A specificity for cellular fibronectin in its effect on cultured chondroblasts

Chick cellular fibronectin has previously been shown to alter the phenotypic properties of cultured chick-embryo vertebral chondroblasts. Over the course of several days, adhesion and spreading on plastic substrata in the presence of serum was stimulated, the morphology of the cells was changed, the synthesis of cartilage-specific type-IV proteoglycan was inhibited, and the synthesis of type-I collagen and fibronectin was induced or stimulated. In the present study, chick plasma fibronectin was isolated and observed to mimic the effect of cellular fibronectin on cell adhesion and spreading. Both kinetic and dose-response relationships were similar between the two isoproteins. In contrast, chick plasma fibronectin, at up to tenfold higher concentrations by weight, did not alter cell morphology or synthesis of type-IV proteoglycan. Control experiments showed that plasma fibronectin could not neutralize cellular fibronectin and that plasma fibronectin did not simply conceal an effect on type-IV proteoglycan production by shifting the balance released into the culture medium. The results suggest that the effect of cellular fibronectin on the differentiated properties of chondroblasts relies on some unique feature not possessed by plasma fibronectin, and thus is not solely dependent on its own ability to stimulate adhesion and spreading.