research initially suggested that, given the right environment, stem cells from mouse muscle could be shown to produce not only muscle, but also components of blood. Later it became clear on rigorous testing that her sample contained two entirely different kinds of stem cells: one that formed blood and one that formed muscle. As she noted, the fact that the two types of stem cells were found in muscle might have interesting therapeutic uses, but in any case, it has not been demonstrated that a single type of stem cell in muscle exhibits this degree of plasticity.

An issue raised at the workshop was the need for more experiments that can show an unequivocal relationship between a stem cell and the tissues that are claimed to have arisen from it. In such an experiment, (an example of which is Krause’s work on HSCs mentioned earlier in this chapter) a single, isolated stem cell would be chemically treated so that it incorporates a chemical “label” that will be passed on to all the cells that arise from it. If the labeled stem cell is injected into a mouse, any cell or tissue that is eventually found to have the label can be assumed to have come from the original single stem cell, and this is the kind of evidence for a definitive relationship that stem cell researchers are seeking.

A second factor that complicates adult stem cell research is that the environment in which stem cells grow or are placed to grow has an important but poorly understood effect on their fate—a theme that was echoed by many speakers at the workshop. For example, Iqbal Ahmad discussed prospects for retinal regeneration, which occurs naturally in goldfish but not in humans. Ahmad has isolated precursor cells in the mammalian eye that can be grown in culture for short periods and will develop into cells that appear to be retinal photoreceptors. If precursor cells from a mouse eye are transplanted into a normal mouse retina, they are not incorporated. In contrast, when transplanted into a diseased retina, the precursors begin to develop into photoreceptor-like cells and integrate into the tissue. Ahmad has not yet determined whether the integrated cells function normally.

What signals does the diseased retina provide that the normal retina does not? The cellular environment has important implications for how cells behave when they grow in a living organism (in vivo) and for what