Granulation Tissue: Definition, Structure, and Function

Granulation Tissue Definition

Granulation tissue is reddish connective tissue that forms on the surface of a wound when the wound is healing. Clinicians observe how granulation tissue is forming on a wound in order to assess how well the injury is being repaired by the body. When too much granulation tissue forms, it is called “proud flesh”.

Function of Granulation Tissue

Granulation tissue forming over a wound means that the body is healthy and working to form a new layer of skin over the part that was torn during injury. It gets its red color from the new blood vessels that are forming to deliver nutrients to the tissue. It also contains a variety of cells that help create a new structure, form new extracellular matrix, destroy damaged cells, protect against infection, and provide nutrients via blood vessels.

Clinicians measure how much a wound has undergone granulation in order to determine where the wound is at in the healing process. They may measure the length, width, and depth of the wound and categorize the tissue that is forming on and around the wound.

Structure of Granulation Tissue

The extracellular matrix of granulation tissue is made by cells called fibroblasts. The fibroblasts form type III collagen, a form of the protein collagen that is found in soft tissues in the body. It is eventually replaced by type I collagen, which is a sturdier type of collagen that is also found in bones, tendons, and organs. Immune cells such as leukocytes (white blood cells) are another type of cell found in granulation tissue. They work to get rid of destroyed cells and also to protect the body against pathogens such as viruses and bacteria. The skin provides a layer of defense against pathogens, and during injury, that barrier is broken. Therefore, it is extremely important for leukocytes to be present to defend the body, and it is also important that a wound heals quickly so that the skin barrier is once again complete.

Additionally, blood vessels must form to provide oxygen and nutrients to both the newly formed cells and to the cells that are helping to create new cells. The process of forming a network of blood vessels is called vascularization, and it is accomplished via the outgrowth of blood vessels already in existence.

This is a microscope image of granulation tissue in an infected wound. It is a network of many cells including fibroblasts, leukocytes, and plasma cells, with capillaries running throughout.

Excess Granulation Tissue— “Proud Flesh”

Sometimes, too much granulation tissue grows over a wound, leaving an excess of bumpy, shiny, red tissue called “proud flesh”. Other terms to describe proud flesh include hypergranulation, hyperplasia of granulation, and exuberant granulation. If too much granulation tissue forms, this can actually hinder the healing process and lead to a long-lasting wound. Proud flesh can be “healthy”, i.e. an overgrowth of normal granulation tissue, or “unhealthy” if it becomes infected. It can be treated with foam dressing, antimicrobials, antibiotics, tapes, creams, silver nitrate, or as a last resort, surgical removal.

There are four main types of wound tissues as described by clinicians: granulating, necrotic, sloughy, and epithelizing. In contrast to granulating tissue, necrotic tissue is dead tissue. Necrotic tissue occurs when cells in or around a wound die from infection or simply old age. Since skin cells only live for three weeks at most, a wound that has been kept under a bandage for weeks will naturally have some necrotic tissue around it. Sloughy tissue is stringy necrotic tissue that is separating from the site of the wound. If there is a lot of sloughy tissue forming around a wound, it is often removed

When granulating tissue forms, it grows over and replaces necrotic tissue. Likewise, epithelizing (or epithelializing) tissue grows over granulating tissue. It is a layer of densely packed keratinocytes and other skin cells. When a wound first forms, it will have a lot of necrotic and sloughy tissue. As it heals, granulating tissue will appear. Then, granulating tissue is replaced by epithelizing tissue. When a wound has a lot of epithelizing tissue, it is well on its way to healing.

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