysis demonstrated a moderate increase (1.7 times) in TGF-beta1 expression 7 days postoperatively; TGF-beta1 expression returned thereafter to near baseline levels. Tbeta-RII mRNA expression was downregulated shortly after osteotomy but then increased, reaching a peak of 1.8 times the baseline level on postoperative day 9. Osteocalcin mRNA expression was dramatically downregulated shortly after osteotomy and remained low during the early phases of fracture repair. Osteocalcin expression trended slowly upward as healing continued, reaching peak expression by day 37 (1.7 times the control level). In contrast, collagen type IalphaI mRNA expression was acutely downregulated shortly after osteotomy, peaked on postoperative days 5, and then decreased at later time points. Histologic samples from animals killed 3 days after osteotomy demonstrated TGF-beta1 protein localized to inflammatory cells and extracellular matrix within the fracture gap, periosteum, and peripheral soft tissues. On postoperative day 7, TGF-beta1 staining was predominantly localized to the osteotomized bone edges, periosteum, surrounding soft tissues, and residual inflammatory cells. By postoperative day 37, complete bony healing was observed, and TGF-beta1 staining was localized to the newly formed bone matrix and areas of remodeling. On postoperative day 3, Tbeta-RII immunostaining localized to inflammatory cells within the fracture gap, periosteal cells, and surrounding soft tissues. By day 7, Tbeta-RII staining localized to osteoblasts of the fracture gap but was most intense within osteoblasts and mesenchymal cells of the osteotomized bone edges. On postoperative day 37, Tbeta-RII protein was seen in osteocytes, osteoblasts, and the newly formed periosteum in the remodeling bone. These observations agree with those of previous in vivo studies of endochondral bone formation, growth, and healing. In addition, these results implicate TGF-beta1 biological activity in the regulation of osteoblast migration, differentiation, and proliferation during mandibular fracture repair. Furthermore, comparison of these data with gene expression during mandibular distraction osteogenesis may provide useful insights into the treatment of poorly healing fractures because distraction osteogenesis has been shown to be effective in the management of these difficult clinical cases.

Biomolecular mechanisms of calvarial bone induction: immature versus mature dura mater

The ability of newborns and immature animals to reossify calvarial defects has been well described. This capacity is generally lost in children greater than 2 years of age and in mature animals. The dura mater has been implicated as a regulator of calvarial reossification. To date, however, few studies have attempted to identify biomolecular differences in the dura mater that enable immature, but not mature, dura to induce osteogenesis. The purpose of these studies was to analyze metabolic characteristics, protein/gene expression, and capacity to form mineralized bone nodules of cells derived from immature and mature dura mater. Transforming growth factor beta-1, basic fibroblast growth factor, collagen type IalphaI, osteocalcin, and alkaline phosphatase are critical growth factors and extracellular matrix proteins essential for successful osteogenesis. In this study, we have characterized the proliferation rates of immature (6-day-old rats, n = 40) and mature (adult rats, n = 10) dura cell cultures. In addition, we analyzed the expression of transforming growth factor beta-1, basic fibroblast growth factor-2, proliferating cell nuclear antigen, and alkaline phosphatase. Our in vitro findings were corroborated with Northern blot analysis of mRNA expression in total cellular RNA isolated from snap-frozen age-matched dural tissues (6-day-old rats, n = 60; adult rats, n = 10). Finally, the capacity of cultured dural cells to form mineralized bone nodules was assessed. We demonstrated that immature dural cells proliferate significantly faster and produce significantly more proliferating cell nuclear antigen than mature dural cells (p < 0.01). Additionally, immature dural cells produce significantly greater amounts of transforming growth factor beta-1, basic fibroblast growth factor-2, and alkaline phosphatase (p < 0.01). Furthermore, Northern blot analysis of RNA isolated from immature and mature dural tissues demonstrated a greater than 9-fold, 8-fold, and 21-fold increase in transforming growth factor beta-1, osteocalcin, and collagen IalphaI gene expression, respectively, in immature as compared with mature dura mater. Finally, in keeping with their in vivo phenotype, immature dural cells formed large calcified bone nodules in vitro, whereas mature dural cells failed to form bone nodules even with extended culture. These studies suggest that differential expression of growth factors and extracellular matrix molecules may be a critical difference between the osteoinductive capacity of immature and mature dura mater. Finally, we believe that the biomolecular bone- and matrix-inducing phenotype of immature dura mater regulates the ability of young children and immature animals to heal calvarial defects.

Cartilage-derived retinoic acid-sensitive protein and type II collagen expression during fracture healing are potential targets for Sox9 regulation

Cartilage-derived retinoic acid-sensitive protein (CD-RAP) and mRNA were examined in the mouse fracture model by immunohistochemistry and Northern blot analysis and compared with the expression of type II collagen. We also studied the expression of the transcription factor Sox9, reported to enhance type II collagen and CD-RAP gene expression in vitro. CD-RAP was first detected in immature chondrocytes on day 5. Intense signals for CD-RAP were found in fracture cartilage on days 7 and 9. CD-RAP decreased at the phase of endochondral ossification. Throughout fracture healing, CD-RAP was detected in cartilage and not in bone or fibrous tissue, thus CD-RAP may be a molecular marker of cartilage formation during fracture healing. Northern blot analysis revealed similar changes in CD-RAP and type II collagen mRNA levels. However, with respect to protein levels, CD-RAP decreased faster than type II collagen implying the stability is lower than type II collagen. Increased levels of Sox9 mRNA and protein were detected on day 5 and coincided with the initial increase of CD-RAP and type II collagen mRNAs. Sox9 mRNA levels declined with the progress of chondrocyte hypertrophy, followed by a concomitant decrease in CD-RAP and type II collagen mRNA levels. These changes in Sox9 expression compared with the cartilage-specific genes (CD-RAP and type II collagen) suggest that cell differentiation during fracture healing may be controlled by specific transcriptional factors which regulate phenotypic changes of the cells.

Rat mandibular distraction osteogenesis: II. Molecular analysis of transforming growth factor beta-1 and osteocalcin gene expression

Distraction osteogenesis is a powerful technique capable of generating viable osseous tissue by the gradual separation of osteotomized bone edges. Although the histologic and ultrastructural changes associated with this process have been extensively delineated, the molecular events governing these changes remain essentially unknown. We have devised a rat model of mandibular distraction osteogenesis that facilitates molecular analysis of this process. Such information has significant clinical implications because it may enable targeted therapeutic manipulations designed to accelerate osseous regeneration. In this study, we have evaluated the expression of transforming growth factor beta-1, a major regulator of osteogenesis during endochondral bone formation and development, and osteocalcin, an abundant noncollagenous extracellular matrix protein implicated in the regulation of mineralization and bone turnover. The right hemimandible of 36 adult male rats was osteotomized, and a customized distraction device was applied. Animals were allowed to recover and, after a 3-day latency period, were distracted at a rate of 0.25 mm twice daily for 6 days followed by a 2- or 4-week consolidation period. Distraction regenerate was harvested after the latency period, days 2, 4, or 6 of distraction, and after 2 or 4 weeks of consolidation and processed for Northern analysis (n = 4 at each time point) and immunohistochemical localization of TGF-beta1 (n = 2 at each time point). Six sham-operated animals (i.e., skin incision without osteotomy) were also killed (immediately postoperatively), and the mandibles were harvested and prepared in a similar fashion. Equal loading and transfer of RNA for Northern analysis was ensured by stripping and probing membranes with a probe against GAPDH (a housekeeping gene). Our results demonstrate that the spatial and temporal patterns of TGF-beta1 mRNA expression and protein production coincide with osteoblast migration, differentiation, and extracellular matrix synthesis. In addition, we demonstrate that TGF-beta1 production may be an important regulator of vasculogenesis during mandibular distraction osteogenesis. Finally, we have shown that osteocalcin gene expression coincides temporally with mineralization during rat mandibular distraction osteogenesis.

Cloning of a novel cDNA expressed during the early stages of fracture healing

Using differential mRNA display (DD-PCR), a novel cDNA, FxC1 (Fracture Callus 1) was isolated from the early stages of a healing fractured femur. Utilizing 5' RACE PCR, a 598-bp full-length cDNA was obtained for FxC1 that contains an open reading frame (ORF) of 243 bp, encoding for an 80 amino acid protein. Within this ORF, a leucine zipper motif was present. In vitro transcription/translation of the full-length cDNA generated the expected 9-kDa protein. Northern analysis reveals that this gene is expressed in calluses harvested from post-fracture day 5, 7 and 10, as well as in several other tissues and bone-derived cell lines. During the differentiation of MC3T3 cells along the osteoblast lineage, FxC1 expression increases 3- to 4-fold during the production and deposition of matrix proteins, suggesting a possible role for this protein in cell differentiation.

Serology of collagen types I and III in normal healing of tibial shaft fractures

OBJECTIVES

To establish that serological indicators of synthesis of collagens I and III in humans with normally healing tibial shaft fractures will display a temporal pattern consistent with the sequence of expression of these collagens observed histologically in animal models of fracture healing.

DESIGN

Prospective.

SETTING

Four district general hospitals in the United Kingdom supported by an academic unit.

PARTICIPANTS

Twenty consecutive patients with isolated tibial shaft fractures were studied. On clinical and radiological grounds, seventeen of the cases united within twenty weeks; these seventeen cases formed the material for this review.

INTERVENTIONS

Nineteen patients were treated conservatively, and a functional brace was applied at five weeks. One patient treated with an external fixator had a functional brace applied at twelve weeks.

MAIN OUTCOME MEASURES

Assays of collagen I carboxy-terminal propeptide (PICP), collagen III amino-terminal propeptide (PIIINP), and collagen I carboxy-terminal telopeptide (ICTP) were made in serum samples taken at standard intervals from twenty-four hours to twenty weeks after fracture.

RESULTS

PICP showed a significant, transient early drop, whereas ICTP rose, indicating early breakdown with uncoupling of collagen I formation and degradation. PIIINP levels rose significantly from day eight to week five, in keeping with early expression of collagen III in experimental fracture callus. PICP levels showed a gradual rise, consistent with later expression of collagen I.

CONCLUSIONS

The changes observed are in keeping with previous histological work on animal fracture models, suggesting that serological measures may reflect events occurring at the fracture site and thus offer a means for noninvasive and dynamic observation of collagen synthesis after fracture. Final proof that such measurements reflect bony healing per se, as opposed to events in other tissues, would require comparison with similar data from a soft tissue injury model.

Mature osteoblasts in human non-union fractures express collagen type III

AIMS

High levels of collagen type III are biochemically detectable in biopsies of non-uniting fractures, and in the serum of patients suffering from this condition. The aim of this study was to determine whether the expression of collagen type III was limited to fibrous tissue in non-unions, or whether some was present in bone.

METHODS

Biopsies from normally healing human fractures and non-unions were examined using in situ hybridisation and immunohistochemistry.

RESULTS

The mesenchymal cell population, which includes fibroblast and osteoblast precursors, expressed mRNA for collagen type III. However, mature osteoblasts on the surface of woven bone varied profoundly between normally healing fractures (in which they were negative or occasionally weakly positive) and non-unions (in which they were strongly positive). Areas of woven bone that had osteoblasts positive for collagen type III mRNA also immunostained positively for the protein.

CONCLUSIONS

This study shows that non-union fracture callus osteoblasts on the surfaces of woven bone exhibit an unusual phenotype: they express collagen type III, a molecule characteristic of an earlier stage of osteoblast differentiation, which is not expressed by osteoblasts on woven bone surfaces of bone that develops normally. This finding may be useful in developing an early clinical test for impending non-union.

The chick type III collagen gene contains two promoters that are preferentially expressed in different cell types and are separated by over 20 kb of DNA containing 23 exons

Type III collagen is present in prechondrogenic mesenchyme, but not in cartilages formed during endochondral ossification. However, cultured chick chondrocytes contain an unusual transcript of the type III collagen gene in which exons 1-23 are replaced with a previously undescribed exon, 23A; this alternative transcript does not encode type III collagen. This observation suggested that, although production of type III collagen mRNA is repressed in chondrocytes, transcription of the type III collagen gene may continue from an alternative promoter. To test this prediction, we isolated and characterized both the upstream and internal promoters of this gene and tested their ability to direct transcription in chondrocytes and skin fibroblasts. The upstream promoter is active in fibroblasts, but inactive in chondrocytes, indicating that repression of type III collagen synthesis during chondrogenesis is transcriptionally mediated. Additionally, sequences in intron 23, preceding exon 23A, function as a highly active promoter in chondrocytes; transcription from this promoter is repressed in fibroblasts. Thus transcriptional control of the type III collagen gene is highly complex, with two promoters separated by at least 20 kb of DNA that are preferentially expressed in different cell types and give rise to RNAs with different structures and functions.

Biocompatibility testing of NiTi screws using immunohistochemistry on sections containing metallic implants

NiTi is one of the most innovative concepts to have appeared in the field of metallic biomaterials in recent years but its biocompatibility remains controversial. We evaluated the biocompatibility of Nitinol screws using immunohistochemistry to observe the distribution of bone proteins during bone remodeling process around NiTi implant. Results were compared with screws made of Vitallium, c.p. titanium, Duplex austenitic-ferritic stainless steel (SAF), and Stainless Steel 316L. Screws were implanted in rabbit tibia for 3, 6, and 12 weeks. Embedding was performed in the hard resin Technovit, and for the immunohistochemical procedure undecalcified sections with bone-anchored implants could thus be used. The immunostaining method developed seemed to be a reliable technique to stain proteins in undecalcified sections. Biocompatibility results of the NiTi screws compared with the other screws showed a slower osteogenesis process characterized by no close contact between implant and bone, disorganized migration of osteoblasts around the implant, and a lower activity of osteonectin synthesis.

Evidence for insufficient chondrocytic differentiation during repair of full-thickness defects of articular cartilage

The main objective of this study was to characterize the cellular phenotypes in the repair tissue of full-thickness defects of articular cartilage by histologic and molecular biologic techniques. Healing of the defects in the articular cartilage of the knee joints of 12 rabbits was analyzed at days 3, 7, 14, 28 and 50 using histology and Northern analysis of mRNA levels for type I, II and III collagens and osteonectin. The cellular source of each mRNA was determined by in situ hybridization. Two novel cDNA clones for rabbit type II and III collagen mRNAs were constructed to obtain species-specific hybridization probes. The repair tissue of full-thickness defects consisted of two types of tissue. At the bottom of the defect, bone-derived cells with high levels of type I collagen and osteonectin mRNA were actively producing new osteoid, while superficially a slow transition from a fibrin clot into undifferentiated mesenchyme with cells containing type III collagen mRNA was observed. This tissue subsequently became fibrocartilaginous, with small groups of cells turning on the transcription of the type II collagen gene and acquiring a phenotype typical for hyaline cartilage. The data suggest that small clusters of cells in the repair tissue of full-thickness articular cartilage defects are capable of turning on an apparently correct chondrocytic phenotype. The low transcription level of the type II collagen gene suggests, however, that insufficient amounts of fundamentally important regulatory factors or progenitor cells are present in the repair tissue. In the future, such factors should be administrable into the joint by novel therapeutic means.

Expression of type VI, IX and XI collagen genes and alternative splicing of type II collagen transcripts in fracture callus tissue in mice

The levels of six mRNAs coding for constituent alpha-chains of three minor collagens of cartilage were analyzed in an experimental fracture model in normal and transgenic Del1 mice harboring a deletion mutation of exon 7 in the type II collagen gene. Reduced and retarded chondrogenesis in Del1 mice was evident in callus samples as reduced mRNA levels for the cartilage specific type IX and XI collagens at days 7 and 9 of fracture healing. Analysis of the calluses for alternative splicing of pro alpha 1(II) collagen mRNA also suggested retarded chondrogenesis in Del1 calluses. Another developmentally regulated step in limb development, a switch between alternative promoters of the alpha 1(IX) collagen gene, was also seen during fracture healing but was less obvious in Del1 calluses. Finally, the current data suggest that the abnormality in bone remodelling in Del1 mice involves activation of the genes coding for alpha 1(XI) and alpha 2(VI) collagens.

Extended expression of cartilage components in experimental pseudoarthrosis

The healing of femoral fractures in an experimental rat pseudoarthrosis model was followed by studying the expression of cartilage specific genes coding for type II and X collagens and aggrecan, soft tissue and bone specific type I collagen, and decorin. Severe impairment of healing was observed with cartilage gene expression continuing until the seventh week and then declining rapidly. The abnormal healing pattern results in an inactive scar-like callus after the ninth week of healing even though house-keeping (e.g., GAPDH) genes are continuously expressed in the tissue. These results could be explained on the basis of continuous chondrogenic stimulus extending much beyond the normal range. If union is not achieved because of mechanical instability, signal of endochondral ossification persists until it becomes exhausted and callus at the fracture gap becomes an inactive fibrous scar. The disturbed matrix gene expression was confirmed by histology.

Collagen turnover after tibial fractures

Collagen turnover after tibial fractures was examined in 16 patients with fracture of the tibial diaphysis and in 8 patients with fracture in the tibial condyle area by measuring sequential changes in serological markers of turnover of types I and III collagen for up to 26 weeks after fracture. The markers were the carboxy-terminal extension peptide of type I procollagen (PICP), the amino-terminal extension peptide of type III procollagen (PIIINP), and the pyridinoline cross-linked carboxy-terminal telopeptide of type I collagen (ICTP). The latter is a new serum marker of degradation of type I collagen. A group comparison showed characteristic sequential changes in the turnover of types I and III collagen in fractures of the tibial diaphysis and tibial condyles. The turnover of type III collagen reached a maximum after 2 weeks in both groups. The synthesis of type I collagen reached a maximum after 2 weeks in the diaphyseal fractures and after 6 weeks in the condylar fractures. The degradation of type I collagen increased after 4 days and reached a maximum at 2 weeks in both groups. The interindividual variation was wide. On a group basis, the turnover of types I and III collagen had levelled of within 26 weeks, although some patients had clearly elevated parameters at the end of the observational period. We suggest that delayed healing of tibial diaphyseal fractures is accompanied by an early increase in the turnover of types I and III collagen.

A standardized experimental fracture in the mouse tibia

The increased use of transgenic mice as experimental animals provides new opportunities to study the biology of fracture repair. We have developed a technique for the production of a standard closed experimental fracture in the mouse tibia. A 0.2 mm stainless-steel rod was introduced into the medullary cavity and the pre-nailed tibial shaft was fractured by an impact device, which resulted in a reproducible transverse or slightly oblique fracture pattern. The intramedullary rod maintained axial alignment, and the fractures united without displacement. On the basis of measurements of callus geometry, four-point bending tests, biochemical analyses, and quantitative histology, the progress of callus formation and remodeling occurred in a predictable sequence of healing phases. The ultimate bending loads of the fractures increased with time, reaching 74% of the strength of intact control tibias in 4 weeks. The stiffness values of the fractures returned to normal levels and, as determined radiographically, the fractures united by external callus in 4 weeks. Radiographically, callus size, cross-sectional callus area, and callus mass peaked at 2 weeks and decreased thereafter, indicating the start of external remodeling. Histologically, the amount of mesenchymal tissue was maximal at days 5 and 7. The callus cartilage area peaked at day 9; at its maximum, it accounted for 46% of the total callus area. Early periosteal formation of membranous new bone, followed by endochondral ossification, resulted in a linear increase of callus bone during the healing process. The healing sequence of the mouse tibial fracture was similar to that seen in the rat tibia.(ABSTRACT TRUNCATED AT 250 WORDS)

Types I and III procollagen extension peptides in serum respond to fracture in humans

Markers of types I and III collagen turnover were measured in serial blood samples in 16 patients with a Colles' fracture. The collagen markers were the carboxy-terminal extension peptide of type I procollagen (PICP) and the amino-terminal extension peptide of type III procollagen (PIIINP). Significant increases were found of PIIINP within 1 week and of PICP within 2 weeks. This sequential appearance of PIIINP and PICP was found to be in agreement with the appearance of types III and I collagen during early fracture healing as demonstrated in previous animal experimental studies. PICP had levelled off after 9 months, whereas PIIINP remained elevated. Osteocalcin, a serum marker of osteoblast activity, increased within 1 week and levelled off after 9 months. Correlations between the change in osteocalcin and those in PICP and PIIINP, respectively, were weak. These new biochemical markers may prove relevant as non-invasive markers of normal and pathological fracture healing in humans.

Collagen synthesis and mineralization in the early phase of distraction bone healing

Corticotomy of the distal radius followed by gradual distraction by external fixation was performed on three sheep. Collagen synthesis and mineral deposition were analysed from sequential biopsies obtained from the center of the distraction area during the first 4 weeks of distraction. The whole distraction area was rapidly filled with organic matrix the amount of which, due to fluctuation in its nonprotein component, initially decreased from 88 to 66% of the level in control bone but gained its initial level in 4 weeks. Total protein in the matrix represented 70% of that in the control bone during the 4-week follow up period while the proportion of collagen of the total protein increased from 53 to 88%, a level comparable with the unoperated bone. Determination of the type of fibrillar collagen by characterization of their cyanogen bromide peptides showed that in the distraction area production of type II collagen does not occur but the heteropolymer type I (alpha 1(I)2 alpha 2(I)1) collagen represents almost totally the collagen synthesized. Deposition of mineral into the distraction gap was detectable already after 2 weeks and increased rapidly after 3 weeks of distraction. The results suggest that unlike in other processes, e.g., direct osteonal and callus-type bone repair, in distraction bone healing gradual distraction of osteotomized bone leads directly to synthesis of mature fibrous organic matrix of bone followed by its rapid mineralization.

Internal remodeling of periosteal new bone during fracture healing

A closed fracture model of the rat tibia was employed to study internal remodeling of periosteal new bone during fracture repair. Static histomorphometric parameters of osteoid surface (or perimeter) and eroded surface (resorption surface) were used as indicators of appositional bone formation and resorption of bone trabeculae, respectively. Intracortical remodeling at the fracture site was evaluated using quantitative tetracycline histology and microradiography. The extents of osteoid and eroded bone surfaces did not differ significantly in the periosteal woven new bone in the early phases of fracture healing. Later on, the periosteal new bone had significantly more osteoid surface than eroded surface (p less than 0.001). The number of osteoclasts also decreased significantly over time during fracture healing (p = 0.028). Cortical bone showed a continuous increase of porosity (p less than 0.01) between 1 and 6 weeks after fracture. These results suggest that there is a time-related change in the balance of periosteal bone formation and resorption during the progress of fracture repair. We hypothesize that this change was related to the restoration of bony continuity. Further studies are, however, needed to indicate the histomorphometric features of periosteal new bone in fracture nonunions.