Effects of Growth Factor Combinations TGF\(\beta\)3, GDF5 and GDF6 on the Matrix Synthesis of Nucleus Pulposus and Nasospetal Chondrocyte Self-Assembled Microtissues  
[PERSON]  
1 Trinity Centre for Biomedical Engineering, Trinity Biomedical Sciences Institute, Trinity College Dublin, The University of Dublin, D02 R590 Dublin, Ireland; [EMAIL_ADDRESS] (S.S.); [EMAIL_ADDRESS] (E.E.M.) [PERSON]  
1 Trinity Centre for Biomedical Engineering, Trinity Biomedical Sciences Institute, Trinity College Dublin, The University of Dublin, D02 R590 Dublin, Ireland; [EMAIL_ADDRESS] (S.S.); [EMAIL_ADDRESS] (E.E.M.) [PERSON]  
1 Trinity Centre for Biomedical Engineering, Trinity Biomedical Sciences Institute, Trinity College Dublin, The University of Dublin, D02 R590 Dublin, Ireland; [EMAIL_ADDRESS] (S.S.); [EMAIL_ADDRESS] (E.E.M.) Footnote 1: email: {[EMAIL_ADDRESS]}  
Footnote 2: email: {[EMAIL_ADDRESS]}  
Footnote 3: email: {[EMAIL_ADDRESS]}  
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disc is insufficient [8] to meet the requirements for successful treatment without significant culture expansion. In contrast, stem cells can be obtained from various sources (bone marrow or adipose tissue) and have been shown to survive and proliferate after implantation into the disc [9]. However, cell leakage at the injection site has been shown to result in osteophyte formation [10]. Mature chondrocytes produce a matrix similar to NP cells with nasospatial chondrocytes (NCs) being recently explored as an alternative non-disc cell source for IVD regeneration. NCs have a higher cell yield and sulphated glycosaminoglycan (sGAG) synthesis [11] when compared to articular chondrocytes (ACs) and MSCs [12]. Recently, [PERSON] et al. also demonstrated that NCs resulted in higher matrix synthesis levels in comparison to NP cells [13]. The use of NCs for IVD regeneration may present many advantages compared to NP cells, including a high cell yield [11], the ability to produce tissue in an age-independent manner [14; 15], enhanced matrix synthesis [13] and can be obtained with minimal donor site morbidity facilitating their autologous use. These intrinsic properties make NCs an attractive non-disc cell source for IVD regeneration. Growth factors regulate IVD homeostasis, including extracellular matrix (ECM) synthesis and degradation, cell differentiation, or apoptosis [16]. Researchers have widely investigated the potential of various growth factor-mediated induction of cells to mimic the expression profile resembling native disc tissue. Studies have shown that the activation of transforming growth factor beta (TGF\(\beta\)) signalling pathways delays IVD degeneration by increasing ECM synthesis [17; 18] and hence members of the TGF\(\beta\) superfamily may be potential candidates for driving disogenic differentiation and phenotype. TGF\(\beta\)3 has been shown to maintain the phenotype of disc cells in organ culture [19] and when encapsulated with MSCs, they induced IVD regeneration in vivo [20]. Growth differentiation factor 5 (GDF5) and GDF6 are members of the bone morphogenetic protein (BMP) family. GDF5 and GDF6 play important roles in the development of bones, limb joints, skull and axial skeleton [21] and are also expressed in developing cartilage, tendons and ligaments [22; 23]. In GDF5/6-knockout mouse models, the vertebral column showed severe lateral curvature and reduced staining of the IVDs indicating lower proteoglycan content [22]. These results suggest that GDF5 and GDF6 are required for normal development and maintenance of the IVD. In pellet culture, GDF5 results in reduced expression of the catabolic enzyme MMP13 in human chondrocytes [24]. There is also evidence to suggest that GDF-5 confers protection against NP cell apoptosis, promotes the synthesis of the main components of the extracellular matrix and can inhibit the activation of the NF-kB signalling pathway, thereby down-regulating the expression of inflammatory cytokines [25].