As some stem cell researchers move ahead with clinical trials of new therapies, they’re facing criticism from others in the field who argue the transition is premature. Tensions between the two factions have been intensified by the spotlight of political controversy and media attention, triggering heated debates in the pages of journals and at meetings.‘‘People have very firm viewpoints on this,’’says Joshua Hare, a cardiologist and director of the Interdisciplinary Stem Cell Institute at the University of Miami’s Miller School of Medicine.‘‘There are different schools of thought because people perceive the stakes as so high.’’Among researchers investigating potential cardiac therapies based on adult stem cells–currently one of the more contentious areas of research–‘‘There’sa huge spectrum of skepticism and concern, and enthusiasm,’’according to Richard Cannon, a cardiologist at the National Heart, Lung, and Blood Institute (NHLBI).‘‘I don’t think I’m overstating the tension there is between basic scientists and clinical investigators, at least in some quarters.’’George Daley, a hematologist and director of the Stem Cell Transplantation Program at Children’s Hospital Boston, attributes that largely to ‘‘a clash of cultures’’between basic scientists and clinical investigators. Hare, who has begun clinical trials using bone marrowderived stem cells to treat heart disease, agrees, saying,‘‘Someone who sees patients and treats patients is going to be much more comfortable taking these therapies into trials, particularly in settings with major unmet needs.’’The debate flared up at the NHLBI’s third Symposium on Cardiovascular Regenerative Medicine late last year where clinical investigators were the targets of criticism during a panel discussion. Cannon, who is investigating the potential of bone marrow-derived stem cells to repair diseased and damaged blood vessels in patients with coronary artery disease, says,‘‘We were pretty much taken to task for endorsing or performing clinical research without knowing everything there is to know about the mechanism of cell-based approaches to heart disease.’’Clinical investigators, Daley says,‘‘are much more tolerant of uncertainties in mechanism, whereas ‘‘arguably the greatest insight into mechanism comes from the stem cell scientific community,’’he adds.‘‘I think that’s where cultural wars will be fought.’’Daley describes himself as ‘‘fairly conservative’’on the matter of when it’s appropriate to begin clinical trials of adult stem cell therapies. Understanding the mechanism of action isn’t an absolute prerequisite, he says, but if it’s not clear,‘‘that does have to give one extra pause’’if there’s even a plausible risk to patients.