Osteoblasts derived from rat bone marrow cells tion of extracellular matrix proteins, mostly composed of were cultured on surface-modified poly(ethylene terephthal-collagen, on all the surfaces. A remarkable result observed ate) films in the presence of ascorbic acid, -glycerophosat this stage was direct deposition of an electron-dense, phate, and dexamethasone. The surfaces employed for cell afibrillar layer of 180 nm thickness onto the surface having culture included the untreated hydrophobic surface and phosphate polymer chains. This layer became much more three modified surfaces possessing immobilized phosphate electron dense after 3 weeks of culture. Energy dispersive polymer chains, collagen molecules, and a thin hydroxy-X-ray microanalysis revealed the presence of calcium phosapatite-deposited layer. They all were produced by photo-phate in this layer. It was further found that the predeposited induced graft polymerization with subsequent surface modi-hydroxyapatite layer on the phosphate polymer-grafted fications of the graft chains. The ultrastructural morphology surface promoted mineral deposition in the extracellular of the substrate/cell interfaces formed in in vitro osteoblast matrix that surrounded cuboid, osteocyte-like cells. © 1997 culture on these substrates was studied by transmission elec-