Articular cartilage was predicted to be one of the first tissues that could successfully be regenerated, but this proved not to be the case. In contrast, bone but also vasculature and cardiac tissues have seen numerous successful reparative approaches, despite consisting of multiple cell and tissue types and thus possessing more complex design requirements. Here, we use bone regeneration successes to highlight cartilage regeneration challenges, namely selecting appropriate cell sources and scaffolds, creating biomechanically suitable tissues, and integrating to native tissue. Also discussed are technologies addressing hurdles of engineering a tissue possessing mechanical properties unmatched in man-made materials and functioning in environments unfavorable to neotissue growth.

Replacement of degenerated cartilage with cell-based cartilage products may offer a long-term solution to halt arthritis' degenerative progression. Chondrocytes are frequently used in cellbased FDA-approved cartilage products; yet human marrow-derived stromal cells (hMSCs) show significant translational potential, reducing donor site morbidity and maintaining their undifferentiated phenotype with expansion. This study sought to investigate the effects of transforming growth factor b1 (TGF-b1), growth/differentiation factor 5 (GDF-5), and bone morphogenetic protein 2 (BMP-2) during postexpansion chondrogenesis in human articular chondrocytes (hACs) and to compare chondrogenesis in passaged hACs with that of passaged hMSCs. Through serial expansion, chondrocytes dedifferentiated, decreasing expression of chondrogenic genes while increasing expression of fibroblastic genes. However, following expansion, 10 ng/mL TGF-b1, 100 ng/mL GDF-5, or 100 ng/mL BMP-2 supplementation during three-dimensional aggregate culture each upregulated one or more markers of chondrogenic gene expression in both hACs and hMSCs. Additionally, in both cell types, the combination of TGF-b1, GDF-5, and BMP-2 induced the greatest upregulation of chondrogenic genes, that is, Col2A1, Col2A1/Col1A1 ratio, SOX9, and ACAN, and synthesis of cartilage-specific matrix, that is, glycosaminoglycans (GAGs) and ratio of collagen II/I. Finally, TGF-b1, GDF-5, and BMP-2 stimulation yielded mechanically robust cartilage rich in collagen II and GAGs in both cell types, following 4 weeks maturation. This study illustrates notable success in using the self-assembling method to generate robust, scaffold-free neocartilage constructs using expanded hACs and hMSCs. STEM CELLS

The ability to replace the knee meniscus requires a material possessing adequate geometrical and biomechanical properties. Meniscal tissue engineering attempts have been unable to produce tissue with collagen content and biomechanical properties, particularly tensile properties, mimicking native menisci. In an effort to obtain the geometric properties and the maturational growth necessary for the recapitulation of biochemical and, thus, biomechanical properties, a scaffoldless cell-based system, the self-assembly process, was used in conjunction with the catabolic enzyme chondroitinase-ABC and TGF-β1. We show that combinations of these agents resulted in maturational growth as evidenced by synergistic enhancement of the radial tensile modulus by 5-fold and the compressive relaxation modulus by 68%, and additive increases of the compressive instantaneous modulus by 136% and Col/WW by 196%. This study shows that tissue engineering can produce a biomaterial that is on par with the biochemical and biomechanical properties of native menisci.

ObjectiveThis study aimed to improve the functional properties of anatomically-shaped meniscus constructs through simultaneous tension and compression mechanical stimulation in conjunction with chemical stimulation.MethodsScaffoldless meniscal constructs were subjected to simultaneous tension and compressive stimulation and chemical stimulation. The temporal aspect of mechanical loadingwas studied by employing two separate five day stimulation periods. Chemical stimulation consisted of the application of a catabolic GAG-depleting enzyme, chondroitinase ABC (C-ABC), and an anabolic growth factor, TGF-β1. Mechanical and chemical stimulation combinations were studied through a full-factorial experimental design and assessed for histological, biochemical, and biomechanical properties following 4 wks of culture.ResultsMechanical loading applied from days 10–14 resulted in significant increases in compressive, tensile, and biochemical properties of meniscal constructs. When mechanical and chemical stimuliwere combined significant additive increases in collagen per wet weight (4-fold), compressive instantaneous (3-fold) and relaxation (2-fold) moduli, and tensile moduli in the circumferential (4-fold) and radial (6-fold) directions were obtained.ConclusionsThis study demonstrates that a stimulation regimen of simultaneous tension and compression mechanical stimulation, C-ABC, and TGF-β1 is able to create anatomic meniscus constructs replicating the compressive mechanical properties, and collagen and GAG content of native tissue. In addition, this study significantly advances meniscus tissue engineering by being the first to apply simultaneous tension and compression mechanical stimulation and observe enhancement of tensile and compressive properties following mechanical stimulation.

The ability to repair damaged cartilage is a major goal of musculoskeletal tissue engineering. Allogeneic (same species, different individual) or xenogeneic (different species) sources can provide an attractive source of chondrocytes for cartilage tissue engineering, since autologous (same individual) cells are scarce. Immune rejection of non-autologous hyaline articular cartilage has seldom been considered due to the popular notion of “cartilage immunoprivilege.” The objective of this study was to determine the suitability of allogeneic and xenogeneic engineered neocartilage tissue for cartilage repair. To address this, scaffold-free tissue engineered articular cartilage of syngeneic (same genetic background), allogeneic, and xenogeneic origin were implanted into two different locations of the rabbit knee (n=3 per group/location). Xenogeneic engineered cartilage and control xenogeneic chondral explants provoked profound innate inflammatory and adaptive cellular responses, regardless of transplant location. Cytological quantification of immune cells showed that, while allogeneic neocartilage elicited an immune response in the patella, negligible responses were observed when implanted into the trochlea; instead the responses were comparable to microfracture-treated empty defect controls. Allogeneic neocartilage survived within the trochlea implant site and demonstrated graft integration into the underlying bone. In conclusion, the knee joint cartilage does not represent an immune privileged site, strongly rejecting xenogeneic but not allogeneic chondrocytes in a location-dependent fashion. This difference in location-dependent survival of allogeneic tissue may be associated with proximity to the synovium.

Objective To describe a surgical technique utilizing a regenerative approach and internal fixation for immediate reconstruction of critical size bone defects following segmental mandibulectomy. Study design Prospective case series Animals Dogs (n=4) that had reconstruction following segmental mandibulectomy for treatment of malignant or benign tumors. Methods Using a combination of extraoral and intraoral approaches, a locking titanium plate was contoured to match the native mandible. Following segmental mandibulectomy, the plate was secured and a compression resistant matrix (CRM) infused with rhBMP-2, implanted in the defect. The implant was then covered with a soft tissue envelope followed by routine intraoral and extraoral closure. Results All dogs that had mandibular reconstruction healed with intact gingival covering over the mandibular defect and had immediate return to normal function and occlusion. Mineralized tissue formation was observed clinically within 2 weeks and solid cortical bone formation within 3 months. Computed tomographic findings at 3 months postoperatively demonstrated that the newly regenerated mandibular bone had ∼50% of the bone density and porosity compared to the contralateral side. No significant complications were noted. Conclusion Mandibular reconstruction using internal fixation and CRM infused with rhBMP-2 is an excellent solution for immediate reconstruction of segmental mandibulectomy defects in dogs. Clinical Relevance In dogs with a segmental mandibulectomy, reconstruction using rhBMP-2 and a CRM should be considered a viable surgical option.

Objective Osteoarthritis affects 1 in 8 American adults over the age of 25 and in the US is a leading cause of chronic disability. Therefore, researchers are pursuing investigations on treating this naturally occurring joint disease. Finding the appropriate animal model for translational purposes is of utmost importance. The aim of the present study is to report the occurrence of naturally-occurring osteoarthritis in the domestic rabbit in the context of an appropriate animal model system. Methods A six-year radiographic retrospective study of domestic rabbits was conducted to assess the presence and severity of naturally occurring osteoarthritis in four major joints: the hip, knee, shoulder, and elbow. Results The rabbit experiences radiographic signs of naturally-occurring osteoarthritis. There was a significant influence of age on the development of the disease. Rabbits begin to have radiographic signs of the disease as early as 1 year of age with older rabbits experiencing over 70% occurrence. The most commonly effected joints were the knee and the hip. In addition, there was certain correlation of weight with the occurrence of osteoarthritis. Conclusion Discovery of new osteoarthritis treatment modalities relies on documenting efficacy in a relevant animal model prior to clinical translation. Despite previous claims on the ability of the rabbit’s cartilage to heal spontaneously and readily repair, we demonstrate that radiographically rabbits demonstrate progressive osteoarthritis. We have found that the rabbit is an excellent spontaneously arising model that may allow a predictable translation of results obtained in bioengineering experiments pertaining to the naturally occurring human disease.

The insufficient healing capacity of articular cartilage necessitates mechanically functional biologic tissue replacements. Using cells to form biomimetic cartilage implants is met with the challenges of cell scarcity and donor site morbidity, requiring expanded cells that possess the ability to generate robust neocartilage. To address this, this study assesses the effects of expansion medium supplementation (bFGF, TFP, FBS) and self-assembled construct seeding density (2, 3, 4 million cells/5 mm dia. construct) on the ability of costochondral cells to generate biochemically and biomechanically robust neocartilage. Results show TFP (1 ng/mL TGF-β1, 5 ng/mL bFGF, 10 ng/mL PDGF) supplementation of serum-free chondrogenic expansion medium enhances the post-expansion chondrogenic potential of costochondral cells, evidenced by increased glycosaminoglycan content, decreased type I/II collagen ratio, and enhanced compressive properties. Low density (2 million cells/construct) enhances matrix synthesis and tensile and compressive mechanical properties. Combined, TFP and Low density interact to further enhance construct properties. That is, with TFP, Low density increases type II collagen content by over 100%, tensile stiffness by over 300%, and compressive moduli by over 140%, compared with High density. In conclusion, the interaction of TFP and Low density seeding enhances construct material properties, allowing for a mechanically functional, biomimetic cartilage to be formed using clinically relevant costochondral cells.