A fibroblast is a cell that makes the structural fibers and ground substance of connective tissue. It has a branched cytoplasm surrounding an elliptical, speckled nucleus having 1 or 2 nucleoli. Active fibroblasts can be recognized by their abundant rough endoplasmic reticulum. Inactive fibroblasts, which are also called fibrocytes, are smaller and spindle shaped. They have a reduced rough endoplasmic reticulum.

Fibroblasts make collagens, glycosaminoglycans, reticular and elastic fibers, and glycoproteins found in the extracellular matrix. In growing individuals fibroblasts are dividing and synthesizing ground substance. Tissue damage stimulates fibrocytes and induces the mitosis of fibroblasts.

Fibroblasts can give rise to other cells, such as bone cells, fat cells, and smooth muscle cells. Note that all of these cells are of mesodermal origin.

See also: fibrocartilage callus