The idea behind this one is that the cells can be swapped for fetal tissue or genetically engineered cells. The HUGE problem with this is that transplanting fetal human cells is scientifically difficult and also politically hard. Even if it can be done most of the cells transplanted in die soon after placement. Also, tissue from other species is usually rejected by the host in a short amount of time. Genetic engineering is still in the infantile stage and it will be a few decades before any good uses can be integrated.
Replacement is one of the least popular ideas but it is still being investigated as a possibility.
The problem that all three of these theories face is that on occasion, a scar forms at the site of the injury and will not let any growth happen at all. There are many studies going into the battle of eliminating the scar so that the theories can be put into practice when they are ready. Also, research has proved that there is only modest scarring and that any of the treatments that are under investigation now will easily solve the problem. There is a line between some scientists that argues whether or not the scar prevents the growth of the connections. But even with all of the bickering science marches on.