The connective tissue in the human body is composed of numerous cell types, all of which develop during embryonic growth from the mesenchyme, a network of primitive cells that have the ability to differentiate into various forms. Scientists believe that a small number of cells from the mesenchyme linger in the blood vessels through adult life, maintaining the ability to differentiate into other cells as needed.

Fibroblasts are long, flat, spindle-shaped cells that produce tropocollagen, the precursor of collagen, and ground substance, an amorphous gel-like material that fills the intracellular spaces in connective tissue. Inactive fibroblasts, embedded in ground substance, are known as fibrocytes and are smaller than their active counterparts. If an organ becomes injured, however, fibrocytes are induced to propagate and migrate to the wound. There they aid in the deposition of fresh collagen, resulting in a scar upon the organ’s surface.

Myofibroblasts are similar to fibroblasts, but also have characteristics of smooth muscle cells. They are most active during the healing of injured tissue and, since myofibroblasts contain a greater amount of contractile proteins than fibroblasts, they have a better ability to actively draw together severed tissues. However, myofibroblast activity late in the healing process can produce gratuitous scarring. Moreover, if they proceed uninhibited, myofibroblasts may contribute to the development of fibrosis of the liver, lungs, and other vital organs.