1.6 Scarring and fibrosis
Scarring and fibrosis are seen when the cells of a tissue are damaged or killed and regeneration of the normal tissue architecture cannot take place. For example, in cirrhosis of the liver, the normal hepatocytes are damaged and do not regenerate effectively. The tissue is repaired and replaced by cells such as fibroblasts, which lay down extracellular matrix components including collagen, which can be seen by appropriate histological stains.
How does collagen appear in H&E staining? What stains show collagen more effectively?
In H&E staining collagen is pale pink and often difficult to differentiate from support cells embedded within it. Masson's trichrome stains collagen blue. Van Gieson stains collagen red/pink.
The cells which carry out the repair vary from one tissue to another. For example, following damage to the CNS, a group of glial cells called astrocytes replace damaged neurons, forming a glial scar. Obviously this scar tissue cannot carry out the normal function of nervous tissue, but it also can actively prevent the tissue from regenerating - neurons do not regrow their axons through glial scars. Similarly scar tissue in the skin will usually lack characteristic features of normal skin, such as hair follicles and sweat glands.
Fibrosis also occurs in some infections, particularly if the infectious agent cannot be cleared, fibroblasts lay down areas of extracellular matrix, which walls off the infection. For example, schistosomiasis (a worm infection) in the liver often results in areas of fibrosis surrounding the individual parasites.
Fibrosis and scarring are end-stages of a pathological process in which the body is unable to regenerate normal tissue and does the best it can by patching up the remaining tissue to limit further damage.