Metalloproteinase inhibition by Batimastat significantly reduced late lumen loss after balloon angioplasty by inhibition of constrictive arterial remodeling, whereas neointima formation was not inhibited by MMP inhibition.

These results suggest that clear differences between chondrocytes from healthy and OA joints exist and that these are not completely abolished during the process of de- and redifferentiation. Therefore, in vitro cartilage regeneration models, which use chondrocytes from OA joints, should be interpreted with care.

Tissue engineering of bone, by combining multipotent stromal cells (MSCs) with osteoconductive scaffolds, has not yet yielded any clinically useful applications so far. The fate and contribution of the seeded cells are not sufficiently clarified, especially at clinically relevant locations. Therefore, we investigated cell proliferation around the spine and at ectopic sites using noninvasive in vivo bioluminescence imaging (BLI) in relation to new bone formation. Goat MSCs were lentivirally transduced to express luciferase. After showing both correlation between MSC viability and BLI signal as well as survival and osteogenic capacity of these cells ectopically in mice, they were seeded on ceramic scaffolds and implanted in immunodeficient rats at two levels in the spine for spinal fusion as well as subcutaneously. Nontransduced MSCs were used as a control group. All rats were monitored at day 1 and after that weekly until termination at week 7. In mice a BLI signal was observed during the whole observation period, indicating survival of the seeded MSCs, which was accompanied by osteogenic differentiation in vivo. However, these same MSCs showed a different response in the rat model, where the BLI signal was present until day 14, both in the spine and ectopically, indicating that MSCs were able to survive at least 2 weeks of implantation. Only when the signal was still present after the total implantation period ectopically, which only occurred in one rat, new bone was formed extensively and the implanted MSCs were responsible for this bone formation. Ectopically, neither a reduced proliferative group (irradiated) nor a group in which the cells were devitalized by liquid nitrogen and the produced extracellular matrix remained (matrix group) resulted in bone formation. This suggests that the release of soluble factors or the presence of an extracellular matrix is not enough to induce bone formation. For the spinal location, the question remains whether the implanted MSCs contribute to the bone regeneration or that the principal mechanism of MSC activity is through the release of soluble mediators.

Cartilage regeneration based on isolated and culture-expanded chondrocytes is studied in a variety of in vitro models, but with varying morphological quality of tissue synthesized. The goal of the present study was to investigate the extent of the influence of expansion and redifferentiation conditions on final tissue morphology by comparing 2 expansion and redifferentiation methods. Chondrocytes from 9 human donors were expanded in medium without growth factor supplementation (basic expansion condition [BEC]) or in medium with basic fibroblast growth factor (bFGF) supplementation (growth factor supplemented expansion condition [GFSEC]). After expansion, cells were either redifferentiated in pellet culture or seeded on collagen type II-coated filters. Post-expansion mRNA levels of collagen type I and II and Sox-5, -6, and 9, measured by semiquantitative real-time polymerase chain reaction (PCR), suggested that expansion in GFSEC results in increased dedifferentiation compared to BEC. However, after 28 days of redifferentiation culture, morphology of tissue synthesized by GFSEC-expanded chondrocytes scored significantly higher on the Bern scale compared to BEC (6.4 +/- 0.3 points vs. 4.5 +/- 0.3 points in pellet culture and 6.0 +/- 0.4 points vs. 4.5 +/- 0.3 points on collagen-coated filters; p < 0.05). Expansion in GFSEC compared to BEC increased proteoglycan (PG) synthesis rate at day 9 (4.0-fold in pellet culture and 1.9-fold on collagen-coated filters; p < 0.01), PG release (6.7-fold in pellet culture and 3.2-fold on collagen-coated filters; p < 0.001), and final PG content at day 28 (1.6-fold in pellet culture and 1.5-fold on collagen-coated filters; p < 0.05). Redifferentiation on collagen-coated filters compared to pellet culture increased PG synthesis rate at day 9 (5.2-fold in BEC-expanded chondrocytes and 2.6-fold in GFSEC-expanded chondrocytes; p < 0.01), PG release (4.2-fold in BEC-expanded chondrocytes and 3.1-fold in GFSECexpanded chondrocytes; p < 0.01), and final PG content (1.3-fold in BEC-expanded chondrocytes and 1.9- fold in GFSEC-expanded chondrocytes; p < 0.01). Moreover, as visualized via electron microscopy, chondrocytes and organization of extracellular matrix cultured on filters was more similar to those found for hyaline cartilage. In conclusion, chondrocyte expansion in GFSEC and redifferentiation on collagen-coated filters resulted in most optimal chondrogenesis.