Fibrosis, also known as fibrotic scarring, is the development of fibrous connective tissue in response to an injury. Fibrosis can be a normal connective tissue deposition or excessive tissue deposition caused by a disease.[2]
Repeated injuries, chronic inflammation and repair are susceptible to fibrosis, where an accidental excessive accumulation of extracellular matrix components, such as collagen, is produced by fibroblasts, leading to the formation of a permanent fibrotic scar.[1][3]
In response to injury, this is called scarring, and if fibrosis arises from a single cell line, this is called a fibroma. Physiologically, fibrosis acts to deposit connective tissue, which can interfere with or totally inhibit the normal architecture and function of the underlying organ or tissue. Fibrosis can be used to describe the pathological state of excess deposition of fibrous tissue, as well as the process of connective tissue deposition in healing.[4] Defined by the pathological accumulation of extracellular matrix (ECM) proteins, fibrosis results in scarring and thickening of the affected tissue—it is in essence a natural wound healing response which interferes with normal organ function.[5]
Physiology
Fibrosis is similar to the process of scarring, in that both involve stimulated fibroblasts laying down connective tissue, including collagen and glycosaminoglycans. The process is initiated when immune cells such as macrophages release soluble factors that stimulate fibroblasts. The most well characterized pro-fibrotic mediator is TGF beta, which is released by macrophages as well as any damaged tissue between surfaces called interstitium. Other soluble mediators of fibrosis include CTGF, platelet-derived growth factor (PDGF), and interleukin 10 (IL-10). These initiate signal transduction pathways such as the AKT/mTOR[6] and SMAD[7] pathways that ultimately lead to the proliferation and activation of fibroblasts, which deposit extracellular matrix into the surrounding connective tissue. This process of tissue repair is a complex one, with tight regulation of extracellular matrix (ECM) synthesis and degradation ensuring maintenance of normal tissue architecture. However, the entire process, although necessary, can lead to a progressive irreversible fibrotic response if tissue injury is severe or repetitive, or if the wound healing response itself becomes deregulated.[5][8]
Anatomical location
Fibrosis can occur in many tissues within the body, typically as a result of inflammation or damage. Common sites of fibrosis include the lungs, liver, kidneys, brain, and heart:
Micrograph showing cirrhosis of the liver. The tissue in this example is stained with a trichrome stain, in which fibrosis is colored blue. The red areas are the nodular liver tissue
Bridging fibrosis – an advanced stage of liver fibrosis, seen in the progressive form of chronic liver diseases. The term bridging refers to the formation of a "bridge" by a band of mature and thick fibrous tissue from the portal area to the central vein. This form of fibrosis leads to the formation of pseudolobules. Long-term exposure to hepatotoxins, such as thioacetamide, carbon tetrachloride, and diethylnitrosamine, has been shown to cause bridging fibrosis in experimental animal models.[9]
Senescence of hepatic stellate cells could prevent progression of liver fibrosis, although has not yet been implemented as a therapy due to risks associated with hepatic dysfunction.[10]
Historically, fibrosis was considered an irreversible process. However, several recent studies have demonstrated reversal in liver and lung tissue,[15][16][17] and in cases of renal,[18] myocardial,[19] and oral-submucosal fibrosis.[20]
Digital Pathology Quantification of Fibrosis
Digital pathology approaches enable quantitative assessment of fibrosis from whole-slide histological images using computational image analysis and machine-learning methods. These techniques measure collagen content, fiber architecture, spatial distribution, and tissue remodeling beyond traditional semi-quantitative staging systems.
Platforms such as those developed by PharmaNest apply AI-assisted analysis to conventional stains (e.g., picrosirius red, Masson's Trichrome) to derive reproducible fibrosis biomarkers[21][22]. Such digital fibrosis quantification methods have been applied across liver, gastrointestinal, pulmonary, renal, and dermatologic tissues. Such methods support translational research and clinical trials by enabling sensitive detection of fibrotic change and therapeutic response [23][24].
↑Zoubek ME, Trautwein C, Strnad P (April 2017). "Reversal of liver fibrosis: From fiction to reality". Best Pract Res Clin Gastroenterol. 31 (2): 129–141. doi:10.1016/j.bpg.2017.04.005. PMID28624101.
↑Chang CH, Juan YH, Hu HC, Kao KC, Lee CS (January 2018). "Reversal of lung fibrosis: an unexpected finding in survivor of acute respiratory distress syndrome". QJM. 111 (1): 47–48. doi:10.1093/qjmed/hcx190. PMID29036729.
↑Liver fibrosis analysis using digital pathology. Miyaaki H, Miuma S, Fukusima M, Sasaki R, Haraguchi M, Nakao Y, Akazawa Y, Nakao K. Med Mol Morphol. 2024 Sep;57(3):161-166. doi: 10.1007/s00795-024-00395-y. Epub 2024 Jul 9. PMID: 38980407.
↑Liver fibrosis phenotyping and severity scoring by quantitative image analysis of biopsy slides. Watson et Al. Liver International, November 27 2023, DOI: 10.1111/liv.15768
↑Digital pathology and artificial intelligence in non-alcoholic steatohepatitis: current status and future directions. Ratziu et Al., Journal of Hepatology, October 2023. doi.org/10.1016/j.jhep.2023.10.015
↑Aramchol improves hepatic fibrosis in metabolic dysfunction–associated steatohepatitis: Results of multimodality assessment using both conventional and digital pathology. Ratziu et Al, Hepatology, Volume 79, issue 4, April 7 2024, DOI: 10.1097/HEP.0000000000000980
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