Effect of different ascorbate supplementations on in vitro cartilage formation in porcine high-density pellet cultures

Articular cartilage has only very limited potential for self-repair and regeneration. For this reason, various tissue engineering approaches have been developed to generate cartilage tissue in vitro. Usually, most strategies require ascorbate supplementation to promote matrix formation by isolated chondrocytes. In this study, we evaluate and compare the effect of different ascorbate forms and concentrations on in vitro cartilage formation in porcine chondrocyte high-density pellet cultures. l-ascorbate, sodium l-ascorbate, and l-ascorbate-2-phosphate were administered in 100 μM, 200 μM, and 400 μM in the culture medium over 16 days. Pellet thickness increased independently from the supplemented ascorbate form and concentration. Hydroxyproline content increased as well, but here, medium concentration of AsAP and low concentration of AsA showed a more pronounced effect. Proteoglycan and collagen formation were evaluated histologically and could be proven in all supplemented cultures. Non-supplemented cultures, however, showed no stable matrix formation at all. Effects on the gene expression pattern of cartilage marker genes (type I and type II collagen, aggrecan, and cartilage oligomeric matrix protein (COMP)) were studied by real-time RT-PCR and compared to non-supplemented control cultures. Expression level of cartilage marker genes was elevated in all cultures showing that dedifferentiation of chondrocytes could be prevented. Again, all supplementations caused a similar effect except for low concentration of AsA, which resulted in an even higher expression level of all marker genes. Besides that, we could not detect a pronounced difference between ascorbate and its derivates as well as between the different concentrations.