A species of flatworms overcomes the aging process to be potentially immortal. Planarian worms have apparently a limitless ability to regenerate. There are two types of planarian worms. Those that reproduce sexually and those that that reproduce asexually, simply dividing in two. Both seems to regenerate indefinitely by growing new muscles, skin, guts and even entire brains over and over again. Planarian worms and their stem cells are somehow able to avoid the aging process and to keep their cells dividing.
Asexual planarian worms have the potential to maintain telomere length during regeneration. They dramatically increase the activity of telomerase when they regenerate, allowing stem cells to maintain their telomeres as they divide to replace missing tissues.
In contrast, sexually reproducing planarian worms do not appears to maintain telomere length in the same way. Al thought sexually reproducing planarian worms also display a indefinitely regenerative capacity, they may eventually exhibit effects of telomere shortening or they are able to use another mechanism to maintain telomeres that would not involve telomerase [22371573; http://www.nottingham.ac.uk/news/pressreleases/2012/february/immortal-worms-defy-ageing.aspx].
Broadly distributed adult plutipotent stem cells underlie the remarkable regenerative abilities of planarians. Regeneration is accomplished by the formation of population of proliferating adult stem cells (neoblasts). These cells are pluripotent (can differentiate into neuronal, intestinal, and other postmitotic cell types) and are distribute throughout the body. A single transplanted clonogenic neoblast is able to restore the regeneration in lethally irradiated hosts [21566185; 23136835].