sentences:  Fibroblasts are activated connective tissue cells characterized by synthesis of proteins of the fibrous matrix, particularly the collagens.When tissue is injured, the predominant mesenchymal cells, the fibroblast, have been believed to be derived from the fibrocyte or possibly from smooth muscle cells lining vessels and glands.Commonly, fibroblasts express smooth muscle actin, a form of actin first found in smooth muscle cells and not found in resting fibrocytes.