New research published early online in the journal Nature Genetics rejects the previously suspected belief that adult stem cells are necessary for successful animal cloning. It shows that cells that have completely matured to a specific type can not only be used for cloning purposes, but may even be a more efficient starting point.
Cloning relies on a process called SCNT, in which the nucleus of a donor cell is transferred into an egg that has been emptied of its genetic material. This egg can then be implanted into a surrogate mother and an exact genetic duplicate of the donor will result if the attempt is successful. Stem cells harvested from embryos, rather than adults give the best results for cloning, but these are not available from human patients.
In 1997 Dolly the sheep was the first mammal to be cloned from a fully mature adult cell. However, since then many scientists have argued that mature cells are too old to be successfully cloned and that Dolly, like many of the cloned animals which followed her, were in fact created using adult stem cells or stem-like cells by accident. Stem cells have the ability to become any type of cell that make up a particular tissue or organ, however they are not always easy to pick out from the cells around them. Many have attributed the limited success of cloning to the theory that only the attempts where stem cells were used by accident succeeded.
Researchers compared the efficiency of cloning mice using blood cells at three different stages of maturation from hematopoietic cells which give rise to all types of red and white blood cells, to granulocytes which are well developed, mature white blood cells. The granulocytes were proved to be the most efficient donor cells with between 35 and 39 per cent producing early embryos, (blastocysts), compared to 4 per cent where hematopoietic cells were used as donors. Moreover, only the granulocytes were able to produce two live cloned pups.
Dr Yang, director of the University of Connecticut's Centre for Regenerative Biology said 'Of the 1,828 nuclear transfers we performed with stem cells, very few could develop to the blastocyst stage and not one clone was produced. With such odds, its hard to believe that Dolly and other cloned animals could have possibly been derived from adult stem cells.' More research will be needed to determine if what has been discovered in relation to hematopoietic cells will hold true for other cell types. However investigators point out that these finding may have important implications for regenerative medicine as they suggest that the potential of adult stem cells in this field may be more limited than previously thought.