CHONDROCYTE SURVIVAL AND DIFFERENTIATION IN-SITU ARE INTEGRIN-MEDIATED

Chondrocytes in specific areas of the chick sternum have different dev elopmental fates, Cephalic chondrocytes become hypertrophic and secret e type X collagen into the extracellular matrix prior to bone depositi on, Middle and caudal chondrocytes remain cartilaginous throughout dev elopment and continue to secrete collagen types II, IX, and XI, The in teraction of integrin receptors with extracellular matrix molecules ha s been shown to affect cytoskeleton organization, proliferation, diffe rentiation, and gene expression in other cell types. We hypothesized t hat chondrocyte survival and differentiation including the deposition into interstitial matrix of type X collagen may be integrin receptor m ediated, To test this hypothesis, a serum-free organ culture sternal m odel that recapitulates normal development and maintains the three-dim ensional relationships of the tissue was developed, We examined chondr ocyte differentiation by five parameters: type X collagen deposition i nto interstitial matrix, sternal growth, actin distribution, cell shap e, and cell diameter changes, Additional sterna were analyzed for apop tosis using a fragmented DNA assay, Sterna were organ cultured with bl ocking antibodies specific for integrin subunits (alpha 2, alpha 3, or beta 1). In the presence of anti-beta 1 integrin (25 mu g/ml, clone W 1B10), type X collagen deposition into interstitial matrix and sternal growth were significantly inhibited, In addition, all chondrocytes we re significantly smaller, the actin was disrupted, and there was a sig nificant increase in apoptosis throughout the specimens, Addition of a nti-alpha 2 (10 mu g/ml, clone P1E6) or anti-alpha 3 (10 mu g/ml, clon e P1B5) integrin partially inhibited type X collagen deposition into i nterstitial matrix; however, sternal growth and cell size were signifi cantly decreased, These data are the first obtained from intact tissue and demonstrate that the interaction of chondrocytes with extracellul ar matrix is required for chondrocyte survival and differentiation. (C ) 1997 Wiley-Liss, Inc.