The principal function of fibroblasts is to form and maintain homeostasis in the intercellular substance of connective tissue. The cells are classified into juvenile (less differentiated and frequently dividing), mature fibroblasts (actively producing components of intercellular substance) and fibrocytes (low synthetic activity but providing for intercellular substance stability).

TEM image, 10,000X (photo courtesy of G. P. Satdykova). A mature fibroblast is branched #1, #2, it is marked by a predominance of euchromatin in the nucleus, as well as by a presence of cytoplasmic mitochondria, Golgi apparatus, and highly developed rough endoplasmic reticulum. The cisterns of the reticulum accumulate protein molecules, which are produced by polyribosomes: procollagen, elastin, and fibrillin. Outside the cells, near the fibroblast surface, these proteins form fibers #1, #2 of intercellular substance.
This fiber formation involves components of amorphous matrix (glycoproteins, glycosaminoglycans), which are also produced by fibroblasts.