Xenografts of articular chondrocytes in the nude mouse

Ectopic models recapitulating morphological and functional features of articular cartilage

BACKGROUND

Articular cartilage is an extremely specialized connective tissue which covers all diarthrodial joints. Implantation of chondrogenic cells without or with additional biomaterial scaffolds in ectopic locationsin vivo generates substitutes of cartilage with structural and functional characteristics that are used in fundamental investigations while also serving as a basis for translational studies.

METHODS

Literature search in Pubmed.

RESULTS AND DISCUSSION

This narrative review summarizes the most relevant ectopic models, among which subcutaneous, intramuscular, and kidney capsule transplantation and elaborates on implanted cells and biomaterial scaffolds and on their use to recapitulate morphological and functional features of articular cartilage. Although the absence of a physiological joint environment and biomechanical stimuli is the major limiting factor, ectopic models are an established component for articular cartilage research aiming to generate a bridge between in vitro data and the clinically more relevant translational orthotopic in vivo models when their limitations are considered.

Ectopic implantation of juvenile osteochondral tissues recapitulates endochondral ossification

Subcutaneous implantation in a mouse can be used to investigate tissue maturation in vivo. Here we demonstrate that this simple model can recapitulate endochondral ossification associated with native skeletal development. By histological and micro-computed tomography analysis we investigated morphological changes of immature bovine osteochondral tissues over the course of subcutaneous implantation in immunocompromised mice for up to 10 weeks. We observed multiple similarities between the ectopic process and native endochondral ossification: (i) permanent cartilage retention in the upper zones; (ii) progressive loss of transient cartilage accompanied by bone formation at the interface; and (iii) remodelling of nascent endochondral bone into mature cancellous bone. Importantly, these processes were mediated by osteoclastogenesis and vascularization. Taken together, these findings advance our understanding of how the simple ectopic model can be used to study phenotypic changes associated with endochondral ossification of native and engineered osteochondral tissues in vivo.

AIMP1 downregulation restores chondrogenic characteristics of dedifferentiated/degenerated chondrocytes by enhancing TGF-β signal

Dedifferentiation and degeneration of chondrocytes critically influences the efficiency of cartilage repair. One of the causes is the defect of transforming growth factor (TGF)-β signaling that promotes chondrogenic differentiation and degeneration. In the present study, we found that aminoacyl-tRNA synthetase-interacting multifunctional protein 1 (AIMP1) negatively regulates TGF-β signaling via interactions with Smad2 and Smad3 in immunoprecipitation assay and luciferase assay. In addition, we observed that the AIMP1 expression level was significantly increased in osteoarthritis (OA) patient-derived degenerated chondrocytes compared with healthy control. So, we hypothesized that downregulation of AIMP1 using small-interfering RNA (siRNA) technology in dedifferentiated (collected at passage #6) and degenerated (obtained from OA-affected areas) chondrocytes could lead to recover TGF-β signaling in both chondrocytes. Indeed, AIMP1 downregulation restored TGF-β signaling by promoting phosphorylation of Smad2 and Smad3, which shows redifferentiated characteristics in both dedifferentiated and degenerated chondrocytes. Additionally, implantation analyses using in vivo mouse model clearly showed that AIMP1 downregulation resulted in the increased chondrogenic potential as well as the enhanced cartilage tissue formation in both dedifferentiated and degenerated chondrocytes. Histological analyses clarified that AIMP1 downregulation increased expression levels of collagen type II (Col II) and aggrecan, but not Col I expression. Taken together, these data indicate that AIMP1 downregulation using siRNA is a novel tool to restore TGF-β signaling and thereby increases the chondrogenic potential of dedifferentiated/degenerated chondrocytes, which could be further developed as a therapeutic siRNA to treat OA.

CARTILAGE TISSUE ENGINEERING

Tissue engineering is a new approach for articular cartilage repair. The aim of the present article was to review the current status of cartilage tissue engineering researches. The scaffold materials used for cartilage tissue engineering, the in vitro, in vivo studies and the clinical trials were all reviewed. Our researches about in vitro cartilage tissue engineering with new type bioactive scaffold and preliminary animal studies results will also be described. The scaffold was tricopolymer made from gelatin, hyaluronan and chondroitin. Chondrocytes seeded in tricopolymer showed in vitro engineered cartilage formation. The engineered cartilage constructs were implanted into knee joints of miniature pigs for animal study.

Engineering of cartilage in recombinant human type II collagen gel in nude mouse model in vivo

OBJECTIVE

Our goal was to test the recombinant human type II collagen (rhCII) material as a gel-like scaffold for chondrocytes in a nude mouse model in vivo.

DESIGN

Isolated bovine chondrocytes (6x10(6)) were seeded into rhCII gels (rhCII-cell) and injected subcutaneously into the backs of nude mice. For comparison, chondrocytes (6x10(6)) in culture medium (Med-cell) and cell-free rhCII gels (rhCII-gel) were similarly injected (n=24 animals, total of three injections/animal). After 6 weeks, the tissue constructs were harvested and analyzed.

RESULTS

Chondrocytes with or without rhCII-gel produced white resilient tissue, which in histological sections had chondrocytes in lacunae-like structures. Extracellular matrix stained heavily with toluidine blue stain and had strongly positive collagen type II immunostaining. The tissue did not show any evidence of vascular invasion or mineralization. The cell-free rhCII-gel constructs showed no signs of cartilage tissue formation. Cartilage tissue produced by Med-cell was thin and macroscopically uneven, while the rhCII-cell construct was smooth and rounded piece of neotissue. RhCII-cell constructs were statistically thicker than Med-cell ones. However, no statistical differences were found between the groups in terms of glycosaminoglycan (GAG) content or biomechanical properties.

CONCLUSIONS

These results show that rhCII-gel provides good expansion and mechanical support for the formation of cartilage neotissue. RhCII material may allow favorable conditions in the repair of chondral lesions.

Osteochondral defects: present situation and tissue engineering approaches

Articular cartilage is often damaged due to trauma or degenerative diseases, resulting in severe pain and disability. Most clinical approaches have been shown to have limited capacity to treat cartilage lesions. Tissue engineering (TE) has been proposed as an alternative strategy to repair cartilage. Cartilage defects often penetrate to the subchondral bone, or full-thickness defects are also produced in some therapeutic procedures. Therefore, in TE strategies one should also consider the need for a simultaneous regeneration of both cartilage and subchondral bone in situations where osteochondral defects are present, or to provide an enhanced support for the cartilage hybrid construct. In this review, different concepts related to TE in osteochondral regeneration will be discussed. The focus is on the need to produce new biphasic scaffolds that will provide differentiated and adequate conditions for guiding the growth of the two tissues, satisfying their different biological and functional requirements.

Reduced chondrogenic potential of adipose tissue derived stromal cells correlates with an altered TGFbeta receptor and BMP profile and is overcome by BMP-6

Recent interest has focused on mesenchymal stem cells (MSC) for tissue engineering and regenerative therapy of cartilage defects. MSC originating from adipose tissue (ATSC) are attractive as they are easily available and abundant. They have similar properties like bone marrow derived MSC (BMSC), except for a reduced chondrogenic potential under standard culture conditions driven by TGFbeta. Aim of this study was to search for possible differences explaining the reduced differentiation capacity of ATSC and to eliminate it by adaptation of induction protocols. Expanded MSC were analyzed for their growth factor and related receptor repertoire and ATSC spheroid cultures were supplemented with BMP-2,-4,-6,-7, TGFbeta, FGFa, FGFb, IGF-1, and PTHrP alone or in combination with TGFbeta. In contrast to BMSC, ATSC showed reduced expression of BMP-2, -4, and -6 mRNA and did not express TGFbeta-receptor-I protein. Consistent with this, increased concentrations of TGFbeta did not improve chondrogenesis of ATSC. BMP6 treatment induced TGFbeta-receptor-I expression and combined application of TGFbeta and BMP-6 eliminated the reduced chondrogenic potential of ATSC inducing a gene expression profile similar to differentiated BMSC. Like in BMSC, chondrogenesis of ATSC was associated with hypertrophy according to premature collagen Type X expression, upregulation of alkaline-phosphatase activity and in vivo calcification of spheroids after ectopic transplantation in SCID mice. In conclusion, a distinct BMP and TGFbeta-receptor repertoire may explain the reduced chondrogenic capacity of ATSC in vitro, which could be compensated by exogenous application of lacking factors. Further studies should now be directed to induce chondrogenesis in the absence of hypertrophy.

Effects of in vitro preculture on in vivo development of human engineered cartilage in an ectopic model

We investigated whether, and under which conditions (i.e., cell-seeding density, medium supplements), in vitro preculture enhances in vivo development of human engineered cartilage in an ectopic nude mouse model. Monolayer-expanded adult human articular chondrocytes (AHACs) were seeded into Hyalograft C disks at 1.3 x 10(7) cells/cm3 (low density) or 7.6 x 10(7) cells/cm3 (high density). Constructs were directly implanted subcutaneously in nude mice for up to 8 weeks or precultured for 2 weeks before implantation. Preculture medium contained either transforming growth factor-beta1 (TGF-beta1, 1 ng/mL), fibroblast growth factor-2, and platelet-derived growth factor (proliferating medium) or TGF-beta1 (10 ng/mL) and insulin (differentiating medium). Both in vitro and after in vivo implantation, constructs derived by cell seeding at high versus low density and precultured in differentiating versus proliferating medium generated more cartilaginous tissues containing higher amounts of glycosaminoglycan and collagen type II and lower amounts of collagen type I, and with higher equilibrium moduli. As compared with direct implantation of freshly seeded scaffolds, preculture of AHAC-Hyalograft C constructs in differentiating medium, but not in proliferating medium, supported enhanced in vivo development of engineered cartilage. The effect of preculture was more pronounced when constructs were seeded at low density as compared with high density. This study indicates that preculture of human engineered cartilage in differentiating medium has the potential to provide grafts with higher equilibrium moduli and enhanced in vivo developmental capacity than freshly seeded scaffolds. These findings need to be validated in an orthotopic model system.

Development and remodeling of engineered cartilage-explant composites in vitro and in vivo

OBJECTIVE

Development and remodeling of engineered cartilage-explant composites were studied in vitro and in vivo.

DESIGN

Individual and interactive effects of cell chondrogenic potential (primary or fifth passage bovine calf chondrocytes), scaffold degradation rate (hyaluronan benzyl ester or polyglycolic acid), and adjacent tissue cell activity and architecture (vital trabecular bone (VB), articular cartilage (AC), devitalized bone (DB) or digested cartilage (DC)) were evaluated over 8 weeks in vitro (bioreactor cultures) and in vivo (ectopic implants).

RESULTS

In vitro, significant effects of cell type on construct adhesive strength (P<0.001) and scaffold type on adhesive strength (P<0.001), modulus (P=0.014), glycosaminoglycans (GAG) (P<0.001), and collagen (P=0.039) were observed. Chondrogenesis was best when the scaffold degradation rate matched the extracellular matrix deposition rate. In vivo, adjacent tissue type affected adhesive strength (P<0.001), modulus (P<0.001), and GAG (P<0.001) such that 8-week values obtained for bone (VB and DB) were higher than for cartilage (AC). In the AC/construct group, chondrogenesis appeared attenuated in the region of the construct close to the AC. In contrast, in the VB/construct group, a 500 microm thick zone of mature hyaline-like cartilage formed at the interface, and signs of active remodeling were present in the bone that included osteoclastic and osteoblastic activity and trabecular rebuttressing; these features were not present in the DB group or in vitro.

CONCLUSIONS

Development and remodeling of composites based on engineered cartilage were mediated in vitro by cell chondrogenic potential and scaffold degradation rate, and in vivo by type of adjacent tissue and time.

Tissue engineering of cartilage using an injectable and adhesive chitosan-based cell-delivery vehicle

OBJECTIVE

Adult articular cartilage shows a limited intrinsic repair response to traumatic injury. To regenerate damaged cartilage, cell-assisted repair is thus viewed as a promising therapy, despite being limited by the lack of a suitable technique to deliver and retain chondrogenic cells at the defect site.

DESIGN

We have developed a cytocompatible chitosan solution that is space-filling, gels within minutes, and adheres to cartilage and bone in situ. This unique combination of properties suggested significant potential for its use as an arthroscopically injectable vehicle for cell-assisted cartilage repair. The primary goal of this study was to assess the ability of this polymer system, when loaded with primary articular chondrocytes, to support cartilage formation in vitro and in vivo. The chitosan gel was cultured in vitro, with and without chondrocytes, as well as injected subcutaneously in nude mice to form subcutaneous dorsal implants. In vitro and in vivo constructs were collectively analyzed histologically, for chondrocyte mRNA and protein expression, for biochemical levels of glycosaminoglycan, collagen, and DNA, and for mechanical properties.

RESULTS

Resulting tissue constructs revealed histochemical, biochemical and mechanical properties comparable to those observed in vitro for primary chondrocytes cultured in 2% agarose. Moreover, the gel was retained after injection into a surgically prepared, rabbit full-thickness chondral defect after 1 day in vivo, and in rabbit osteochondral defects, up to 1 week.

CONCLUSIONS

The in situ-gelling chitosan solution described here can support in vitro and in vivo accumulation of cartilage matrix by primary chondrocytes, while persisting in osteochondral defects at least 1 week in vivo.

Animal models for cartilage reconstruction

Animal models are widely used to develop and evaluate tissue-engineering techniques for the reconstruction of damaged human articular cartilage. For the purpose of this review, these model systems will include in vitro culture of animal cells and explants, heterotopic models of chondrogenesis, and articular cartilage defect models. The objectives for these preclinical studies are to engineer articular cartilage for the functional restoration of a joint surface that appears anatomically, histologically, biologically, biochemically, and mechanically to resemble the original joint surface. While no animal model permits direct application to humans, each is capable of yielding principles on which decisions can be made that might eventually translate into a human application. Clearly, the use of animal models has and will continue to play a significant role in the advancement of this field. Each animal model has specific advantages and disadvantages. The key issue in the selection of an appropriate animal model is to match the model to the question being investigated and the hypothesis to be tested. The purpose of this review is to discuss issues regarding animal model selection, the benefits and limitations of these model systems, scaffold selection with emphasis on polymers, and evaluation of the tissue-engineered articular cartilage.

Phenotypic plasticity of human articular chondrocytes

BACKGROUND

Progenitor cells in mesenchymal tissues are important in the maintenance of tissue homeostasis and regeneration capacity. Articular cartilage is a tissue with a very low capacity for repair. One explanation could be the lack of chondrogenic progenitor cells within the adult tissue. As a test of chondrogenic differentiation potential, we examined the ability of isolated chondrocytes to take on several phenotypic identities within the mesenchymal lineage by applying culture techniques and markers used in the study of the phenotypic plasticity of marrow-derived mesenchymal stem cells (MSCs).

METHODS

Culture-expanded human articular chondrocytes were analyzed for chondrogenic, adipogenic, and osteogenic capacity in defined in vitro culture systems. The osteochondrogenic potential of cells loaded into porous calcium-phosphate ceramic cubes implanted into mice was also determined.

RESULTS

The different assays demonstrated that culture-expanded chondrocytes have the potential to form cartilage in pellet mass cultures, to form adipose cells in dense monolayer cultures, and to form a calcium-rich matrix in an osteogenic assay. In the in vitro assays, a variability of phenotypic plasticity was demonstrated among the donors. In contrast with MSCs, chondrocytes formed cartilage only (and not bone) in the in vivo osteochondrogenic assay.

CONCLUSIONS

These results suggest that, within articular cartilage, there are chondrogenic cells that exhibit a level of phenotypic plasticity that is comparable with that of MSCs. However, there was a difference in the expression of bone in the in vivo assay.

Effects of cell concentration and growth period on articular and ear chondrocyte transplants for tissue engineering

This study determined the effects of chondrocyte source, cell concentration, and growth period on cartilage production when isolated porcine cells are injected subcutaneously in a nude mouse model. Chondrocytes were isolated from both ear and articular cartilage and were suspended in Ham's F-12 medium at concentrations of 10, 20, 40, and 80 million cells per cubic centimeter. Using the nude mouse model, each concentration group was injected subcutaneously in 100-microl aliquots and was allowed to incubate for 6 weeks in vivo. In addition, cells suspended at a fixed concentration of 40 million cells per cubic centimeter were injected in 100-microl aliquots and were incubated for 1, 2, 3, 4, 5, 6, 9, and 12 weeks. Each concentration or time period studied contained a total of eight mice, with four samples harvested per mouse for a final sample size of 32 constructs. All neocartilage samples were analyzed by histologic characteristics, mass, glycosaminoglycan level, and DNA content. Control groups consisted of native porcine ear and articular cartilage. Specimen mass increased with increasing concentration and incubation time. Ear neocartilage was larger than articular neocartilage at each concentration and time period. At 40 million cells per cubic centimeter, both ear and articular chondrocytes produced optimal neocartilage, without limitations in growth. Specimen mass increased with incubation time up to 6 weeks in both ear and articular samples. No significant variations in glycosaminoglycan content were found in either articular or ear neocartilage, with respect to variable chondrocyte concentration or growth period. Although articular samples demonstrated no significant trends in DNA content over time, ear specimens showed decreasing values through 6 weeks, inversely proportional to increase in specimen mass. Although both articular and ear sources of chondrocytes have been used in past tissue-engineering studies with success, this study indicates that a suspension of ear chondrocytes injected into a subcutaneous location will produce biochemical and histologic data with greater similarity to those of native cartilage. The authors believe that this phenomenon is attributable to the local environment in which isolated chondrocytes from different sources are introduced. The subcutaneous environment of native ear cartilage accommodates subcutaneously injected ear chondrocyte transplants better than articular transplants. Native structural and biochemical cues within the local environment are believed to guide the proliferation of the differentiated chondrocytes.

Molecular markers predictive of the capacity of expanded human articular chondrocytes to form stable cartilage in vivo

OBJECTIVE

To establish a model and associated molecular markers for monitoring the capacity of in vitro-expanded chondrocytes to generate stable cartilage in vivo.

METHODS

Adult human articular chondrocytes (AHAC) were prepared by collagenase digestion of samples obtained postmortem and were expanded in monolayer. Upon passaging, aliquots of chondrocyte suspensions were either injected intramuscularly into nude mice, cultured in agarose, or used for gene expression analysis. Cartilage formation in vivo was documented by histology, histochemistry, immunofluorescence for type II collagen, and proteoglycan analysis by 35S-sulfate incorporation and molecular sieve chromatography of the radiolabeled macromolecules. In situ hybridization for species-specific genomic repeats was used to discriminate human-derived from mouse-derived cells. Gene expression dynamics were analyzed by semiquantitative reverse transcription-polymerase chain reaction.

RESULTS

Intramuscular injection of freshly isolated AHAC into nude mice resulted in stable cartilage implants that were resistant to mineralization, vascular invasion, and replacement by bone. In vitro expansion of AHAC resulted in the loss of in vivo cartilage formation. This capacity was positively associated with the expression of fibroblast growth factor receptor 3, bone morphogenetic protein 2, and alpha1(II) collagen (COL2A1), and its loss was marked by the up-regulation of activin receptor-like kinase 1 messenger RNA. Anchorage-independent growth and the reexpression of COL2A1 in agarose culture were insufficient to predict cartilage formation in vivo.

CONCLUSION

AHAC have a finite capacity to form stable cartilage in vivo; this capacity is lost throughout passaging and can be monitored using a nude mouse model and associated molecular markers. This cartilage-forming ability in vivo may be pivotal for successful cell-based joint surface defect repair protocols.

The science of tissue engineering

This article reviews the development of tissue engineering during the last decade. The science began to fully develop in association with efforts to combine viable cells with biocompatible material. The history and scope of this new field are presented. Basic principles of cell biology, materials, and technologies are discussed. Future challenges in the field are presented.

Pretreatment with platelet derived growth factor-BB modulates the ability of costochondral resting zone chondrocytes incorporated into PLA/PGA scaffolds to form new cartilage in vivo

Optimal repair of chondral defects is likely to require both a suitable population of chondrogenic cells and a biodegradable matrix to provide a space-filling structural support during the early stages of cartilage formation. This study examined the ability of chondrocytes to support cartilage formation when incorporated into biodegradable scaffolds constructed from copolymers (PLG) of polylactic acid (PLA) and polyglycolic acid (PGA) and implanted in the calf muscle of nude mice. Scaffolds were fabricated to be more hydrophilic (PLG-H) or were reinforced with 10% PGA fibers (PLG-FR), increasing the stiffness of the implant by 20-fold. Confluent primary cultures of rat costochondral resting zone chondrocytes (RC) were loaded into PLG-H foams and implanted intramuscularly. To determine if growth factor pretreatment could modulate the ability of the cells to form new cartilage, RC cells were pretreated with recombinant human platelet derived growth factor-BB IPDGF-BB) for 4 or 24 h prior to implantation. To assess whether scaffold material properties could affect the ability of chondrogenic cells to form cartilage, RC cells were also loaded into PLG-FR scaffolds. To determine if the scaffolds or treatment with PDGF-BB affected the rate of chondrogenesis, tissue at the implant site was harvested at four and eight weeks post-operatively, fixed, decalcified and embedded in paraffin. Sections were obtained along the transverse plane of the lower leg, stained with haematoxylin and eosin, and then assessed by morphometric analysis for area of cartilage, area of residual implant, and area of fibrous connective tissue formation (fibrosis). Whether or not the cartilage contained hypertrophic cells was also assessed. The amount of residual implant did not change with time in any of the implanted tissues. The area occupied by PLG-FR implants was greater than that occupied by PLG-H implants at both time points. All implants were surrounded by fibrous connective tissue, whether they were seeded with RC cells or not. The amount of fibrosis was reduced at eight weeks for both implant types. When RC cells were present, the amount of fibrosis was less than seen in cell-free scaffolds. Pretreatment with PDGF-BB caused a slightly greater degree of fibrosis at four weeks than was seen if untreated cells were used in the implants. However, at eight weeks, if the cells had been exposed to PDGF-BB for 24 h, fibrosis was comparable to that seen associated with cell-free scaffolds. The cells supported an equivalent area of cartilage formation in both scaffolds. PDGF-BB caused a time-dependent decrease in cartilage formation at four weeks, but at eight weeks, there was a marked increase in cartilage formation in PDGF-BB-treated cells that was greatest in cells exposed for 4 h compared to those exposed for 24 h. Moreover, PDGF-BB decreased the formation of hypertrophic cells. The results indicate that in this model, RC cells produce cartilage; pretreatment of the RC cells with PDGF-BB promotes retention of a hyaline-like chondrogenic phenotype; and the material properties of the implant do not negatively impact on the ability of the cells to support chondrogenesis.

An overview of tissue engineered bone

Numerous important developments in tissue engineering of new bone during the last 10 years are reviewed. Early efforts to combine cells with biocompatible materials are described and applications of this technology are presented with particular focus on uses in orthopaedics and maxillofacial surgery. Basic principles of tissue engineering focusing on cell biology and materials science as used currently in the field are presented. Finally, future challenges are outlined from the perspective of integrating technologies from medicine, biology, and engineering in hopes of translating tissue engineering to clinical applications.

Injectable cartilage using polyethylene oxide polymer substrates

This study demonstrates that polyethylene oxide gels, which are biocompatible and biodegradable synthetic polymers, can be utilized for the encapsulation of isolated chondrocytes and maintenance of three-dimensional spatial support for new tissue development. Chondrocytes isolated from the glenohumeral and humeroradioulnar joints of a calf were added to a 20% polyethylene oxide solution in Ham's F-12 medium to generate a final cellular density of 10 x 10(6)/mL. The polymer-chondrocyte constructs were injected through a 22-gauge needle in 500-microliters aliquots subcutaneously in 12 nude mice and incubated for 6 and 12 weeks in vivo. Histologic and biochemical analyses including deoxyribonucleic acid and glycosaminoglycan quantitative analyses confirmed the presence of actively proliferating chondrocytes with production of a well-formed cartilaginous matrix in the transplanted samples. Control specimens from eight implantation sites consisting of chondrocytes alone or polyethylene oxide substrates did not demonstrate any gross or histologic evidence of neocartilage formation. These findings demonstrate the potential use of an injectable and moldable polymer substrate that can support cell proliferation and matrix synthesis after subcutaneous transplantation for neocartilage generation. The use of functional biologic tissue substitutes may serve as an alternative solution to current methods of augmentation or reconstruction of structural craniofacial contour deformities.

Chondrocyte-fibrin matrix transplants for resurfacing extensive articular cartilage defects

Cartilage resurfacing by chondrocyte implantation, with fibrin used as a vehicle, was examined in large (12 mm) full-thickness articular cartilage defects in horses. Articular chondrocytes, isolated from a 9-day-old foal, were mixed with fibrinogen and injected with thrombin, in a 1:1 mixture, into 12 mm circular defects on the lateral trochlea of the distal femur of eight normal horses. The contralateral femoropatellar (knee) joint served as a control in which the defect was left empty. Synovial fluid from the femoropatellar joints was sampled on days 0, 4, 7, 30, 120, and 240 postoperatively. Groups of four horses were killed at 4 or 8 months postoperatively, and the repair tissue was evaluated by gross and histologic examination with use of hematoxylin and eosin and safranin O staining and by autoradiography. Biochemical analyses included quantitation of proteoglycan, total collagen, and type-II collagen in the repair tissue. Grossly, grafted defects had improved filling of the cartilage lesions; histologically, these areas consisted of differentiated chondrocytes in the deep and middle zones. The cellular arrangement in these zones resembled that of hyaline cartilage. The control defects contained poorly attached fibrous tissue throughout. Grafted tissue at 8 months had increased proteoglycan synthesis evident by both safranin O staining and autoradiography. Glycosaminoglycan quantitation by dye-binding assay confirmed a significantly elevated glycosaminoglycan content in grafted defects (58.8 micrograms/mg of dry weight) compared with control defects (27.4 micrograms/mg; p < 0.05). Similarly, the levels of chondroitin sulfate/dermatan sulfate was significantly elevated in the grafted defects, and this was the predominant glycosaminoglycan epitope present. There was a statistically significant (p < 0.05) increase in type-II collagen in the grafted tissue at 8 months (61.2% grafted; 25.1% control). This resurfacing attempt with use of allograft chondrocytes, secured in large full-thickness articular defects with polymerized fibrin, resulted in an improved cartilage surface in comparison with the control defects, a significantly greater aggrecan level, and a significantly higher proportion of type-II collagen.

Differences in cartilage formed intramuscularly or in joint surface defects by syngeneic rat chondrocytes isolated from the articular-epiphyseal cartilage complex

Syngeneic rat chondrocytes isolated from the articular-epiphyseal cartilage complex were suspended in hyaluronic acid and transplanted intramuscularly or into joint surface defects. Transplants were fixed in ruthenium hexammonium trichloride and embedded in glycol methacrylate. In cartilage nodules produced intramuscularly, chondrocyte hypertrophy and matrix calcification were observed after 2 wk. Partial ossification occurred after 4 wk and the cartilage was almost completely replaced by an ossicle after 8 wk. Only small, dispersed groups of chondrocytes remained within the ossicle. In cartilage formed in joint surface defects a superficial and a deep zone were distinguished. Chondrocytes in the superficial zone did not hypertrophy and cartilage remained unossified. In the deep zone matrix calcification and bone formation occurred. These processes were, however, retarded in comparison with intramuscular transplants. Thus, either intraarticular environment exerted an inhibitory effect on chondrocyte hypertrophy and matrix calcification or articular chondrocytes present among transplanted cells accumulated close to the joint lumen and reconstructed normal articular cartilage.

SV40-immortalization of rabbit articular chondrocytes: alteration of differentiated functions

Cell lines were established from rabbit articular chondrocytes following transfection with a plasmid encoding SV40 early function genes. This resulted in cell immortalization (130 passages have been completed for the oldest cell line) with acquisition of characteristics of partial transformation such as reduced serum requirements for normal and clonal growth. The immortalized chondrocytes, called SVRAC, did not form multilayer foci when maintained in postconfluent culture. Their ability to form colonies in soft agar was not increased in comparison with normal chondrocytes, but they were weakly tumorigenic in nude mice. SVRAC lost the ability to synthesize type II collagen and Alcian blue-stainable matrix, which are markers of the differentiated chondrocyte phenotype, and synthesized predominantly type I collagen. Studies of collagen gene expression showed that pro alpha 1 (II) mRNA was undetectable, whereas pro alpha 1 (I) collagen mRNA was expressed even in late passage cultures. Unlike normal dedifferentiated chondrocytes, SVRAC were unable to re-express the differentiated phenotype in response to tridimensional culture or microfilament depolymerization. Cell lines obtained from chondrocytes transfected either in primary culture or just after release of cells from cartilage displayed the same behaviour. Thus SV40 early genes were able to immortalize rabbit articular chondrocytes, but the resulting cell lines displayed an apparently irreversibly dedifferentiated phenotype. These cell lines can be used as models to identify regulatory pathways that are required for the maintenance or reexpression of differentiated function in chondrocytes.

Matrix vesicles as a marker of endochondral ossification

Endochondral ossification in bone development and repair, and in induced bone formation in mesenchymal tissues, involves recruitment of mesenchymal cells, their differentiation into chondrocytes, and calcification of the cartilagenous matrix. Stimulation of proteoglycan synthesis is used as a biochemical marker of chondrogenesis, however it does not distinguish among chondrogenic phenotypes. Chondrocytes derived from the resting zone and adjacent growth zone cartilage of the costochondral junction of young rats, produce matrix vesicles in culture which are enriched in alkaline phosphatase specific activity with respect to the plasma membrane. Matrix vesicles isolated from cultures of neonatal rat muscle mesenchymal cells are not enriched in this enzyme activity. Alkaline phosphatase in matrix vesicles produced by growth zone chondrocytes is stimulated by 1,25(OH)2D3; enzyme in matrix vesicles produced by resting zone chondrocytes is stimulated by 24,25(OH)2D3; enzyme in matrix vesicles isolated from mesenchymal cell cultures is responsive to neither metabolite. Matrix vesicle phospholipase A2 is stimulated by 1,25(OH)2D3 in growth zone chondrocytes cultures; inhibited by 24,25(OH)2D3 in resting zone chondrocyte cultures; and is unaffected by either metabolite in mesenchymal cell cultures. These observations suggest that matrix vesicle production, as defined by alkaline phosphatase enrichment, and responsiveness of matrix vesicle enzymes to vitamin D metabolites, can be used as markers of phenotypic maturation during chondrogenesis in vivo and in vitro.

Induction of bone xenografts of rabbit growth plate chondrocytes in the nude mouse

Subcutaneous transplantation of growth plate chondrocytes isolated enzymatically from the proximal tibia of 6-week-old rabbits into athymic (nu/nu) mice resulted in the formation of cartilaginous nodules. Calcification of the matrix was first seen after 48 hrs, and endochondral ossification at 12 days. The mineral first occurred about hypertrophic cells. Histochemical alkaline phosphatase activity was concentrated in pericellular collars at the same location. Immunofluorescence examination with rabbit anti-mouse lymphocyte serum disclosed that the bulk of the osteoblasts was derived from the mouse. A small quantity of mouse antigen was present in the cartilage matrix at its junction with bone. It presumably diffused into the cartilaginous interface from the host, but the possibility that some chondrocytes were of murine origin has not been excluded. Five of six grafts of cells grown to confluence in monolayer culture for 10 to 14 days became ossified. The ability to induce mineralization declined in subculture. Chondrocytes killed by heating to 56 degrees did not induce calcified cartilage or bone.

Considerations in the use of nude mice for cancer research

Transplants of human tumors in nude mice have shown a progressive increase during the past 15 years as an experimental model for cancer research. A variety of factors, including relatively fragile health, have been identified that require appropriate experimental controls if the investigator is obtain consistent results. Not all tumors grow in nude mice. The frequency of tumor 'take' varies according to tumor origin, tumor type, inoculation site, age and conditioning of the mouse host, and a variety of other factors. Manipulation of these variables has led to successful propagation of almost every known variety of human malignancy. Following transplant, changes in characteristics have been documented, but the frequency and degree of such changes remains uncertain. Tumor growth rate probably increases after transplantation, requiring great care in the interpretation of chemotherapy experiments, but biochemical characteristics may be more stable. The nude mouse offers great interest as a model for the in vivo study of metastasis, as a number of experimental variables, mainly immunological, have been shown to affect this process. Spontaneous tumors have been shown to arise in these animals, but the controversy over their frequency relative to the thymus-bearing background strain is unresolved. We conclude that the nude mouse/tumor xenograft model, while requiring meticulous experimental controls, is nevertheless an extremely useful tool for cancer research.