Abstract
OBJECTIVE: If dedifferentiated chondrocytes could be induced to redifferentiate in vitro, then we might thereby be furnished with a population of phenotypically stable cells for autologous implantation in reconstructive surgery. We therefore investigated the redifferentiation capabilities of chondrocytes which, having migrated from alginate beads to form a monolayer, were subsequently passaged. We also characterized the molecular traits of irreversibly dedifferentiated cells. METHODS: Human chondrocytes that had migrated from alginate beads to form a monolayer (passage 1) were passaged seven times (passages 2-8). Cells from each passage were then recultivated in alginate beads. We assessed the synthesis of type-II collagen, cartilage-specific proteoglycans, adhesion molecules (integrins), signaling proteins (Src-homology collagen [Shc] and extracellular-signal-regulated kinase [Erk]) and the apoptosis marker 'activated' caspase-3 in monolayer or secondary alginate cultures. RESULTS: The synthesis of cartilage-specific type-II collagen, alpha 3-integrin, Shc and activated Erk1/2 decreased rapidly after four passages in monolayer culture. Up to passage 4, cells redifferentiated in alginate culture. However, between passages 5 and 8, cells began to produce activated caspase-3; these cells not only failed to redifferentiate when recultivated in alginate, but underwent apoptosis. CONCLUSION: We conclude that the loss of chondrogenic potential by chondrocytes maintained in monolayer culture is associated with a decrease in the synthesis of cartilage markers and with a suppressed activation of key signaling proteins in the Ras-mitogen-activated protein kinase pathway (Shc and Erk1/2). These events lead to apoptosis. A decrease in Shc/Erk expression/interaction could serve as a recognition marker for irreversibly dedifferentiated chondrocytes in tissue engineering.