The requirements for cartilage disc formation by self-assembling MSCs are not well understood (Table 1 ). We showed that the ECM coating and seeding density are important determinants of functional cartilage disc development by self-assembling hMSCs. Type I collagen enhanced chondrogenesis and promoted disc formation, whereas high cell denseness improved tissue properties but resulted in less frequent disk formation. Comparing pellet and disc cultures, we recognized compositional and morphological differences during in vitro tradition and following in vivo implantation. We also demonstrated that prolonged chondrogenic induction in vitro expedited endochondral ossification of cartilage discs in vivo.
The importance of ECM for chondrogenic differentiation associated with MSCs has been studied in vivo and in vitro. Developing studies revealed interesting dynamics of ECM formation since the prechondrogenic mesenchyme was enriched with type I collagen whereas the mature cartilage was enriched with kind II collagen [ 26 , 27 ]. Although both collagen varieties have been used for in vitro chondrogenic induction of MSCs, a recent study found important differences between the early plus late stage ECM during chondrogenesis [ 28 – 30 ]. Early-stage ECM was rich in type I collagen and other cell-binding proteins, and strongly induced chondrogenesis of MSCs [ 28 – 30 ]. In contrast, late-stage ECM was rich in type II collagen, deficient in type I collagen and other cell-binding proteins, and resulted in poor chondrogenic induction of MSCs [ 30 ]. Due to alternative splicing, mature chondrocytes synthesize type IIB procollagen that lacks a TGF-β -binding chordin-like domain [ 31 ]. Thus, it was proposed that will type II collagen in ECM produced during past due chondrogenesis disrupted TGF-β -mediated chondrogenesis [ 30 ].
Interestingly, we observed similar distinctions between the cartilage discs formed on membranes coated along with type I collagen (Col1) versus type II collagen (Col2). Col1 discs exhibited better tissue properties since revealed by biochemical, histological, and mechanical analyses. The indegent ECM deposition and mechanical properties of Col2 dvds can be attributed to early deficits in the gene expression associated with cartilage markers. In agreement with the developmental studies, we all found uniform expression of type I collagen inside the first week of chondrogenic induction, which confirms the importance during early chondrogenesis of hMSCs.
Furthermore, Col1 coating enabled disc development most reproducibly. In contrast, self-assembling hMSCs on uncoated walls condensed and failed to form discs. Disc formation upon type II collagen was less frequent than upon type I collagen. Although we could not compare joining forces, our results suggest that type I collagen is a better anchor for the self-assembling hMSCs to resist moisture build-up or condensation forces. Cell seeding density was shown to modulate the fibrous connective tissue cartilage formation by MSCs cultured in scaffold [ 32 , 33 ]. Here, we showed that increasing the cell seeding density improved the compressive modulus, sGAG content, plus thickness of the discs significantly. However , increasing the cellular seeding density also promoted condensation and yielded much less frequent disc formation. During mesenchymal condensation, MSCs go through actomyosin contraction and cytoskeletal rearrangement [ 34 ]. Most likely the increased condensation forces at high cell densities overcame the anchoring forces between the membrane ECM as well as the attached cells.
Disc tradition formed hyaline cartilage while pellet culture formed fibrocartilage with more type I collagen. This outcome was related to the fibrogenic tensile forces associated with the increase in surface area from the pellet, but not the disc, which grew in thickness [ 8 ]. Disc culture also enhanced collagen network maturation [ 35 ]. We confirmed these findings and discovered that whereas pellet culture formed fibrocartilage with thick type I collagen at the surface, and disc lifestyle formed hyaline cartilage with uniform deposition of sGAG and type II collagen. Furthermore, hMSCs differentiated within disc culture expressed more COL2A1 relative to ACAN within pellet culture. Consequently, the discs exhibited a higher COL relative to sGAG content than the pellets. Morphologically, the disks were stratified and resembled the articular cartilage, along with lubricin lining only the top surface.
As the primary goal of the overall study has been to investigate a method for cartilage formation by hMSCs as well as implications in vitro and in vivo, we chose to work with a source of hMSCs that has been well characterized, shown to undergo strong differentiation, and extensively published. This enabled us in order to consistently generate cartilage tissues in different formats for within vitro and in vivo analyses and compare with previous function. However , the heterogeneity of MSCs is well recorded and the findings from this study will need to be verified with cellular material isolated from different donors and sources such as the bone fragments marrow, adipose tissue, and synovium. The lack of donor plus cell source diversity is a limitation of this study that people seek to address in future investigations.
Although cartilage pellets and discs formed through self-assembling hMSCs both achieved structural integrity, other strategies may be necessary to grow tissues for large-scale therapeutic apps. One such method for forming large and anatomically shaped the fibrous connective tissue cartilage is the fusion of pellets atop bone substrate that people demonstrated in our previous study [ 7 ]. Foreseeably, bigger discs could also be formed by inducing cartilage formation through self-assembling hMSCs on a larger coated membrane. However , the studies suggest that improving the thickness and mechanical real estate of the discs comes at a cost of a lower yield associated with disc formation, necessitating the use of scaffolds for the cultivation associated with larger tissue constructs.
With respect to the extent of chondrogenic induction, pellets formed from MSCs are replaced by fibrous tissue in vivo or even undergo endochondral ossification [ 10 , 36 ]. To the best of the knowledge, the in vivo fate of the cartilage dvds has not been studied. Thus, we assessed the in vivo implications of different culture regimens and found that both pellets and the discs underwent endochondral ossification following extented chondrogenic induction, with remarkably different outcomes. Endochondral ossification of the pellets resembled callus maturation during long bone tissue repair, whereby bone enveloped the pellet and the cartilage loss progressed into the bulk of the tissue. Instead, endochondral ossification of the discs resembled epiphyseal cartilage maturation throughout skeletal development, whereby bone formed at the bottom and the cartilage loss progressed toward the surface [ 37 ].
Recent studies showed that hypertrophic induction following chondrogenic induction promoted terminal chondrocyte differentiation through hMSCs in vitro and enhanced endochondral ossification within vivo. [ 11 , 38 ]. Here, we showed that extented chondrogenic induction of self-assembling hMSCs in disc tradition similarly expedited endochondral ossification in vivo. After 4 weeks of implantation, discs cultured for 10 several weeks exhibited greater mineral volume and density than dvds cultured for 6 weeks or 8 weeks. Adult bone and marrow stroma were present in the disks cultured for 10 weeks but not 6 weeks or even 8 weeks, despite the same duration of implantation. Oddly enough, discs cultured for 6 weeks mineralized at the edge whereas discs cultured for 10 weeks mineralized all over the place. This suggests early peripheral maturation that could be resulted through the normal compressive force at the insert wall [ 39 ]. These results demonstrate that the in vitro culture routine can modulate endochondral ossification of cartilage formed simply by self-assembling hMSCs in vivo.
As the primary goal of the in vivo study has been to evaluate cartilage stability, we only evaluated the cells outcomes after 4 weeks of ectopic implantation. Nevertheless , it is likely that the tissues would undergo further endochondral ossification with prolonged implantation. A recent 8-week implantation study at the fate of chondrocytes terminally differentiated from hMSCs discovered that chondrocytes released from lacunae during endochondral ossification underwent a reversion of differentiation to become marrow stromal cells within the ossicle [ 40 ]. With prolonged implantation, it is possible that terminally differentiated chondrocytes in both the disks and the pellets could have undergone a similar reversion, leading to the particular formation of bone and marrow stroma.
However , the presence of osteopontin and mineral pointed out that during early stages of ectopic implantation, the within vivo environment promoted terminal differentiation of chondrocytes together a chondrogenic pathway. While a switch to osteogenic difference would also have resulted in the deposition of osteopontin plus mineral, Movat’ s pentachrome staining showed that the tissue were residing in cartilage matrix, and were unlikely to get undergone a reversion of chondrogenic differentiation.
During in vitro culture, we failed to observe a significant decrease in DNA content up to 6 days of chondrogenic differentiation in disc or pellet lifestyle. This suggests that there was no progressive loss of cells throughout chondrogenic induction. Likewise, histological observations confirmed that the lacunae in the cartilage tissues remained well nucleated after implantation, suggesting the persistence of viable chondrocytes. However , previous studies suggest that terminal differentiation of chondrocytes can result in apoptosis, which mediates cartilage turnover [ 41 ].
The long-term in vivo stability associated with cartilage formed from MSCs remains a challenge [ 3 , 42 ]. Recent studies found that modulation of Wnt signaling and hypoxia can enhance the in vivo stability associated with cartilage formed by MSCs in pellet culture [ 43 , 44 ]. Conceivably, trophic and biophysical stimuli can enhance the disc culture and enable the growth of a practical, organized and stable articular cartilage in vitro through MSCs. As the articular cartilage is anisotropic, implementation associated with spatiotemporal control during in vitro culture could recapitulate native gradients and further improve tissue organization and balance [ 45 ].