15502-m-pleumeekers

TFGβ1 [272]. In co-cultures, differences between h MSC-cell sources appear less clear. Acharya et al . demonstrated enhanced chondrocyte proliferation capacity and improved sGAG formation in pellets containing h BMSC/ b ACs compared to h AMSC/ b ACs. [80] Besides, 3 independent co-culture studies using AMSCs only showed limited or decreased effects of MSCs on chondrogenesis. [243-245] Such effect was hardly seen in co-culture studies using BMSCs only, which may propose that, compared to BMSCs, AMSCs seem less efficient in co- culture, Although we could not find a general beneficial effect of h BMSCs in co-cultures compared to h AMSCs in vitro and in vivo , we did show that in vitro , h BMSC/ b ACs outperformed h AMSC/ b ACs and hypertrophic gene expression was lower in h BMSC/bACs. True dissimilarities between h AMSCs and h BMSCs in co-culture are unfortunately hard to expose, as h MSC-cultures are highly heterogeneous and distinct population subsets will probably interfere with the reciprocal communication pathways in co-culture. Therefore, the purification of distinct subsets of h MSCs might enhance the particular capability of h AMSCs and h BMSCs in co-culture by eliminating interfering cells with limited potential, or even cells with inhibitory activity. Future research still needs to clarify whether the trophic role of MSCs in co-culture is truly a general MSC-characteristic produced by a distinct subset of the MSC- population or dependent on the original origin of the MSCs. Our data and that of others emphasize the importance of paracrine signaling pathways in co-culture comparatively to juxtacrine or gap-junctional signaling. Although the importance of direct cell-cell contact is still unclear in literature [263], such signaling pathways remained less important in our study, since alginate hydrogel impedes direct cell-cell contact and in pellet culture no beneficial effect of direct cell-cell contact was observed. On the contrary, b ACs produced less cartilage matrix in Transwell® system with h MSCs and the amount of cartilage matrix was further reduced in h MSC-conditioned medium. Although direct cell-cell contact seems less significant than paracrine signaling, it seems correspondingly important to secure a certain cell-cell distance for optimal cell communication. Furthermore, for optimal cell communication and subsequent cartilage regeneration, an optimal cell density and ratio of MSCs to ACs is imperative. Additionally, for cell-based cartilage repair, it would be ideal to only use low numbers of primary chondrocytes. Although Puelacher et al already recommended cell densities greater than 20x10 6 cells per milliliter [239], we could not increase the cell seeding density over 4x10 6 cells per milliliter, as the size of our experimental set-up did not enable higher densities. Additionally, we have replaced 80% of the b ACs by h MSCs (at a 4:1 ratio), as described previously. [74, 246] However, no consensus on optimal co-culture ratios is yet available. Future research needs to clarify if we could increase cell density while further reduce the number of primary chondrocytes (increase the MSC-chondrocyte-ratio) without inhibiting cartilage matrix production and stability. The species mismatch limited the translation of presented basic research to clinical application. However, the species mismatch was chosen to be able to discriminate between the role of the different cell types. We do not expect huge differences in fully human co- culture models, as both xenogeneic and autologous co-culture models have resulted in comparable outcomes, indicating that in both models comparable mechanisms are likely operational. [74] Our results confirmed previously published results of h MSCs combined with 103 AMSCs OR BMSCs FULFILL A TROPHIC ROLE IN CO-CULTURE 5