Integrins in ageing cartilage tissue in vitro.

Matrix-cell interactions are of great importance for numerous cell functions whereby integrins play an essential role as transmitters of extracellular signals. In cultures of ageing cartilage tissue (organoid or high density cultures) cartilage cells occur on the surface of which thick fibrils of collagen type I are deposited. Since integrins, in their role as receptors, cause an interaction between matrix components and cell membrane, we tried to demonstrate immunomorphologically (light and electron microscopically) the corresponding integrin receptors for collagen type I (beta 1 alpha 1 and beta 1 alpha 2) on the surface of these ageing cartilage cells. Cultures of normal, i.e. young cartilage tissue exhibit only beta 1 alpha 3- and beta 1 alpha 5-receptors; labelling against the integrins beta 1 alpha 1 and beta 1 alpha 2 is not possible in this case. Our results show that after the occurrence of thick fibrils cartilage cells express new receptors (beta 1 alpha 1 and beta 1 alpha 2) on the cell membrane. Thus, in ageing or dedifferentiating cartilage tissue it is not only the synthesis programme of matrix components (e.g. instead of collagen type II >> collagen type I) which changes but also the integrins (instead of alpha 3/beta 1, alpha 5/beta 1 >> alpha 1/beta 1, alpha 2/beta 1) so that new collagen types can be bound. These findings may also serve for a better understanding and interpretation of cartilage changes in vivo during ageing and under pathological conditions.