Pannus is an abnormal layer of fibrovascular tissue or granulation tissue. Common sites for pannus formation include over the cornea, over a joint surface (as seen in rheumatoid arthritis), or on a prosthetic heart valve. Pannus may grow in a tumor-like fashion, as in joints where it may erode articular cartilage and bone.
In common usage, the term pannus is often used to refer to a panniculus (a hanging flap of tissue).
Pannus in rheumatoid arthritis
The term "pannus" is derived from the Latin for "tablecloth". Inflammation and exuberant proliferation of the synovium leads to formation of pannus and destruction of cartilage, bone, tendons, ligaments, and blood vessels. Basically, the hypertrophied synovium is called pannus. Pannus tissue is composed of aggressive macrophage- and fibroblast-like mesenchymal cells, macrophage-like cells and other inflammatory cells that release collagenolytic enzymes.
In people suffering from rheumatoid arthritis, pannus tissue eventually forms in the joint affected by the disease, causing loss of bone and cartilage. From Autoimmunity and Disease by Harley Y. Tse and Michail K. Shaw:
Chronic stages of the disease typically coincide with the formation of a structure known as a pannus. A pannus is a membrane of granulation tissue composed of mesenchyme- and bone marrow-derived cells. Formation of the pannus stimulates the release of IL-1, platelet-derived growth factor, prostaglandins, and substance P by macrophages, which ultimately cause cartilage destruction and bone erosion.
Pannus in ophthalmology
In ophthalmology, pannus refers to the growth of blood vessels into the peripheral cornea. In normal individuals, the cornea is avascular. Chronic local hypoxia (such as that occurring with overuse of contact lenses) or inflammation may lead to peripheral corneal vascularization, or pannus. Pannus may also develop in diseases of the corneal stem cells, such as aniridia. It is often resolved by peritomy.
- Farlex Partner Medical Dictionary © Farlex 2012 
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- Furuzawa-Carballeda, J.; Macip-Rodríguez, P.M.; Cabral, A.R. (2008). "Osteoarthritis and rheumatoid arthritis pannus have similar qualitative metabolic characteristics and pro-inflammatory cytokine response". Retrieved December 5, 2015.