Cell, matrix changes and alpha-smooth muscle actin expression in repair of the canine meniscus

Silencing of NOTCH3 Signaling in Meniscus Smooth Muscle Cells Inhibits Fibrosis and Exacerbates Degeneration in a HEYL-Dependent Manner

The mechanisms of meniscus fibrosis and novel ways to enhance fibrosis is unclear. This work reveals human meniscus fibrosis initiated at E24 weeks. Smooth muscle cell cluster is identified in embryonic meniscus, and the combined analysis with previous data suggests smooth muscle cell in embryonic meniscus as precursors of progenitor cells in the mature meniscus. NOTCH3 is constantly expressed in smooth muscle cells throughout embryogenesis to adulthood. Inhibition of NOTCH3 signaling in vivo inhibits meniscus fibrosis and exacerbates degeneration. Continuous histological sections show that HEYL, NOTCH3 downstream target gene, is expressed consistently with NOTCH3. HEYL knockdown in meniscus cells attenuated the COL1A1 upregulation by CTGF and TGF-β stimulation. Thus, this study discovers the existence of smooth muscle cells and fibers in the meniscus. Inhibition of NOTCH3 signaling in meniscus smooth muscle cells in a HEYL-dependent manner prevented meniscus fibrosis and exacerbated degeneration. Therefore, NOTCH3/HEYL signaling might be a potential therapeutic target for meniscus fibrosis.

Combining TGF-β1 and Mechanical Anchoring to Enhance Collagen Fiber Formation and Alignment in Tissue-Engineered Menisci

Recapitulating the collagen fiber structure of native menisci is one of the major challenges in the development of tissue-engineered menisci. Native collagen fibers are developed by the complex interplay of biochemical and biomechanical signals. In this study, we optimized glucose and transforming growth factor-β1 (TGF-β1) concentrations in combination with mechanical anchoring to balance contributions of proteoglycan synthesis and contractile behavior in collagen fiber assembly. Glucose had a profound effect on the final dimensions of collagen-based constructs. TGF-β1 influenced construct contraction rate and glycosaminoglycan (GAG) production with two half-maximal effective concentration (EC₅₀) ranges, which are 0.23 to 0.28 and 0.53 to 1.71 ng/mL, respectively. At concentrations less than the EC₅₀, for the GAG production and contraction rate, TGF-β1 treatment resulted in less organized collagen fibers. At concentrations greater than the EC₅₀, TGF-β1 led to dense, disorganized collagen fibers. Between the two EC₅₀ values, collagen fiber diameter and length increased. The effects of TGF-β1 on fiber development were enhanced by mechanical anchoring, leading to peaks in fiber diameter, length, and alignment index. Fiber diameter and length increased from 7.9 ± 1.4 and 148.7 ± 16.4 to 17.5 ± 2.1 and 262.0 ± 13.0 μm, respectively. The alignment index reached 1.31, comparable to that of native tissue, 1.40. These enhancements in fiber architecture resulted in significant increases in tensile modulus and ultimate tensile stress (UTS) by 1.6- and 1.4-fold. Correlation analysis showed that tensile modulus and UTS strongly correlated with collagen fiber length, diameter, and alignment, while compressive modulus correlated with GAG content. These outcomes highlight the need for optimization of both biochemical and biomechanical cues in the culture environment for enhancing fiber development within tissue-engineered constructs.

The origin and distribution of CD68, CD163, and αSMA+ cells in the early phase after meniscal resection in a parabiotic rat model

We previously reported that circulating peripheral blood-borne cells (PBCs) contribute to early-phase meniscal reparative change. Because macrophages and myofibroblasts are important contributors of tissue regeneration, we examined their origin and distribution in the reparative meniscus. Reparative menisci were evaluated at 1, 2, and 4 weeks post-meniscectomy by immunohistochemistry to locate monocytes and macrophages (stained positive for CD68 and CD163), and myofibroblasts (stained positive for αSMA). Of the total number of cells, 13% were CD68⁺ at 1 week post-meniscectomy, which decreased to 1% by 4 weeks post-meniscectomy; of these, almost half of CD68⁺ cells (49.4%: 98.8% as PBCs) were green fluorescent protein (GFP)-positive post-meniscectomy (1, 2, and 4 weeks), indicating that the majority of CD68⁺ cells were derived from PBCs. Of the total cells, 6% were CD163⁺ at 1 week post-meniscectomy, which decreased to 1% by week 4. Of the CD163⁺ cells, the majority were GFP-positive (42.5%: 85.0% as PBCs) after 1 week; however, this decreased significantly over time, which indicates that the majority of CD163⁺ cells are derived from PBCs during the early phase of meniscal reparative change, but are derived from resident cells at later time points. Of the total cells, 38% were αSMA⁺ at 1 week post-meniscectomy, which decreased to 3% by 4 weeks. The proportion of GFP-positive αSMA⁺ cells was 2.8% after 1 week, with no significant change over time, which indicates that the majority of αSMA⁺ cells originated from resident cells. Here, we describe the origin and distribution of macrophages and myofibroblasts during meniscal reparative change.

Characteristics of meniscus progenitor cells migrated from injured meniscus

Serious meniscus injuries seldom heal and increase the risk for knee osteoarthritis; thus, there is a need to develop new reparative therapies. In that regard, stimulating tissue regeneration by autologous stem/progenitor cells has emerged as a promising new strategy. We showed previously that migratory chondrogenic progenitor cells (CPCs) were recruited to injured cartilage, where they showed a capability in situ tissue repair. Here, we tested the hypothesis that the meniscus contains a similar population of regenerative cells. Explant studies revealed that migrating cells were mainly confined to the red zone in normal menisci: However, these cells were capable of repopulating defects made in the white zone. In vivo, migrating cell numbers increased dramatically in damaged meniscus. Relative to non-migrating meniscus cells, migrating cells were more clonogenic, overexpressed progenitor cell markers, and included a larger side population. Gene expression profiling showed that the migrating population was more similar to CPCs than other meniscus cells. Finally, migrating cells equaled CPCs in chondrogenic potential, indicating a capacity for repair of the cartilaginous white zone of the meniscus. These findings demonstrate that, much as in articular cartilage, injuries to the meniscus mobilize an intrinsic progenitor cell population with strong reparative potential. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1966-1972, 2017.

Meniscus, articular cartilage and nucleus pulposus: a comparative review of cartilage-like tissues in anatomy, development and function

The degradation of cartilage in the human body is impacted by aging, disease, genetic predisposition and continued insults resulting from daily activity. The burden of cartilage defects (osteoarthritis, rheumatoid arthritis, intervertebral disc damage, knee replacement surgeries, etc.) is daunting in light of substantial economic and social stresses. This review strives to broaden the scope of regenerative medicine and tissue engineering approaches used for cartilage repair by comparing and contrasting the anatomical and functional nature of the meniscus, articular cartilage (AC) and nucleus pulposus (NP). Many review papers have provided detailed evaluations of these cartilages and cartilage-like tissues individually but none have comprehensively examined the parallels and inconsistencies in signaling, genetic expression and extracellular matrix composition between tissues. For the first time, this review outlines the importance of understanding these three tissues as unique entities, providing a comparative analysis of anatomy, ultrastructure, biochemistry and function for each tissue. This novel approach highlights the similarities and differences between tissues, progressing research toward an understanding of what defines each tissue as distinctive. The goal of this paper is to provide researchers with the fundamental knowledge to correctly engineer the meniscus, AC and NP without inadvertently developing the wrong tissue function or biochemistry.

Identification and characterization of adult mouse meniscus stem/progenitor cells

Meniscal damage is a common problem that accelerates the onset of knee osteoarthritis. Stem cell-based tissue engineering treatment approaches have shown promise in preserving meniscal tissue and restoring meniscal function. The purpose of our study was to identify meniscus-derived stem/progenitor cells (MSPCs) from mouse, a model system that allows for in vivo analysis of the mechanisms underlying meniscal injury and healing. MSPCs were isolated from murine menisci grown in explant culture and characterized for stem cell properties. Flow cytometry was used to detect the presence of surface antigens related to stem cells, and qRT-PCR was used to examine the gene expression profile of MSPCs. Major proteins associated with MSPCs were localized in the adult mouse knee using immunohistochemistry. Our data show that MSPCs have universal stem cell-like properties including clonogenicity and multi-potentiality. MSPCs expressed the mesenchymal stem cell markers CD44, Sca-1, CD90, and CD73 and when cultured had elevated levels of biglycan and collagen type I, important extracellular matrix components of adult meniscus. MSPC also expressed significant levels of Lox and Igf-1, genes associated with the embryonic meniscus. Localization studies showed staining for these same proteins in the superficial and outer zones of the adult mouse meniscus, regions thought to harbor endogenous repair cells. MSPCs represent a novel resident stem cell population in the murine meniscus. Analysis of MSPCs in mice will allow for a greater understanding of the cell biology of the meniscus, essential information for enhancing therapeutic strategies for treating knee joint injury and disease.

The Cellular Effect of a Single Interrupted Suture on Tendon

The effects on cell and matrix morphology of a single interrupted suture are described in rabbit (vascular) and mouse (avascular) digital flexor tendons. This model of tendon injury is reproducible and suitable for quantitative histological analysis. Tendons analysed at day 1, 3, 5, 7 and 14 after wounding demonstrated a well-demarcated “acellular zone” around the suture within 24 hours and persisting over 14 days. The placement of an untied suture in tendon did not produce this effect but tying and releasing the tied knot did. The rapidity of onset suggests that cells move from the zone of injury into less mechanically strained tissue. The acellular zone was apparent in rabbit hind paw flexor tendon which is vascularised and the corresponding tendon in mouse which has no intrinsic blood vessels. This phenomenon highlights biological events that must be considered in parallel with the current trend for multistrand locking flexor tendon suture repairs.

Meniscus tear surgery and meniscus replacement

OBJECTIVE

the menisci are easily injured and difficult to repair. The aim of this study was to analyze the current state of meniscal surgery aimed at preserving morphology and conserving the biomechanics of the knee to prevent joint degeneration.

METHODOLOGY

a search of the electronic medical literature database Medline was conducted, from http://www.ncbi.nlm.nih.gov/pubmed. The search was not limited by language. Candidate articles were identified by searching for those that included the keywords meniscus, surgery, suture, implant, allograft. The limits were included for clinical research and clinical trials. Basic research was not included. The studies selected were evaluated and classified in three different categories: basic science, reconstruction (suture and meniscectomy) and implants (scaffolds and allograft).

RESULTS

the consequences of meniscectomy performed at a young age can lead to a joint cartilage degeneration twenty years later. There are few surgical options for the repair of meniscal injuries in order both to preserve the meniscus and to ensure the long term survival of the knee joint, meniscectomy, repair, suturing the tear, or reconstruction, when a meniscal allograft or synthetic substitute is used to replace the meniscus, but the biomechanical properties of the native meniscus are not reproduced entirely by the scaffolds that exist today.

CONCLUSION

therapies that successfully repair or replace the meniscus are therefore likely to prevent or delay osteoarthritis progression.

From repair to regeneration: biomaterials to reprogram the meniscus wound microenvironment

When the field of tissue engineering first arose, scaffolds were conceived of as inert three-dimensional structures whose primary function was to support cellularity and tissue growth. Since then, advances in scaffold and biomaterial design have evolved to not only guide tissue formation, but also to interact dynamically with and manipulate the wound environment. At present, these efforts are being directed towards strategies that directly address limitations in endogenous wound repair, with the goal of reprogramming the local wound environment (and the cells within that locality) from a state that culminates in an inferior tissue repair into a state in which functional regeneration is achieved. This review will address this approach with a focus on recent advances in scaffold design towards the resolution of tears of the knee meniscus as a case example. The inherent limitations to endogenous repair will be discussed, as will specific examples of how biomaterials are being designed to overcome these limitations. Examples will include design of fibrous scaffolds that promote colonization by modulating local extracellular matrix density and delivering recruitment factors. Furthermore, we will discuss scaffolds that are themselves modulated by the wound environment to alter porosity and modulate therapeutic release through precise coordination of scaffold degradation. Finally, we will close with emerging concepts in local control of cell mechanics to improve interstitial cell migration and so advance repair. Overall, these examples will illustrate how emergent features within a biomaterial can be tuned to manipulate and harness the local tissue microenvironment in order to promote robust regeneration.

A new method for meniscus repair using type I collagen scaffold and infrapatellar fat pad

AIM

The aim of this study was to investigate a new method for meniscal repair by combinative transplantation with type I collagen scaffold and infrapatellar fat pad.

METHODS

Two-mm cylindrical defects at the anterior part of bilateral medial menisci were prepared in nine Japanese white rabbits. The 18 knees were equally divided into three groups: I, no treatment; II, collagen scaffold transplantation; and III, collagen scaffold and infrapatellar fat pad transplantation. Another three rabbits (six knees) underwent sham surgery and served as controls. Rabbits were sacrificed at eight weeks after transplantation. Surface area of the medial meniscus was evaluated using macrophotographs. Ishida score for meniscal regeneration was used for assessment. To evaluate the composition of regenerated tissue, immunohistochemistry was analyzed with anti-type I and anti-type II collagen antibodies, and anti-Ki67 antibody. To investigate the effects of collagen scaffold on human meniscus, cells were isolated from human meniscus and infrapatellar fat pad, and cultured with collagen scaffold for three weeks. After that, gene expression was evaluated by using quantitative real-time polymerase chain reaction.

RESULTS

In group I, the meniscus shrank anterior to posterior, and the surface area was significantly less than that of normal meniscus. However, the surface area was maintained in group III. Ishida score and Ki67-positive cell ratio in group III were significantly higher than that in any other group, and staining with type I and type II collagen was similar to that of the control. Expression of matrix metalloproteinase was significantly lower in cocultures of collagen scaffold, meniscus cell, and infrapatellar fat pad cell than in monocultured meniscus cell, and expression of interleukin-1β was not increased.

CONCLUSION

This new method for meniscal repair by combinative transplantation with type I collagen scaffold and infrapatellar fat pad showed meniscal regeneration and potential for suppressing inflammation.

Repair of dense connective tissues via biomaterial-mediated matrix reprogramming of the wound interface

Repair of dense connective tissues in adults is limited by their intrinsic hypocellularity and is exacerbated by a dense extracellular matrix (ECM) that impedes cellular migration to and local proliferation at the wound site. Conversely, healing in fetal tissues occurs due in part to an environment conducive to cell mobility and division. Here, we investigated whether the application of a degradative enzyme, collagenase, could reprogram the adult wound margin to a more fetal-like state, and thus abrogate the biophysical impediments that hinder migration and proliferation. We tested this concept using the knee meniscus, a commonly injured structure for which few regenerative approaches exist. To focus delivery and degradation to the wound interface, we developed a system in which collagenase was stored inside poly(ethylene oxide) (PEO) electrospun nanofibers and released upon hydration. Through a series of in vitro and in vivo studies, our findings show that partial digestion of the wound interface improves repair by creating a more compliant and porous microenvironment that expedites cell migration to and/or proliferation at the wound margin. This innovative approach of targeted manipulation of the wound interface, focused on removing the naturally occurring barriers to adult tissue repair, may find widespread application in the treatment of injuries to a variety of dense connective tissues.

Fibrochondrogenic potential of synoviocytes from osteoarthritic and normal joints cultured as tensioned bioscaffolds for meniscal tissue engineering in dogs

Meniscal tears are a common cause of stifle lameness in dogs. Use of autologous synoviocytes from the affected stifle is an attractive cell source for tissue engineering replacement fibrocartilage. However, the diseased state of these cells may impede in vitro fibrocartilage formation. Synoviocytes from 12 osteoarthritic (“oaTSB”) and 6 normal joints (“nTSB”) were cultured as tensioned bioscaffolds and compared for their ability to synthesize fibrocartilage sheets. Gene expression of collagens type I and II were higher and expression of interleukin-6 was lower in oaTSB versus nTSB. Compared with nTSB, oaTSB had more glycosaminoglycan and alpha smooth muscle staining and less collagen I and II staining on histologic analysis, whereas collagen and glycosaminoglycan quantities were similar. In conclusion, osteoarthritic joint—origin synoviocytes can produce extracellular matrix components of meniscal fibrocartilage at similar levels to normal joint—origin synoviocytes, which makes them a potential cell source for canine meniscal tissue engineering.

Comparison of meniscal fibrochondrocyte and synoviocyte bioscaffolds toward meniscal tissue engineering in the dog

Tissue engineering is a promising field of study toward curing the meniscal deficient stifle; however the ideal cell type for this task is not known. We describe here the extraction of synoviocytes and meniscal fibrochondrocytes from arthroscopic debris from six dogs, which were cultured as tensioned bioscaffolds to synthesize meniscal-like fibrocartilage sheets. Despite the diseased status of the original tissues, synoviocytes and meniscal fibrochondrocytes had high viability at the time of removal from the joint. Glycosaminoglycan and collagen content of bioscaffolds did not differ. Meniscal fibrochondrocyte bioscaffolds contained more type II collagen, but collagen deposition was disorganized, with only 30-40% of cells viable. The collagen of synoviocyte bioscaffolds was organized into sheets and bands and 80-90% of cells were viable. Autologous, diseased meniscal fibrochondrocytes and synoviocytes are plausible cell sources for future meniscal tissue engineering research, however cell viability of meniscal fibrochondrocytes in the tensioned bioscaffolds was low.

Correspondence of high-frequency ultrasound and histomorphometry of healing rabbit Achilles tendon tissue

OBJECTIVES

Static and dynamic high-frequency ultrasound of healing rabbit Achilles tendons were set in relationship to histomorphometric analyses at three and six weeks post-surgery.

MATERIALS AND METHODS

Twelve New Zealand White rabbits received a clean-cut Achilles tendon laceration (the medial and lateral Musculus gastrocnemius) and were repaired with a four-strand Becker suture. Six rabbits got additionally a tight polyester urethane tube at the repair site in order to vary the adhesion extent. Tendons were analysed by static and dynamic ultrasound (control: healthy contralateral legs). The ultrasound outcome was corresponded to the tendon shape, tenocyte and tenoblast density, tenocyte and tenoblast nuclei width, collagen fibre orientation and adhesion extent.

RESULTS

The spindle-like morphology of healing tendons (ultrasound) was confirmed by the swollen epitenon (histology). Prediction of adhesion formation by dynamic ultrasound assessment was confirmed by histology (contact region to surrounding tissue). Hyperechogenic areas corresponded to acellular zones with aligned fibres and hypoechogenic zones to not yet oriented fibres and to cell-rich areas.

CONCLUSIONS

These findings add new in-depth structural knowledge to the established non-invasive analytical tool, ultrasound.

Growth factor treated tensioned synoviocyte neotissues: towards meniscal bioscaffold tissue engineering

Meniscal injury is a common cause of osteoarthritis, pain, and disability in dogs and humans, but tissue-engineered bioscaffolds could be a treatment option for meniscal deficiency. The objective of this study was to compare meniscus-like matrix histology, composition, and biomechanical properties of autologous tensioned synoviocyte neotissues (TSN) treated with fetal bovine serum (TSNfbs) or three chondrogenic growth factors (TSNgf). Fourth passage canine synoviocytes from 10 dogs were grown in hyperconfluent monolayer culture, formed into TSN, and then cultured for 3 weeks with 17.7% FBS or three human recombinant TSNgf (bFGF, TGF-β1, and IGF-1). Cell viability was determined with laser microscopy. Histological architecture and the composition of fibrocartilage matrix were evaluated in TSN by staining tissues for glycosaminoglycan (GAG), α-smooth muscle actin, and collagen 1 and 2; quantifying the content of GAG, DNA, and hydroxyproline; and measuring the gene expression of collagens type 1α and 2α, the GAG aggrecan, and transcription factor Sry-type Homeobox Protein-9 (SOX9). Biomechanical properties were determined by materials testing force-deformation curves. The TSN contained components and histological features of mensical fibrocartilage extracellular matrix. Growth factor-treated TSN had higher DNA content but lower cell viability than TSNfbs. TSNgf had greater fibrocartilage-like matrix content (collagen 2 and GAG content with increased collagen 2α and SOX9 gene expression). Additionally, TSNgf collagen was more organized histologically and so had greater tensile biomechanical properties. The results indicate the potential of TSN when cultured with growth factors as implantable bioscaffolds for the treatment of canine meniscal deficiency.

In vitro synthesis of tensioned synoviocyte bioscaffolds for meniscal fibrocartilage tissue engineering

Background

Meniscal injury is a common cause of lameness in the dog. Tissue engineered bioscaffolds may be a treatment option for meniscal incompetency, and ideally would possess meniscus- like extracellular matrix (ECM) and withstand meniscal tensile hoop strains. Synovium may be a useful cell source for meniscal tissue engineering because of its natural role in meniscal deficiency and its in vitro chondrogenic potential. The objective of this study is to compare meniscal -like extracellular matrix content of hyperconfluent synoviocyte cell sheets (“HCS”) and hyperconfluent synoviocyte sheets which have been tensioned over wire hoops (tensioned synoviocyte bioscaffolds, “TSB”) and cultured for 1 month.

Results

Long term culture with tension resulted in higher GAG concentration, higher chondrogenic index, higher collagen concentration, and type II collagen immunoreactivity in TSB versus HCS. Both HCS and TSB were immunoreactive for type I collagen, however, HCS had mild, patchy intracellular immunoreactivity while TSB had diffuse moderate immunoreactivity over the entire bisocaffold. The tissue architecture was markedly different between TSB and HCS, with TSB containing collagen organized in bands and sheets. Both HCS and TSB expressed alpha smooth muscle actin and displayed active contractile behavior. Double stranded DNA content was not different between TSB and HCS, while cell viability decreased in TSB.

Conclusions

Long term culture of synoviocytes with tension improved meniscal- like extra cellular matrix components, specifically, the total collagen content, including type I and II collagen, and increased GAG content relative to HCS. Future research is warranted to investigate the potential of TSB for meniscal tissue engineering.

Culture of canine synoviocytes on porcine intestinal submucosa scaffolds as a strategy for meniscal tissue engineering for treatment of meniscal injury in dogs

Meniscal injury is a common cause of canine lameness. Tissue engineered bioscaffolds may be a treatment option for dogs suffering from meniscal damage. The aim of this study was to compare in vitro meniscal-like matrix formation and biomechanical properties of porcine intestinal submucosa sheets (SIS), used in canine meniscal regenerative medicine, to synoviocyte-seeded SIS bioscaffold (SSB), cultured with fetal bovine serum (SSBfbs) or chondrogenic growth factors (SSBgf). Synoviocytes from nine dogs were seeded on SIS and cultured for 30days with 17.7% fetal bovine serum or recombinant chondrogenic growth factors (IGF-1, TGFβ1 and bFGF). The effect on fibrochondrogenesis was determined by comparing mRNA expression of collagen types Iα and IIα, aggrecan, and Sry-type homeobox protein-9 (SOX9) as well as protein expression of collagens I and II, glycosaminoglycan (GAG), and hydroxyproline. The effect of synoviocyte seeding and culture conditions on biochemical properties was determined by measuring peak load, tensile stiffness, resilience, and toughness of bioscaffolds. Pre-culture SIS contained 13.6% collagen and 2.9% double-stranded DNA. Chondrogenic growth factor treatment significantly increased SOX9, collagens I and IIα, aggrecan gene expression (P<0.05), and histological deposition of fibrocartilage extracellular matrix (GAG and collagen II). Culture with synoviocytes increased SIS tensile peak load at failure, resilience, and toughness of bioscaffolds (P<0.05). In conclusion, culturing SIS with synoviocytes prior to implantation might provide biomechanical benefits, and chondrogenic growth factor treatment of cultured synoviocytes improves in vitro axial meniscal matrix formation.

Cell study of the three areas of the meniscus: effect of growth factors in an experimental model in sheep

Meniscus had two areas with different vascular supply. Cells of the two areas and the synovium were monolayer cultivated. We analyzed the expression of genes of Col1, Col 2A, MMP-2, MMP-13, and aggrecan in a baseline state and after incubation with VEGF, TGF-β, FGF, and IGF. We found that the growth factors used produced a major increase in the MMP-13 in all three areas. In the vascular area, the stimulation of MMP-3 was produced by FGF, while in the synovial and avascular areas, it was caused by TGF-β. MMP-2 was only stimulated in the synovial area by IGF. Col 2A was stimulated in the synovial area by VEGF, and in the avascular area by TGF-β, FGF, and IGF, whereas Col 1 was stimulated in the avascular area by IGF, FGF, and VEGF. The vascular or avascular areas of the meniscus, behave differently in terms of repair, and their cells express different factors. The growth factors act in a different way in each meniscal area.

CD34 and SMA expression of superficial zone cells in the normal and pathological human meniscus

The aim of this study was to evaluate histological changes in torn (0.5-27 weeks after injury) and osteoarthritic (OA) knee menisci versus normal menisci after PAS-AB, SAF-O-FG, and immunostaining for CD34, CD31, and smooth muscle actin (SMA). Cell layers in the superficial zone and the cell density in the deep zone of the menisci were counted. In the superficial zone of normal menisci, cells expressing CD34 were demonstrated. CD34(+) CD31(-) cells were absent in OA menisci and disappeared in torn menisci as a function of time. In contrast, an increase of SMA(+) cells combined with an increase of cell layers was observed in the superficial zone of torn menisci. SMA(+) cells were absent in normal and OA menisci. The predominant tissue type in torn menisci evolved from fibrocartilage-like to fibrous-like tissue as a function of time, whereas in OA menisci it became cartilage-like. The response of the superficial zone was reflected by the decrease of CD34(+) and the increase of SMA(+) cells in torn menisci and the transformation of a fibrous-like into a cartilage-like surface layer in OA menisci. These results potentially illustrate the contribution of CD34(+) cells to the homeostasis of meniscus tissue.

Migration responses of outer and inner meniscus cells to applied direct current electric fields

Injuries to the inner regions of the knee meniscus do not heal and can result in degenerative changes to the articular surface, ultimately leading to osteoarthritis. A possible stimulus to enhance meniscus healing is to use electric fields that induce galvanotaxis. In this study, a novel characterization of the effects of direct current electric fields on migration characteristics of meniscus cells was performed. Primary and passaged inner and outer meniscus cells were exposed to varying electric field strengths from 0 to 6 V/cm. Cell migration was tracked using time lapse digital photography, and cell displacement and cathodal direct velocity were quantified. Cytoskeletal staining was performed to examine actin distribution and nuclear content. Cell adhesion strength was quantified as a function of wall shear stress. Meniscus cells exhibited cathodal migration and cell elongation perpendicular to the applied electric field accompanied by actin reorganization. Outer meniscus cells migrated quicker and exhibited lower adhesion strengths when compared to inner meniscus cells. Passaged cells exhibited higher migration characteristics when compared to primary cells. Overall, this study demonstrated that electric fields can significantly enhance and direct meniscus cell migration and suggests the potential for their incorporation in strategies of meniscus repair and tissue engineering.

Interleukin-1, tumor necrosis factor-alpha, and transforming growth factor-beta 1 and integrative meniscal repair: influences on meniscal cell proliferation and migration

Introduction

Interleukin-1 (IL-1) and tumor necrosis factor-α (TNF-α) are up-regulated in injured and osteoarthritic knee joints. IL-1 and TNF-α inhibit integrative meniscal repair; however, the mechanisms by which this inhibition occurs are not fully understood. Transforming growth factor-β1 (TGF-β1) increases meniscal cell proliferation and accumulation, and enhances integrative meniscal repair. An improved understanding of the mechanisms modulating meniscal cell proliferation and migration will help to improve approaches for enhancing intrinsic or tissue-engineered repair of the meniscus. The goal of this study was to examine the hypothesis that IL-1 and TNF-α suppress, while TGF-β1 enhances, cellular proliferation and migration in cell and tissue models of meniscal repair.

Methods

A micro-wound assay was used to assess meniscal cell migration and proliferation in response to the following treatments for 0, 24, or 48 hours: 0 to 10 ng/mL IL-1, TNF-α, or TGF-β1, in the presence or absence of 10% serum. Proliferated and total cells were fluorescently labeled and imaged using confocal laser scanning microscopy and the number of proliferated, migrated, and total cells was determined in the micro-wound and edges of each image. Meniscal cell proliferation was also assessed throughout meniscal repair model explants treated with 0 or 10 ng/mL IL-1, TNF-α, or TGF-β1 for 14 days. At the end of the culture period, biomechanical testing and histological analyses were also performed. Statistical differences were assessed using an ANOVA and Newman-Keuls post hoc test.

Results

IL-1 and TNF-α decreased cell proliferation in both cell and tissue models of meniscal repair. In the presence of serum, TGF-β1 increased outer zone cell proliferation in the micro-wound and in the cross section of meniscal repair model explants. Both IL-1 and TNF-α decreased the integrative shear strength of repair and extracellular matrix deposition in the meniscal repair model system, while TGF-β1 had no effect on either measure.

Conclusions

Meniscal cell proliferation in vivo may be diminished following joint injury due to the up-regulation of inflammatory cytokines, thereby limiting native cellular repair of meniscal lesions. Therefore, therapies that can promote meniscal cell proliferation have promise to enhance meniscal repair and improve tissue engineering strategies.

Osteochondral integration of multiply incised pure cartilage allograft: repair method of focal chondral defects in a porcine model

BACKGROUND

A focal cartilage lesion has limited capacity to heal, and the repair modalities used at present are still unable to provide a universal solution. Pure cartilage graft implantation appears to be a simple option, but it has not been applied widely as cartilage will not reattach easily to the subchondral bone.

HYPOTHESIS

We used a multiple-incision technique (processed chondrograft) to increase cartilage graft surface. We hypothesized that pure cartilage graft with augmented osteochondral fusion capacity may be used for cartilage repair and we compared this method with other repair techniques.

STUDY DESIGN

Controlled laboratory study.

METHODS

Full-thickness focal cartilage defects were created on the medial femoral condyle of 9-month-old pigs; defects were repaired using various methods including bone marrow stimulation, autologous chondrocyte implantation, and processed chondrograft. After the repair, at weeks 6 and 24, macroscopic and histologic evaluation was carried out.

RESULTS

Compared with other methods, processed chondrograft was found to be similarly effective in cartilage repair. Defects without repair and defects treated with bone marrow stimulation appeared slightly irregular with fibrocartilage filling. Autologous chondrocyte implantation produced hyalinelike cartilage, although its cellular organization was distinguishable from the surrounding articular cartilage. Processed chondrograft demonstrated good osteochondral integration, and the resulting tissue appeared to be hyaline cartilage.

CONCLUSION

The applied cartilage surface processing method allows acceptable osteochondral integration, and the repair tissue appears to have good macroscopic and histologic characteristics.

CLINICAL RELEVANCE

If further studies confirm its efficacy, this technique could be considered for human application in the future.

Role of deleted in colon carcinoma in osteoarthritis and in chondrocyte migration

OBJECTIVE

The concept of the chondrocyte as a stationary cell surrounded by an apparently impenetrable matrix has been challenged by in vitro observations in recent years. Chondrocyte migration may have a role in remodelling of the cartilage and pathological conditions. Candidate molecules are repellent factors for the regulation of chondrocyte migration, which are expressed in fetal and adult cartilage. We analysed the potential role of the receptor deleted in colon carcinoma (DCC) in chondrocytes, as this may exert attractive activities.

METHODS

Gene expression was determined by quantitative RT-PCR and immunohistochemistry, and gene regulation by electro mobility shift assay and chromatin immunoprecipitation. Functional assays on migration and differentiation were done after cell treatment and transfection.

RESULTS

DCC was shown to be specifically up-regulated in OA compared with normal chondrocytes in vitro and in vivo. Promoter analysis and transfection studies showed that the up-regulation of DCC in OA chondrocytes may be mediated by the transcription factors Sox9 and AP-2. Netrin-1, the ligand of DCC, was revealed to induce the migration of OA chondrocytes specifically. Expression of DCC in healthy chondrocytes by transient transfection significantly induced cell migration and chemotaxis to Netrin-1. DCC expression had no influence on cell differentiation; however, induction of MMP1 and -3 expression was observed.

CONCLUSION

Strong differential expression of DCC in OA compared with normal chondrocytes hints of a possible role of DCC in the pathophysiology of OA. The strong impact of the DCC receptor on cellular mobility of chondrocytes in vitro suggests a major relevance of migratory activities in physiological and pathological conditions of cartilage. However, definite proof of chondrocyte movements in vivo still has to be established.

Development and characterization of an acellular porcine medial meniscus for use in tissue engineering

The objectives of this study were to characterize fresh porcine menisci and develop a decellularization protocol with a view to the generation of a biocompatible and biomechanically functional scaffold for use in tissue engineering/regeneration of the meniscus. Menisci were decellularized by exposing the tissue to freeze-thaw cycles, incubation in hypotonic tris buffer, 0.1% (w/v) sodium dodecyl sulfate in hypotonic buffer plus protease inhibitors, nucleases, hypertonic buffer followed by disinfection using 0.1% (v/v) peracetic acid and final washing in phosphate-buffered saline. Histological, immunohistochemical, and biochemical analyses of the decellularized tissue confirmed the retention of the major structural proteins. There was, however, a 59.4% loss of glycosaminoglycans. The histoarchitecture was unchanged, and there was no evidence of the expression of the major xenogeneic epitope, galactose-alpha-1,3-galactose. Biocompatibility of the acellular scaffold was determined by using contact cytotoxicity and extract cytotoxicity tests. Decellularized tissue and extracts were not cytotoxic to cells. Biomechanical properties were determined by indentation and tensile tests, which confirmed the retention of biomechanical properties following decellularization. In conclusion, this study has generated data on the production of a biocompatible, biomechanically functional scaffold for use in meniscal repair.

Fibrochondrogenesis in two embryonic stem cell lines: effects of differentiation timelines

Human embryonic stem cells (hESCs) are an exciting cell source for fibrocartilage engineering. In this study, the effects of differentiation time and cell line, H9 versus BG01V, were examined. Embryoid bodies (EBs) were fibrochondrogenically differentiated for 1, 3, or 6 weeks and then used to engineer tissue constructs that were grown for an additional 4 weeks. Construct matrix was fibrocartilaginous, containing glycosaminoglycans (GAGs) and collagens I, II, and VI. A differentiation time of 3 or 6 weeks produced homogeneous constructs, with matrix composition varying greatly with cell line and differentiation time: from 2.6 to 17.4 microg of GAG per 10(6) cells and from 22.3 to 238.4 microg of collagen per 10(6) cells. Differentiation for 1 week resulted in small constructs with poor structural integrity that could not be mechanically tested. The compressive stiffness of the constructs obtained from EBs differentiated for 3 or 6 weeks did not vary significantly as a function of either differentiation time or cell line. In contrast, the tensile properties were markedly greater with the H9 cell line, 1,562-1,940 versus 32-80 kPa in the BG01V constructs. These results demonstrate the dramatic effects of hESC line and differentiation time on the biochemical and functional properties of tissue-engineered constructs and show progress in fibrocartilage tissue engineering with an exciting new cell source.

Interleukin-1 and tumor necrosis factor alpha inhibit repair of the porcine meniscus in vitro

OBJECTIVE

Injury or removal of the knee meniscus leads to progressive joint degeneration, and current surgical therapies for meniscal tears seek to maximally preserve meniscal structure and function. However, the factors that influence intrinsic repair of the meniscus are not well understood. The goal of this study was to investigate the capacity of meniscus tissue to repair a simulated defect in vitro and to examine the effect of pro-inflammatory cytokines on this process.

METHODS

Cylindrical explants were harvested from the outer one-third of medial porcine menisci. To simulate a full-thickness defect, a central core was removed and reinserted immediately into the defect. Explants were cultured for 2, 4, or 6 weeks in serum-containing media in the presence or absence of interleukin-1 (IL-1) or tumor necrosis factor alpha (TNF-alpha), and meniscal repair was investigated using mechanical testing and fluorescence confocal microscopy.

RESULTS

Meniscal lesions in untreated samples showed a significant capacity for intrinsic repair in vitro, with increasing cell accumulation and repair strength over time in culture. In the presence of IL-1 or TNF-alpha, no repair was observed despite the presence of abundant viable cells.

CONCLUSIONS

This study demonstrates that the meniscus exhibits an intrinsic repair response in vitro. However, the presence of pro-inflammatory cytokines completely inhibited repair. These findings suggest that increased levels of pro-inflammatory cytokines post-injury or under arthritic conditions may inhibit meniscal repair. Therefore, inhibition of these cytokines may provide a means of accelerating repair of damaged or injured menisci in vivo.

Chondrocyte moves: clever strategies?

GOALS

To review the literature on chondrocyte movements and to develop plausible hypothesis for further work.

DESIGN

Chondrocyte movements are herein defined as translocations of the cell body. A brief overview of cell migration in other cell types is presented to set the stage for a discussion of chondrocyte moves; this includes a discussion of the challenges that cells find when moving within tissues. Reports of isolated chondrocyte migration in vitro (isolated cell systems) and ex vivo (cartilage organ cultures) are then summarized, followed by a discussion of recent studies that infer chondrocyte movements in vivo.

RESULTS

Investigators from different laboratories have observed chondrocyte motility in vitro. I became interested in the question of whether articular chondrocytes retained their phenotype during their migratory excursions. We devised a simple method to separate migratory and stationary chondrocytes and then showed that migratory chondrocytes synthesized collagen II but not I--consistent with a differentiated phenotype. Our time-lapse video microscopy studies showed that the cells displayed appropriate movement kinetics, albeit with low speed and directionality. Similarly, others have presented data consistent with slow movement of chondrocytes out of cartilage explants. It is important to decipher whether these in vitro movements reflect physiological states and if so, which events are simulated. Examples of in vivo studies that have inferred chondrocyte movements include those describing rotational or gliding movements of chondrocytes in the proliferative zone of the growth plate and its importance in the growth process; and the notion that chondrocytes move from the cartilage endplates to the nucleus pulposus (NP) in the spine of rabbits and rats during development. Such studies are consistent with the hypothesis that chondrocytes exhibit highly controlled and specialized movements during tissue growth and remodeling in vivo. On the other hand, the cartilage explant studies elicit interest in the possibility that matrix injuries resulting in disruption of the collagen network of adult cartilages provide a permissive environment for chondrocyte motility.

CONCLUSIONS

The case for in vivo chondrocyte motility remains to be proven. However, the in vitro and in vivo data on chondrocyte movements present an argument for further thought and studies in this area.

Self-assembly of fibrochondrocytes and chondrocytes for tissue engineering of the knee meniscus

Chondrocyte self-assembly in high-density scaffoldless culture has shown success in producing articular cartilage constructs, and a similar process could be applied to fibrocartilage tissue engineering. Three cell combinations were compared in self-assembly culture-100% chondrocytes, 100% meniscal fibrochondrocytes, and 50:50 co-cultures of fibrochondrocytes and chondrocytes with the goal of creating a proteoglycan, collagen I, and collagen II matrix similar to native meniscus. Two culture surfaces were also compared for self-assembly: agarose-coated wells and tissue culture plastic. After 4 weeks, the resulting self-assembled chondrocyte constructs were 10.24+/-0.63 mm in diameter and 0.96+/-0.14 mm thick, weighing 84.5+/-7.2 mg. Co-culture constructs were smaller and weighed 22.5+/-1.0 mg. In contrast, the fibrochondrocyte constructs contracted into spheres weighing 1.3+/-0.3 mg. Immunostaining showed collagen II in the chondrocyte constructs, both collagen I and collagen II in the co-cultures, and only collagen I in the fibrochondrocyte constructs. Collagen densities for chondrocyte, co-culture, and fibrochondrocyte constructs were 41+/-3, 38+/-3, and 20+/-2 microg/mg dry weight, and glycosaminoglycan densities were 230+/-2, 80+/-6, and 10+/-1 microg/mg dry weight, respectively. Self-assembled co-cultures, with their mixed collagen I and II matrix and robust gross characteristics, appear promising for tissue engineering of the knee meniscus.

Repair response of the inner and outer regions of the porcine meniscus in vitro

BACKGROUND

The menisci are essential intra-articular structures that contribute to knee function, and meniscal injury or loss is associated with joint degeneration. Tears of the outer vascularized zone have a greater potential for repair than do tears in the inner avascular region.

OBJECTIVE AND HYPOTHESIS

Develop an in vitro explant model to examine the hypothesis that differences exist in the intrinsic repair response between the outer and inner region of the meniscus.

STUDY DESIGN

Controlled laboratory study.

METHODS

Cylindrical explants were harvested from the outer one third and inner two thirds of medial porcine menisci. To simulate a full-thickness defect, a central core was removed and reinserted immediately. Explants were cultured for 2, 4, or 6 weeks, and meniscal healing was investigated using mechanical testing, histologic analysis, and fluorescence confocal microscopy.

RESULTS

Over the 6-week culture period, meniscal explants exhibited migration of cells into the repair site, followed by increased tissue formation that bridged the interface. The repair strength increased significantly over time, with no differences between the 2 regions.

CONCLUSION

The findings show that explants from the avascular inner zone and vascular outer zone of the meniscus exhibit similar healing potential and repair strength in vitro.

CLINICAL RELEVANCE

These findings support the hypothesis that the regional differences in meniscal repair observed clinically are owed to the additional vascular supply of the outer meniscus rather than intrinsic differences between the extracellular matrix and cells from these 2 areas.

Enhanced integrative repair of the porcine meniscus in vitro by inhibition of interleukin-1 or tumor necrosis factor α

OBJECTIVE

To examine the hypotheses that increasing concentrations of interleukin-1 (IL-1) or tumor necrosis factor alpha (TNFalpha) inhibit the integrative repair of the knee meniscus in an in vitro model system, and that inhibitors of these cytokines will enhance repair.

METHODS

Explants (8 mm in diameter) were harvested from porcine medial menisci. To simulate a full-thickness defect, a 4-mm-diameter core was removed and reinserted. Explants were cultured for 14, 28, or 42 days in the presence of 0-1,000 pg/ml of IL-1 or TNFalpha. Explants were also cultured in the presence of IL-1 or TNFalpha with IL-1 receptor antagonist (IL-1Ra) or TNF monoclonal antibody (mAb). At the end of the culture period, biomechanical testing, cell viability, and histologic analyses were performed to quantify the extent of repair.

RESULTS

Mechanical testing revealed increased repair strength, cell accumulation, and tissue formation at the interface over time under control conditions. Pathophysiologic concentrations of both IL-1 and TNFalpha significantly decreased repair strength, cell migration, and tissue formation at the interface. The addition of IL-1Ra or TNF mAb to explants prevented the effects of IL-1 or TNFalpha, respectively.

CONCLUSION

Our findings document that physiologically relevant concentrations of IL-1 and TNFalpha inhibit meniscal repair in vitro and therefore may also inhibit meniscal repair during arthritis or following joint injury. The finding that IL-1Ra and TNF mAb promoted integrative meniscal repair in an inflammatory microenvironment suggests that intraarticular delivery of IL-1Ra and/or TNF mAb may be useful clinically to promote meniscal healing following injury.

Comparative spatial and temporal localisation of perlecan, aggrecan and type I, II and IV collagen in the ovine meniscus: an ageing study

This is the first study to immunolocalise perlecan in meniscal tissues and to demonstrate how its localisation varied with ageing relative to aggrecan and type I, II and IV collagen. Perlecan was present in the middle and inner meniscal zones where it was expressed by cells of an oval or rounded morphology. Unlike the other components visualised in this study, perlecan was strongly cell associated and its levels fell significantly with age onset and cell number decline. The peripheral outer meniscal zones displayed very little perlecan staining other than in small blood vessels. Picrosirius red staining viewed under polarised light strongly delineated complex arrangements of slender discrete randomly oriented collagen fibre bundles as well as transverse, thick, strongly oriented, collagen tie bundles in the middle and outer meniscal zones. The collagen fibres demarcated areas of the meniscus which were rich in anionic toluidine blue positive proteoglycans; immunolocalisations confirmed the presence of aggrecan and perlecan. When meniscal sections were examined macroscopically, type II collagen localisation in the inner meniscal zone was readily evident in the 2- to 7-day-old specimens; this became more disperse in the older meniscal specimens. Type I collagen had a widespread distribution in all meniscal zones at all time points. Type IV collagen was strongly associated with blood vessels in the 2- to 7-day-old meniscal specimens but was virtually undetectable at the later time points (>7 month).

Characterisation of human knee meniscus cell phenotype

OBJECTIVE

Studies on the biology of the human meniscus cell are scarce. The objective of our studies was to assess survival/proliferation of human meniscus cells in different culture conditions and to characterize the extracellular matrix (ECM) produced by these cells in these artificial environments. The composition of this ECM offers a variable to define the distinct meniscus cell phenotype.

MATERIALS AND METHODS

Human meniscus cells were isolated enzymatically from visually intact lateral and medial knee menisci. Cells were cultured in monolayer conditions or in alginate gel. The composition of the cell-associated matrix (CAM) accumulated by the isolated cells during culture was investigated and compared to the CAM of articular chondrocytes cultured in alginate using flow cytometry with fluorescein isothiocyanate-conjugated monoclonal antibodies against type I collagen, type II collagen and aggrecan. Additional cell membrane markers analysis was performed to further identify the different meniscus cell populations in the alginate culture conditions and meniscus tissue sections. Proliferation was analyzed using the Hoechst 33258 dye method. In some experiments, the effect of TGFbeta1 on some of these variables was investigated.

RESULTS

The CAM of monolayer cultured meniscus cells is composed of high amounts of type I and II collagen and low amounts of aggrecan. A major population of alginate cultured meniscus cells on the other hand synthesized a CAM containing high amounts of type I collagen, low amounts of type II collagen and high amounts of aggrecan. This population is CD44+CD105+CD34-CD31-. In contrast, a minor cell population in the alginate culture did not accumulate ECM and was mainly CD34+. The CAM of alginate cultured articular chondrocytes is composed of low amounts of type I collagen, high amounts of type II collagen and aggrecan. The expression of aggrecan and of type II collagen was increased by the addition of TGFbeta1 to the culture medium. The proliferation of meniscus cells is increased in the monolayer culture conditions. Cell numbers decrease slightly in the alginate culture, but can be increased after the addition of TGFbeta1.

CONCLUSION

These results demonstrate that the human meniscus is populated by different cell types which can be identified by a distinct CAM composition and membrane marker expression. Unlike the monolayer culture conditions, the alginate culture conditions appear to favor a more fibrochondrocyte-like cell accumulating a CAM resembling the native tissue composition. This CAM composition is distinctly different from the CAM composition of phenotypically stable articular cartilage chondrocytes cultured in the same alginate matrix.

Spatial organization of types I and II collagen in the canine meniscus

The meniscus of the knee joint is a fibrocartilage mainly composed of type I collagen and smaller amounts of type II collagen. The distribution of type II collagen in the canine meniscus and its spatial relationship to type I collagen was examined by immunohistochemistry and confocal microscopy. Dorsal and coronal slices of the mid-section of medial and lateral menisci from the knee joints of skeletally mature dogs were predigested with Streptomyces hyaluronate lyase and bacterial Protease enzyme XXIV. Monoclonal antibodies against type I collagen (CP17L) and type II collagen (II-II6B3) and an anti-type II collagen polyclonal antibody (AB759) were employed. The staining for type II collagen in the extracellular matrix of hyaline articular cartilage was diffuse without any identifiable spatial organization. In striking contrast, type II collagen in the fibrocartilage of the meniscus stained as an organized network. Type II collagen was distributed throughout the meniscus with the exception of the outer zone containing the blood vessels. Coronal and dorsal staining of the meniscus showed bundles of circumferential fibrils of type I that colocalized with type II collagen in specific sites. These bundles were enwrapped in a second organizational fibrillar system of types I and II collagen that also colocalized. Bundles of circumferential fibrils appeared in cross-section in coronal sections as dots within the interstitial spaces framed by the network of types I and II collagen of the second system. Confocal overlays showed that types I and II collagens were superimposed, suggesting a close spatial proximity between the two collagens. The cells were confined to the types I and II collagen fibrils that enwrapped the bundles. A striking feature of the radial tie fibers was patches of type II collagen without colocalized type I collagen. Our study reveals a unique network of type II collagen in fibrocartilage of the meniscus that serves as a morphological distinction between fibro- and hyaline cartilage.

Alpha-smooth muscle actin in pathological human disc nucleus pulposus cells in vivo and in vitro

That a contractile actin isoform has been found in cells of other cartilage tissues in healing and disease states prompted this investigation of the presence of alpha-smooth muscle actin (alpha-SMA) in pathological human intervertebral disc tissue. The presence of this isoform has been reported in human intervertebral disc specimens obtained at autopsy from subjects for whom there were no reported symptoms. An objective of this study was to evaluate the cell density and percentage of alpha-SMA-containing cells in pathological nucleus pulposus tissue obtained from lumbar disc surgery from 17 patients. Additionally, explants of nucleus pulposus material were cultured to determine how alpha-SMA expression changed with time in vitro. Seventy-six 5-mm diameter explants (approximately 2 mm thick) pooled from six lumbar surgeries were cultured for 1, 2, 4, or 6 weeks. Microtomed sections of paraffin-embedded specimens were stained with hematoxylin and eosin or a monoclonal antibody to alpha-SMA. Histologically, cells were categorized as to alpha-SMA phenotype (positive or negative), and the areal cell density was determined. The evaluation of the cultured nucleus pulposus explants also included documentation of the percentage of cells that were round or elongated and the percentage of the cells that were part of a group (group: >/= 2 cells). Every nucleus pulposus section exhibited the presence of alpha-SMA-containing cells, which accounted for approximately 24 percent of the cells in vivo. In vivo, the cell density was significantly higher in older individuals (p = 0.02). The average time for cell outgrowth from the explants was 8.6 days. Approximately 10-15 percent of the cells in the explants stained positive for alpha-SMA. The time in culture had no significant effect on any of the outcome measures except the percentage of alpha-SMA-containing cells that were round (p = 0.008), with values decreasing through 4 weeks and then slightly rising at 6 weeks. The role of alpha-SMA in intervertebral disc pathology warrants further investigation.

Tissue engineering of the meniscus

Meniscus lesions are among the most frequent injuries in orthopaedic practice and they will inevitably lead to degeneration of the knee articular cartilage. The fibro-cartilage-like tissue of the meniscus is notorious for its limited regenerative capacity. Tissue engineering could offer new treatment modalities for repair of meniscus tears and eventually will enable the replacement of a whole meniscus by a tissue-engineered construct. Many questions remain to be answered before the final goal, a tissue-engineered meniscus is available for clinical implementation. These questions are related to the selection of an optimal cell type, the source of the cells, the need to use growth factor(s) and the type of scaffold that can be used for stimulation of differentiation of cells into tissues with optimal phenotypes. Particularly in a loaded, highly complex environment of the knee, optimal mechanical properties of such a scaffold seem to be of utmost importance. With respect to cells, autologous meniscus cells seems the optimal cell source for tissue engineering of meniscus tissue, but their availability is limited. Therefore research should be stimulated to investigate the suitability of other cell sources for the creation of meniscus tissue. Bone marrow stroma cells could be useful since it is well known that they can differentiate into bone and cartilage cells. With respect to growth factors, TGF-beta could be a suitable growth factor to stimulate cells into a fibroblastic phenotype but the problems of TGF-beta introduced into a joint environment should then be solved. Polyurethane scaffolds with optimal mechanical properties and with optimal interconnective macro-porosity have been shown to facilitate ingrowth and differentiation of tissue into fibro-cartilage. However, even these materials cannot prevent cartilage degeneration in animal models. Surface modification and/or seeding of cells into the scaffolds before implantation may offer a solution for this problem in the future.This review focuses on a number of specific questions; what is the status of the development of procedures for lesion healing and how far are we from replacing the entire meniscus by a (tissue-engineered) prosthesis. Subquestions related to the type of scaffold used are: is the degree of tissue ingrowth and differentiation related to the initial mechanical properties and if so, what is the influence of those properties on the subsequent remodelling of the tissue into fibro-cartilage; what is the ideal pore geometry and what is the optimal degradation period to allow biological remodelling of the tissue in the scaffold. Finally, we will finish with our latest results of the effect of tear reconstruction and the insertion of prostheses on articular cartilage degradation.

Biology of Fibrocartilage Cells

Fibrocartilage is an avascular tissue that is best documented in menisci, intervertebral discs, tendons, ligaments, and the temporomandibular joint. Several of these sites are of particular interest to those in the emerging field of tissue engineering. Fibrocartilage cells frequently resemble chondrocytes in having prominent rough endoplasmic reticulum, many glycogen granules, and lipid droplets, and intermediate filaments together with and actin stress fibers that help to determine cell organization in the intervertebral disc. Fibrocartilage cells can synthesize a variety of matrix molecules including collagens, proteoglycans, and noncollagenous proteins. All the fibrillar collagens (types I, II, III, V, and XI) have been reported, together with FACIT (types IX and XII) and network-forming collagens (types VI and X). The proteoglycans include large, aggregating types (aggrecan and versican) and small, leucine-rich types (decorin, biglycan, lumican, and fibromodulin). Less attention has been paid to noncollagenous proteins, although tenascin-C expression may be modulated by mechanical strain. As in hyaline cartilage, matrix metalloproteinases are important in matrix turnover and fibrocartilage cells are capable of apoptosis.

Effects of selected growth factors on porcine meniscus in chemically defined medium

Some evidence shows that selected growth factors can increase proteoglycan synthesis and that fibrin clot aids in the repair of meniscal tears. A significant (P<.05) dose-dependent stimulation of proteoglycan synthesis was found with platelet-derived growth factor AB (PDGF-[AB]), transforming growth factor-beta1 and bone morphogenetic protein-7 (osteogenic protein-I). In separate experiments, a significant increase in cellularity was found in the deep and superficial aspects of the explants treated with PDGF-(AB) compared with those treated with basal medium alone. The current study provides data as to which growth factors might prove most useful in the maintenance and repair of menisci.

Contractile actin expression in torn human menisci

The human meniscus is subject to injury that necessitates repair or removal. Many aspects of the cellular response to injury have not been well characterized. The purpose of this study was to describe the cellular distributions within the torn human meniscus. In addition to evaluating the cell density in selected regions, we investigated the cellular expression of a contractile actin isoform that has recently been found in the intact human meniscus. Included as a contemporaneous comparative group were torn human meniscal allografts. We hypothesized that a hypercellular surface zone would be found in the torn menisci, with a higher percentage of cells in this peripheral region expressing alpha-smooth muscle actin compared with other locations in the interior of the remnant. The rationale for this hypothesis was based on prior immunohistochemical investigations of the distribution of alpha-smooth muscle actin-containing cells in the torn human anterior cruciate ligament. Eighteen torn meniscal specimens were obtained from 17 patients, 0.5 to 84 months after injury, and four torn allograft meniscal samples were retrieved from three patients, 11 to 49 months after implantation. Microtomed sections of paraffin-embedded tissue were stained with hematoxylin and eosin and a monoclonal antibody to alpha-smooth muscle actin. The cell density and percentage of cells containing alpha-smooth muscle actin were determined in the following zones: synovial, vascular, hypercellular with loose collagen, hypocellular with dense collagen, and organized collagen. A cellular layer that resembled synovium was present on the surface of all but two of the specimens. Vascular regions were often continuous with the synovium abutting the more interior loose collagen zones. The total cell density was greatest in each of the zones closest to the periphery (synovium, vascular, and loose collagen; p < 0.001), when compared to the interior of the tissue. The synovium-like layer was found to have the highest percentage of alpha-smooth muscle actin-expressing cells and the highest alpha-smooth muscle actin-containing cell density (p < 0.05). Similar results were found for the torn allograft menisci. These findings confirm the working hypothesis and suggest that the torn human meniscus is capable of mounting a reparative response, including the proliferation of cells capable of contributing to wound closure. This underscores the importance of providing a bridging scaffold into which such cells can migrate.

Enhanced cartilage tissue engineering by sequential exposure of chondrocytes to FGF-2 during 2D expansion and BMP-2 during 3D cultivation

Bovine calf articular chondrocytes, either primary or expanded in monolayers (2D) with or without 5 ng/ml fibroblast growth factor-2 (FGF-2), were cultured on three-dimensional (3D) biodegradable polyglycolic acid (PGA) scaffolds with or without 10 ng/ml bone morphogenetic protein-2 (BMP-2). Chondrocytes expanded without FGF-2 exhibited high intensity immunostaining for smooth muscle alpha-actin (SMA) and collagen type I and induced shrinkage of the PGA scaffold, thus resembling contractile fibroblasts. Chondrocytes expanded in the presence of FGF-2 and cultured 6 weeks on PGA scaffolds yielded engineered cartilage with 3.7-fold higher cell number, 4.2-fold higher wet weight, and 2.8-fold higher wet weight glycosaminoglycan (GAG) fraction than chondrocytes expanded without FGF-2. Chondrocytes expanded with FGF-2 and cultured on PGA scaffolds in the presence of BMP-2 for 6 weeks yielded engineered cartilage with similar cellularity and size, 1.5-fold higher wet weight GAG fraction, and more homogenous GAG distribution than the corresponding engineered cartilage cultured without BMP-2. The presence of BMP-2 during 3D culture had no apparent effect on primary chondrocytes or those expanded without FGF-2. In summary, the presence of FGF-2 during 2D expansion reduced chondrocyte expression of fibroblastic molecules and induced responsiveness to BMP-2 during 3D cultivation on PGA scaffolds.