A novel exogenous concentration-gradient collagen scaffold augments full-thickness articular cartilage repair.

Mimura T., Imai S., Kubo M., Isoya E., Ando K., Okumura N., Matsusue Y. A novel exogenous concentration-gradient collagen scaffold augments full-thickness articular cartilage repair. Osteoarthritis and Cartilage 2008;16(9):1083-91.

http://dx.doi.org/10.1016/j.joca.2008.02.003

Background

Articular cartilage has poor regenerative properties because it is avascular and aneural. Furthermore, chondrocytes, the cells responsible for matrix synthesis and maintenance, are essentially immobilized in lacunae (small spaces in the cartilage), so they are unable to migrate to the site of injury and promote healing. Thus, partial defects in the cartilage show little healing if any. However, if the injury penetrates the subchondral bone, cartilage can be regenerated through migration and differentiation of mesenchymal stem cells (MSCs) or other mesenchymal progenitor cells. Small diameter (less than 3 mm) full-thickness defects can be repaired in this manner. On the other hand, large diameter defects (larger than 5 mm) are filled with fibrous tissue, which is much weaker than the original articular cartilage. Previous studies have used collagen gels and membranes seeded with chondrocytes to treat the defect. This study uses a novel concentration gradient (CG) collagen type I gel to recruit MSCs from the host (Japanese white rabbit) to the center of the defect to enhance repair.

Summary

To create a concentration gradient (CG) collagen gel, the authors formed composites of two gels with different concentrations of collagen type I. Two composites were made: a 33% CG gel (0.18% and 0.24% collagen) and a 50% CG gel (0.18% and 0.27% collagen).

In order to demonstrate the superiority of the CG gel, the researchers created artificial defects in the cartilage of the rabbits and implanted a 33% or 50% CG gel, or a non-composite gel (0.18% collagen). Histological sections were prepared at set time points (1, 2, 3, 4, 8, 12 weeks post operation). To detect MSCs, the animals were injected with BrdU one hour prior to sacrifice. BrdU will be incorporated by proliferating cells, thus they will not stain chondrocytes.

Shown in Fig. 7 are the histological sections for week 2. An interesting result is that the 33% CG gel has more BrdU cells in the central region than the 50% CG gel does, indicating higher levels of cell migration.

At week 3, the researchers found that the 33% CG gel had significantly fewer BrdU-positive cells in the peripheral regions compared to the control and 50% CG gel. Taken together with the result from week 2, the researchers suggested that the 33% CG gel was superior at recruitment of MSCs.

The authors suggest that migration occurs via haptotaxis (migration towards higher density of adhesion sites). To study the haptotactic effect of collagen, combinations of collagen gels were studied in in vitro migration assays. Because the CG gel was eventually to be implanted into a Japanese white rabbit, MSCs were taken from those rabbits to be used in the experiment.

Interestingly, cell migration peaks at a certain CG and then drops later. The cause of this behavior may be the reduced mobility at higher binding site densities, which competes with the haptotactic effect of the collagen and decreases the amount of cell migration. This would also explain the result seen in Fig. 7, where the higher CG gel appears to induce less migration.

Articular cartilage tissue formation is also evaluated. The researchers used a specific histological grading scale to evaluate the cartilage at 4, 8, and 12 weeks post-op. While the 33% CG gel has superior characteristics over the control in the 4 and 8 week period, no significant difference was noted at the 12 week period. Furthermore, no significant difference was noted between the 33% and 50% CG gels.

Critique

What is peculiar about this work is that at the 12 week postoperative time point, there is no significant difference between a non-composite collagen gel and the two CG gels. This raises the question of whether increasing migration of MSCs into the defect zone is of any help at all in the long term. The authors claim that though there may not be significant differences in between the histological grades, the 33% CG gel leads at every time point. However, it might be possible that the grading scale they used (shown in the paper) may give unnecessary weight to certain aspects of the tissue.

Another issue that ought to have been addressed is the lack of consistency between histological sections analyses. In the paper, data for the central region of the defect is only shown for week 2, while peripheral region data is only shown for week 3. The authors did not explain why certain data was omitted or why they chose to analyze those particular regions at those specific time points.