Neural transplantation and stem cells.

Recent results have raised important questions on our ability to amplify stem cell populations in sufficient numbers as to be useful for therapy. Several reports have indicated that human stem cell populations harvested from the adult have low or undetectable telomerase levels, age in culture, and may not be propagated indefinitely. Other groups have shown that stem cells age and as such, their properties will have changed depending on the age of the individual from which they are harvested, and the time for which they are propagated in culture. Other groups have shown that cells maintained in culture may undergo alterations as they are propagated, and that these alterations may alter the predicted behavior of stem cells. Yet others have shown that human cells differ from their counterparts in other species in significant ways and have identified important difficulties in assessing cells in a xeno environment. Clinical colleagues have identified issues of variability and difficulties in the long-term follow-up that is being requested. Researchers in the stem cell field focused on translational work need to develop a practical plan that takes into account such difficulties while developing manufacturing protocols, designing animal studies, or developing trial protocols. Such proactive planning will be critical in ensuring a successful transition from the bench to the clinic.