Healing: Encyclopedia II - Healing - Healing by repair

Healing - Healing by repair

Healing must happen by repair in the case of injury to cells that are unable to regenerate (e.g. cardiac muscle or neurons). Also, damage to the collagen network (e.g. by enzymes or physical destruction), or its total collapse (as can happen in an infarct) cause healing to take place by repair.

Soon after injury, a wound healing cascade is unleashed. This cascade is usually said to take place in three phases: the inflammatory, proliferative, and maturation stages.

In the inflammatory phase, macrophages and other phagocytic cells kill bacteria, debride damaged tissue and release chemical factors such as growth hormones that encourage fibroblasts epithelial cells and endothelial cells which make new capillaries to migrate to the area and divide.

In the proliferative phase, immature granulation tissue containing plump active fibroblasts and forms. Fibroblasts quickly produce abundant type III collagen, which fills the defect left by an open wound. Granulation tissue moves, as a wave, from the border of the injury towards the center.

As granulation tissue matures, the fibroblasts produce less collagen and become more spindly in appearance. They begin to produce the much stronger type I collagen. Some of the fibroblasts mature into myofibroblasts which contain the same type of actin found in smooth muscle, which enables them to contract and reduce the size of the wound.

During the maturation phase of wound healing, unnecessary vessels formed in granulation tissue are removed by apoptosis, and type III collagen is largely replaced by type I. Collagen which was originally disorganized is cross-linked and aligned along tension lines. This phase can last a year or longer. Ultimately a scar made of collagen, containing a small number of fibroblasts is left.

Adapted from the Wikipedia article "Healing by repair", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki