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Granulation tissue is newconnective tissue and microscopic blood vessels that form on the surfaces of a wound during the healing process.[1] Granulation tissue typically grows from the base of a wound and is able to fill wounds of almost any size. Examples of granulation tissue can be seen inpyogenic granulomas andpulp polyps. Its histological appearance is characterized by proliferation of fibroblasts and thin-walled, delicate capillaries (angiogenesis), and infiltrated inflammatory cells in a loose extracellular matrix.
During the migratory phase ofwound healing, granulation tissue is:
Granulation tissue is composed of tissue matrix supporting a variety of cell types,[3] most of which can be associated with one of the following functions:
An excess of granulation tissue (caro luxurians) is informally referred to as hypergranulation or "proud flesh".[4]
The extracellular matrix of granulation tissue is created and modified byfibroblasts.[5] Initially, it consists of a network oftype-III collagen, a weaker form of the structural protein that can be produced rapidly. This is later replaced by the stronger, long-strandedtype-I collagen, as evidenced inscar tissue.
The main immune cells active in the tissue aremacrophages andneutrophils, although otherleukocytes are also present.[6] These work tophagocytize old or damagedtissue, and protect the healing tissue from pathogenic infection. This is necessary both to aid the healing process and to protect against invading pathogens, as the wound often does not have an effective skin barrier to act as a first line of defense.
It is necessary for a network of blood vessels to be established as soon as possible to provide the growing tissue with nutrients, to take away cellular wastes, and transport new leukocytes to the area.Fibroblasts, the main cells that deposit granulation tissue, depend on oxygen to proliferate and lay down the newextracellular matrix.[7]
In vascularisation, also calledangiogenesis,endothelial cells quickly grow into the tissue from older, intactblood vessels.[8] These branch out in a systematic way, forminganastomoses with other vessels.
