Fibroblasts were cultured 4 hr (A) and 16 hr (B) on 1.5 mg/ml and 4 mg/ml collagen matrices and collagen-coated coverslips in PDGF and serum media. At the end of incubations, samples were fixed and stained for actin. On 1.5 mg/ml collagen matrices, PDGF medium favored formation of dendritic extensions. Serum medium delayed cell spreading. After 16 hr, cells in PDGF medium were dispersed. Cells in serum medium were clustered. Formation of dendritic extensions, delayed cell, and cell clustering did not occur on collagen-coated coverslips. Scale bar, 50 μm.