This episode highlighted some important factors in stem cell regeneration. The show talked about the success that could be seen with partial paraplegia. Total paraplegia does not get this response. The same can be said for partial cartilage loss in Osteoarthritis and total cartilage loss.
The reason for this relates to the way stem cells work. Stem cells respond to signals from other cells. If the signal is for repair that is what the response does. If the cell is apoptotic (undergoing an orderly cell death) then as part of apoptosis the stem cell will receive a signal to create a replacement cell. If the cell is already dead and gone there will be no apoptotic signal and no new cell. This helps to explain why adult stem cells do not grow bone, cartilage or other tissues where it is not needed. Embryonic stem cells retain the ability to grow new cells without an apoptotic signal.