A paper in the journal Stem Cell Reports has reignited debate among a section of the stem cell research community. The study calls into question the existence of a type of cell that, if validated, would have great potential for use in regenerative medicine.
The cells, called very small embryonic-like cells (VSELs), were first reported in 2006 by Dr Mariusz Ratajczak at the University of Louisville, USA. Since then, several publications have questioned whether the cells exist, with the most recent study led by Professor Irving Weissman at Stanford University, also in the USA.
However, proponents of VSELs argue that others are simply failing in the difficult task of harvesting the correct cells. Dr Ratajczak told Nature that Professor Weissman 'has never visited my lab to witness exactly how we carry out the method', while Dr Wojciech Wojakowski of the Medical University of Silesia in Katowice, Poland, told the journal that 'other investigators are just not managing to catch the right cells'.
Part of the difficulty may lie in the size of the reported cells, thought to be less than six micrometers in diameter, and the fact that they are extremely rare.
Dr Ratajczak and others report that VSELs, which were discovered in mouse bone marrow, can transform into a range of other cell types, including muscle, nerve, and blood. Such properties are characteristic of embryonic stem cells. Normally adult stem cells are more restricted in the types of cells that they can become. Thus, the discovery of VSELs, if validated, would represent a game-changing development.
Proponents also suggest VSELs to be an ethical pluripotent stem cell source because their preparation does not require the destruction of embryos. The Vatican supports VSEL research and donated US$1 million to the Stem For Life Foundation, an offshoot of the New York-based NeoStem company that promotes the use of adult stem cells over embryonic stem cells.
Professor Weissman's paper does not signal the end of the debate. Indeed, the arguments may intensify as two major clinical trials using the cells are planned. One of these is scheduled to start in Europe while a second trial backed by NeoStem is awaiting approval by the US Food and Drug Administration.
Professor Robin Lovell-Badge, head of developmental genetics at the Medical Research Council National Institute for Medical Research, who was not involved in the study, commented: 'VSELs make little sense biologically'.
He also questioned the rationale behind allowing human trials: 'Would you really want to put something in your body that has not been described in detail let alone understood?'