Cultures of rat dermal fibroblasts are very useful for a wide range of cellular and molecular studies. This is mainly because fibroblasts are one of easiest types of cells to grow in culture, and their durability makes them amenable to a wide variety of manipulations ranging from studies employing gene transfection to microinjection. Fibroblasts secrete a nonrigid extracellular matrix that is rich in type I and/or type III collagen. There is evidence showing that fibroblasts in different organs are intrinsically different. Dermal fibroblasts also secrete large quantities of hyaluronan in response to inflammatory stimuli. During wound healing, dermal fibroblasts switch from a migratory, repopulating phenotype to a contractile, matrix-reassembling phenotype.