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The skin has various regeneration and repair mechanisms.
These are employed to eliminate any damage caused by external influences
and to restore lost function. |
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Reactions
of the horny layer
The action of external mechanical, physical or chemical
irritants causes the horny layer to thicken. Typical examples
are the thickening found after intensive UV radiation and the formation
of calluses on areas subject to mechanical stress (palms of the hands
and soles of the feet).
Regeneration following UV-related damage
Intense UV-exposure causes primary damage to the genetic material.
Secondary damage is inflicted on the cell proteins and membranes by
UV-induced free radicals. The skin is known to have many mechanisms
for the repair of damaged DNA. In humans the most important are the
excision repair and post-replication repair mechanisms: The excision
repair mechanism is based on recognition, removal and replacement
of the damaged DNA segment. This way mutations are prevented as long
as the repair mechanism is not overburdened or defective. The post-replication
repair mechanism, on the other hand, works around the damaged DNA
segment, meaning that it is ignored when the genetic code is read.
Only later is the damage repaired. This mechanism is so faulty however,
that often more mutations are caused by the repair than by the original
radiation damage.
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Chemically-induced
calluses can ensue from repeated bathing with lipid-dissolving solvents
and water.
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Regeneration
following skin injury
The layer of epidermal mother cells - the basal layer - ensures a
steady renewal of the epidermis, through continual cell division (proliferation).
If an injury is confined to the uppermost skin layer, this damage,
known as erosion, can heal without scarring. If the damage reaches
the dermis (e.g. an ulcer) and thus involves the basal membrane, then
healing is usually accompanied by scar formation. In this case destroyed
skin cells are replaced by connective tissue. Wound-healing follows
in several consecutive stages: in the first phase coagulating blood
forms a membrane with a hard surface that adheres to the wound (crust).
In the following skin cleansing stage, autolysis
and phagocytosis of the damaged and dying
cells takes place. In parallel connective tissue fibres
are dissolved by enzymes. Mobile immune cells and phagocytes become
active and lymphatic fluids flow into the wound. |
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Autolysis is the destruction
of dead or dying cells by lysosomal enzymes produced by the cells
themselves. Phagocytosis is the active ingestion of particles by phagocytes.
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In the building or proliferation
phase, epithelization of the wound base occurs, including the formation
of capillary buds, new connective tissue and collagen fibres. Cell
division during the proliferation phase can be stimulated and supported
by the application of topicals such as dexpanthenol. This is conducive
to a better and faster healing. |
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