Regeneration occurs because of unspecialized cells, many invertebrates (organisms without backbones) have unspecialized cells that can grow into any specific tissues type and allows for the regrowth.
Most of our cells are specialized and can only reproduce into similar cell types, stem cells offer hope of regeneration because they are undifferentiated (not specialized) and can become any type of cell. This is why stem cell research is so important, we may be able to one day re-grow body parts or organs, or even recover from spinal injuries.
The starfish depicted above and in the video to the right are exhibiting regeneration of lost limbs.
Earlier this century oyster fisherman in the long island sound noticed that starfish were eating their precious oysters. Each time they would catch starfish in their traps they would chop them up and throw them overboard. Unknowingly they were doubling and tripling the starfish populations and this ended up decimating the oyster population, ultimately putting themselves out of business and leading to the end of oyster fishing in the Long island sound for many years.
Regeneration is the growth of an entire new organism from just a part of another organism.
The small worm in the pictures to the left are called planarian, a flatworm. These worms were cut into three pieces and observed over the next week. There is a picture of them 7 days later. Notice the head part, on the top, is growing a new tail. The middle has already grown new eye spots and a tail and the original tail part, on the bottom, is growing a new torso. This regrowth of missing parts is called regeneration. In one to two more weeks time we would be able to see two fully developed planarians.