Recent studies have suggested that stem cells are able to cross primordial tissue barriers. Their ability to respond to unrelated microenvironmental signals strongly suggest that they have greater potential than previously imagined especially for their future clinical use for the regeneration of tissues or even perhaps organ systems. In particular there is an intriguing reciprocal relationship between the hematopoietic and neuronal stem cell systems. Both stem cell pools appear to respond to microenvironmental signals that are not developmental related. These intriguing observations have led to a number of studies that have attempted to explore this apparent phenomenon of plasticity associated with both hematopoietic and neuronal stem cell populations.