Healing
For the village, see Healing, North East LincolnshireHealing is the process whereby the
cells in the
body regenerate and repair to reduce the size of a damaged or
necrotic area.Healing incorporates both the removal of necrotic
tissue (demolition), and the replacement of this tissue.
The replacement can happen in two ways:
* by
regeneration: the necrotic cells are replaced by the same tissue as was originally there.
* by
repair: injured tissue is replaced with
scar tissue.
Most
organs will heal using a mixture of both mechanisms.
In order for an
injury to be healed by regeneration, the cell type that was destroyed must be able to replicate. Most cells have this ability, although it is believed that
cardiac muscle cells and
neurons are two important exceptions.
Cells also need a
collagen framework along which to grow. Alongside most cells there is either a
basement membrane or a
collagenous network made by
fibroblasts that will guide the cells' growth. Since
ischaemia and most
toxins do not destroy collagen, it will continue to exist even when the cells around it are dead.
Example of regeneration
Acute tubular necrosis (ATN) in the
kidney is a case in which cells heal completely by regeneration. ATN occurs when the
epithelial cells that line the kidney are destroyed by either a lack of
oxygen (such as in
hypovolemic shock, when blood supply to the kidneys is dramatically reduced), or by toxins (such as some
antibiotics,
heavy metals or
carbon tetrachloride).
Although many of these epithelial cells are dead, there is typically patchy necrosis, meaning that there are patches of epithelial cells still alive. In addition, the collagen framework of the tubules remains completely intact.
The existing epithelial cells can replicate, and, using the basement membrane as a guide, eventually bring the kidney back to normal. After regeneration is complete, the damage is undetectable, even
microscopically.
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.
*
Wound healing - this is important
cosmetically and functionally, as
scar formation, especially at a
joint, can impair movement.
*
Bone healing - the healing of a
fracture takes a long time, has many potential problems, and a great deal can be done surgically to overcome these problems.
*
Brain healing - Such as in a
cerebral infarct (a stroke). Not only can neurons not divide, but a collagen scar will not form.
*
Nerve healing - Even though neurons cannot replicate, a
nerve is often just a collection of
axons, and the cell itself doesn't die.
It is interesting to note that some itching may accompany the healing process in the affected area.
*
How wounds heal and tumors form With this simple Flash demonstration, Harvard professor Donald Ingber explains how wounds heal, why scars form, and how tumors develop. Presented by Children's Hospital Boston.
*
Wound Healing*
Wound Healing and Repair* Lorenz H.P. and Longaker M.T.
Wounds: Biology, Pathology, and Management. Stanford University Medical Center.
* Romo T. and McLaughlin L.A. 2003.
Wound Healing, Skin. Emedicine.com.
* Rosenberg L. and de la Torre J. 2003.
Wound Healing, Growth Factors. Emedicine.com.
