Fibroblasts are the cells that make up the structural framework or stroma composed of the extracellular matrix and collagen in animal tissues. These cells are the most common type of connective tissue in animals, and are important for wound healing. Fibroblast transfection is a commonly used method in molecular and cell biology research, fibroblast transfection reagents are commercially available for both fibroblast cell lines and primary cells. As depicted below, fibroblast cells are large and flat with elongated processes protruding from the body of each cell creating the spindle-like appearance of the cell. The nucleus of the fibroblast is oval.
Fibroblasts are derived from primitive mesenchyme, like all connective tissue cells. Their ability to express the filament protein vimentin alludes to their mesodermal origin.
Connective tissue is one of four main types of tissue in the body. The others are epithelial, muscle and nervous tissue. Connective tissue is a fibrous tissue made largely of collagen, the most abundant protein in mammals. There are many kinds of connective tissue including loose, dense, elastic, reticular and adipose connective tissue. In addition, there are embryonic connective tissues, bone, cartilage and blood.
Fibroblasts and the Extracellular Matrix
Dense connective tissue is used to form ligaments and tendons. Ligaments are rope-like tissue bundles that attach bones to each other and are found in joints whereas tendons attach bones to the surrounding skeletal muscle tissue. Dense connective tissue also makes up the dermis. The tissue matrix is made almost entirely of collagen, which is the most abundant protein in mammals. Interspersed between the collagen fibers of dense connective tissue are fibroblasts which produce a collagen subunit, tropocollagen, used to construct larger collagenous aggregates.
Fibroblasts also produce glycoproteins and polysaccharides for the ground substances, a gel-like material that surrounds collagen fibers of dense connective tissue. This forms an “extracellular matrix” which contributes to the structural integrity of ligaments and tendons and determines the physical properties of connective tissue. In addition, fibroblasts have a tissue repair function and wounds stimulate fibroblast production. Collagen’s ubiquity makes fibroblasts the most common cells of connective tissue in mammals.
Fibroblasts and Fibrocytes
Fibroblasts come in various shapes and sizes, as well as in an activated and non-activated form. Fibroblasts are the activated form (the suffix “blast” refers to a metabolically active cell), while fibrocytes are considered less active. However, sometimes both fibroblasts and fibrocytes are not designated as being different and are simply referred to as fibroblasts. Morphologically, fibroblasts can be distinguished from fibrocytes by their abundance of rough endoplasmic reticulum and relatively larger size.