Articular chondrocytes and mesenchymal stem cells seeded on biodegradable scaffolds for the repair of cartilage in a rat osteochondral defect model
Introduction
While a number of treatment options currently exist for the repair of articular cartilage defects, these options primarily lead to short-term functional repair, but are not capable of achieving stable, long-term repair of the tissue [1], [2]. Autologous chondrocyte implantation (ACI) is generally one of the most often-used procedures for the treatment of cartilage defects, and has been shown to have some success in repairing the damaged tissue [1], [3]. However, the isolation of appropriate numbers of autologous chondrocytes is not without challenges. Chondrocytes are present in relatively low densities in native articular cartilage [4], and the isolation of sufficient numbers would lead to large donor site morbidity [5]. Furthermore, the in vitro expansion of chondrocytes is associated with a rapid dedifferentiation of the cells into a more fibroblastic phenotype, which ultimately leads to the production inferior tissue [6]. Thus, numerous approaches have been investigated in order to enhance the chondrogenic phenotype of expanded cells or to reduce the demand for chondrocytes in the treatment of articular cartilage defects [7].
Co-cultures of articular chondrocytes and mesenchymal stem cells (MSCs) are one approach that has been proposed to reduce the demand for articular chondrocytes and thus improve articular cartilage treatments [8], [9], [10], [11]. When co-cultured with MSCs, articular chondrocytes have been observed to undergo enhanced proliferation and matrix production [9], [12], [13], [14]. This effect, which has been shown to be independent of MSC source or culture condition [15], would allow for the use of reduced numbers of chondrocytes to achieve an equal chondrogenic outcome [11]. Furthermore, the co-cultured cell population has been demonstrated to be more sensitive to chondrogenic stimuli, such as transforming growth factor-β3 (TGF-β3), and to produce a phenotype that is more stable after the removal of the stimuli, compared to monocultures of either cell type [8]. While the beneficial effects of MSCs on chondrocytes are crucial to the performance of these co-cultures, chondrocytes have similarly been demonstrated to have beneficial effects on MSCs, which mitigates some disadvantages associated with MSC chondrogenesis. The chondrogenesis of MSCs is challenged by the eventual hypertrophy and mineralization of these cells after extended culture in chondrogenic conditions [16]. However, co-culture with articular chondrocytes has been demonstrated to reduce the hypertrophy of MSCs in culture [10], [17], [18]. Thus, the advantages of co-cultures of articular chondrocytes and MSCs for the in vitro generation of articular cartilage is well-documented; however the use of this cell population for in vivo repair of articular cartilage defects has not been investigated.
The objective of the present study was to investigate the use of co-cultures of articular chondrocytes and bone marrow-derived MSCs for the in vivo repair of articular cartilage in a rat osteochondral defect. We hypothesized that the use of co-cultures of chondrocytes and MSCs would lead to equal or greater cartilage repair compared to chondrocytes alone, thus allowing for the use of reduced numbers of chondrocytes. Therefore, we implanted electrospun poly(ɛ-caprolactone) (PCL) scaffolds, seeded with MSCs, chondrocytes, or co-cultures of chondrocytes and MSCs into the trochlear groove of rats and evaluated the tissue repair via histology and microcomputed tomography.
Section snippets
Study design
The groups investigated in this study are outlined in Table 1. Briefly, bovine articular chondrocytes and rat bone marrow-derived MSCs were seeded onto electrospun PCL scaffolds to create three separate experimental groups. The AC group consisted of articular chondrocytes seeded in monoculture at a density of 40,000 cells per scaffold; the MSC group consisted of MSCs seeded in monoculture at a density of 40,000 cells per scaffold. The CC group consisted of articular chondrocytes and MSCs seeded
μCT imaging and analysis
μCT analysis (Fig. 1) found mineralized bone regeneration in the empty, blank, and MSC samples that resulted in 28–35% of the defect site filled with new bone. No effect of the PCL scaffold or MSCs was observed on the total mineralized bone volume in the defect. In the AC and CC samples, negligible mineralized bone growth was observed with a total bone volume of only 0.75–1.0% of the defect site. Thus, significantly lower mineralized bone volume was detected in the AC and CC samples compared to
Discussion
Clinically, autologous chondrocytes have long been recognized for their ability to repair chondral defects when transplanted in vivo [3]. Currently, the ACI procedure involves initial biopsy of autologous cartilage for the isolation of articular chondrocytes, expansion of cells in vitro, and implantation of expanded cells into the defect, which is sealed by a layer of periosteum sutured over top [1]. While the technique is widely used in the clinic, the use of autologous chondrocytes presents
Conclusions
This study demonstrated the ability of co-cultures of articular chondrocytes and MSCs to repair cartilage defects in the rat trochlear groove defect. These results have important implications for cartilage tissue engineering, as they demonstrate that such co-cultures could be used to reduce the total number of chondrocytes needed for cartilage repair, while still achieving an equivalent level of cartilage repair.
Acknowledgments
This work was supported by the National Institutes of Health grant R01 AR057083.
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