Proteoglycan and collagen accumulation by passaged chondrocytes can be enhanced through side-by-side culture with primary chondrocytes.

Identifying a source of sufficient numbers of chondrocytes for cartilage tissue engineering is a major factor limiting its use clinically. Previously we demonstrated that combined coculture of passaged dedifferentiated articular chondrocytes with primary bovine chondrocytes will induce their redifferentiation. In this study we determine whether these two cell types have to be in contact, whether human chondrocytes respond similarly, and whether the ability of primary cells to influence passaged cells depends on the age of the donor. Coculture of primary and passaged bovine chondrocytes grown on filter inserts placed in the same culture well but not in direct contact resulted in the passaged cells accumulating matrix rich in proteoglycans and type II collagen. There was upregulation of type II collagen and Sox9 and decrease in type I collagen gene expression in the passaged cells, to levels not significantly different from those of primary chondrocytes. Passaged chondrocytes obtained from older animals responded similarly to cells from younger animals. Further, passaged human chondrocytes were also induced to form cartilage tissue when placed in side-by-side culture with bovine chondrocytes; these data suggest that a soluble factor(s) may be responsible for redifferentiation of passaged chondrocytes and that it is not species specific. The responsiveness of human chondrocytes to this factor(s) suggests that this approach may be suitable to overcome the problem of limited chondrocyte numbers for cartilage tissue engineering.