Plasticity of Stem Cells: Cell-fusion Versus Transdifferentiation

Stem cell research is still at a very early stage of its development and we are only beginning to unravel how stem cells influence the organism to keep its balance. There are many regulation systems involved which dictated the replacement of damaged, aged or diseased tissues with new cells. This review will focus on the hotly debated issue, whether the observed plasticity needed for tissue regeneration and replacement results from cell fusion or if transdifferentiation prevails. First, some basics of stem cell biology will be revised, followed by defining the terms of chimerism, fusion and transdifferentiation. A suggestion will be made on how and when the three terms should be used when dealing with the topic of stem cell based therapy. With the help of some selected publications, this review wants to convince the reader that fusion of cells does occur in organisms for various reasons, but that transdifferentiation of stem cells seems to prevail when speaking of tissue regeneration and maintenance of homeostasis. Some comments will be devoted to the linkage between stem cells and aging, diseased or tumourigenic tissues.