Chondrogenesis is the developmental process by which undifferentiated mesenchymal cells undergo differentiation and begin secreting a cartilage matrix consisting of type 11 collagen, proteoglycan, and non-collagenous proteins such as aggrecan. Before differentiating, mesenchymal cells undergo condensation and form extensive cell-cell contacts. Precartilage condensation and chondrogenic differentiation can be stimulated in vitro. The work presented in this thesis examines the pattern of integrin subunit expression and the role of fibronectin matrix synthesis in the formation of precartilage condensations using an in vitro model of chondrogenesis. Integrins act as receptors for components of the extracellular matrix. Therefore, changes in the pattern of integrin subunits as the chondrogenic matrix is secreted by differentiating cells provide clues with respect to their function. Several integrin subunits appeared to vary in their expression during chondrogenesis. a l integrin was downregulated during the first three days in culture while alO was expressed starting at day 5 in concert with type II collagen. a5 integrin was initially upregulated during the first 3 days in culture and then deereased in expression. This pattern correlated with fibronectin expression in the in vitro model. Other integrin subunits appeared more stable in their expression patterns. Precartilage condensation is associated with fibronectin production. Fibronectin matrix synthesis is dependent upon rho GTPase activity, interfibrillar associations and integrin activity. Experiments in which cells were pretreated with C3 transferase to interfere with rho activity also prevented precartilage condensation and matrix production, as did inhibition of the downstream effector rho kinase. The monoclonal antibody 9D2 and the functional upstream domain protein of Streptococcus pyogenes, reagents, which interfere with fibronectin fibrillogenesis, also prevented condensation in the aggregate model of chondrogenesis. This work thus suggests that the formation of precartilage condensations is highly dependent upon fibronectin matrix assembly. Fibronectin contains multiple binding domains for other eomponents of the extracellular matrix. Thus, fibroneetin may act as a scaffolding protein upon which other matrix components are added to the developing cartilage matrix. Furthermore, those matrix components may be incorporated into the matrix using specific integrin receptors during chondrogenesis.