Fibrosis

Fibrosis is the excessive formation of fibrous connective tissue, rich in collagen and other extracellular matrix proteins, in an organ or tissue. Laying down matrix is a normal step in healing: after an injury, fibroblasts become activated, many turn into contractile cells called myofibroblasts, and these secrete collagen and draw the wound edges together. Once repair is complete, the extra matrix is removed. Fibrosis develops when this repair program fails to switch off, usually because injury or inflammation is repeated or long-lasting.

Signals such as the growth factor TGF-beta, released by macrophages and damaged cells, keep myofibroblasts active, and stiff scar tissue gradually takes the place of working tissue, distorting the organ's structure and impairing its function. Almost any organ can be affected. Cirrhosis is advanced scarring of the liver; idiopathic pulmonary fibrosis stiffens the lungs and makes breathing steadily harder; chronic kidney disease ends in a scarred, shrunken kidney; and after a heart attack, dead heart muscle is replaced by a scar. Together, fibrotic diseases account for a large share of deaths in industrialized countries.

Fibrosis was long assumed to be permanent, but it can partly regress once its cause is removed, as liver scarring sometimes does after hepatitis C is cured. A few drugs that slow lung fibrosis are in use, and many more are being tested.