Concentration-dependent effects of fibronectin adsorbed on hydroxyapatite surfaces on osteoblast adhesion

Fibronectin (Fn) is an essential protein that is involved in cell attachment, migration, and differentiation. It is known that the conformation of Fn changes depending on its surrounding environment or on the surface properties of the substrate to which it is adsorbed. Here, we show that Fn adopts different conformations in physiological solutions and can play different roles in osteoblast adhesion on hydroxyapatite (HAp) depending on its concentration. The results of in situ observations showed that the adsorption configuration of Fn was an oblate ellipsoidal structure at low concentrations and a fibrillar structure at high concentrations, suggesting that individual Fn molecules attach to HAp with a side-on or end-on orientation, under low and high concentrations, respectively. Osteoblasts adhered more to the HAp surface under low concentrations of adsorbed Fn than under high concentrations. These observations suggest that the Fn concentration gradient is important for regulating biomaterial-cell interactions in regenerative medicine.