The quick stage involves increased supply of blood,local blood vessels, antibodies and clotting materials, and white cells to fight local bacterial invaders; this results in a local clot, which scabs over, in a few days. If there is infection or dirt, the white cells predominate, and there is more flow of blood and serum, and inflammation, causing pus formation, which can well delay matters. This is the emergency repair phase.
The slower, more definitive phase involves migration by newly regenerated tissues, which, if the wound is slight and clean, can lead to virtually no scarring. If the gap from the injury is too wide, the tissues form a scaffolding of collagen fibres across the scab, and the result is a scar, which slowly contracts. This scar tissue has no blood supply, and so is effectively biologically inert like hair or toe nail!
However, over very long times, local phagocytes( scavenging cells) might be able to nibble away at the edges of scar tissue, allowing for a slow replacement by underlying active tissues, like skin or membranes. Because a scar forms an effective repair, replacement is not a high priority, as long as surface integrity has been achieved, and since, as I have said, scar is inert, bloodless tissue, removal would be very slow from the margins, and often, if the scar is substantial, does not happen at all. The tendency of scars to turn pale after initial inflammation shows the withdrawal of active attempts by the body to repair further, which is why most scars go pale and shrink to some extent. Similar principles operate internally
In the case of tissues which do not readily regenerate, like nervous tissues, scars form permanently in default of new nerves.
http://www.madsci.org/posts/archives/aug98/901896791.Me.r.html