After how many passages in monolayer culture do the chondrocytes start to de-differentiate?

Mature chondrocytes from articular cartilage are postmitotic producers of extracellular matrix components like collagen types II IX and XI and proteoglycans. Upon release from the tissue of origin and seeding in monolayer culture cells re-enter the cell cycle and proliferate. After a period of approx. 1-3 weeks they will gradually start dedifferentiation and will adopt a fibroblastic phenotype. Dedifferentiation can be detected by decreased collagen type II or increased collagen type I expression as well as by the appearence of the fibroblast marker Thy-1/CD90. Re-differentiation can be induced by changing from conventional monolayer culture to a 3-D culture system.