Researchers say they have found the strongest evidence to date that human pluripotent stem cells -- cells that can give rise to all tissues of the body -- will develop normally once transplanted into an embryo.

Pluripotent stem cells for use in regenerative medicine or biomedical research come from two sources: embryonic stem cells, derived from fertilized egg cells; and induced pluripotent stem cells, where skin cells are 'reset' to their original form. The promise (bordering on hype in the case of human embryonic stem cells, with promises a decade ago of curing Alzheimer's if they just got more money) is that they might repair various organs and tissues, particularly those that have poor regenerative capacity, such as the heart, brain and pancreas. 

Some concerns have been that the cells may not incorporate properly into the body and hence not proliferate or distribute themselves as intended, resulting in tumors. A new study suggests that this will not be the case and that stem cells, when transplanted appropriately, are likely to be safe for use in regenerative medicine.  

The best way to test how well stem cells would incorporate into the body is to transplant them into an early-stage embryo and see how they develop. As this cannot be done ethically in humans, scientists use mouse embryos. The gold standard test, developed in Cambridge in the 1980s, involves putting the stem cells into a mouse blastocyst, a very early stage embryo after fertilisation, then assessing stem cell contribution to the various tissues of the body..

Previous research has not succeeded in getting human pluripotent stem cells to incorporate into embryos but the new results show that it is possible to successfully transplant human pluripotent stem cells into the mouse embryo and that they then develop and grow normally. If so, human pluripotent stem cells are equivalent to an embryonic counterpart. Attempts to incorporate human pluripotent stem cells had failed because the stem cells had not been matched to the correct stage of embryo development: the cells needed to be transplanted into the mouse embryo at a later stage than was previously thought (a stage of embryo development known as gastrulation). Once transplanted at the correct stage, the stem cells went on to grow and proliferate normally, to integrate into the embryo and to distribute themselves correctly across relevant tissues.