Together, these form the structural framework of tissues in animals and play an important role in tissue repair. The primary function of fibroblasts is the maintenance of structural integrity within the connective tissue. Fibroblasts secrete the extracellular matrix precursors required for the formation of connective tissue and various fibres.
Fibroblasts are originally derived from primitive mesenchyme and therefore display the filament protein vimentin, which acts as a marker of mesodermal origin. In some cases, epithelial cells may also produce fibroblasts, a process which is referred to as epithelial–mesenchymal transition (EMT). Conversely, fibroblasts sometimes undergo mesenchymal–epithelial transition (MET) to produce epithelia. This process is seen as part of normal development, tissue repair and tumor growth.
Inactive fibroblasts are called fibrocytes. Active fibroblasts are rich in endoplasmic reticulum and have a branched cytoplasm that surrounds an oval, spotted nucleus whereas fibcrocytes are smaller and spindle-shaped.
Fibroblasts produce the glycosaminoglycans, collagens, elastic fibers, reticular fibres and glycoproteins that can be seen in the extracellular matrix. When tissue damage has occurred, the fibrocytes are stimulated to undergo mitosis or multiplication by replication and division.