Embryonic stem (ES) cells are a promising source of new neurons to replace those that are lost through disease and injury. Functional recovery is the ultimate goal, but the cells must first be targeted to the right place, differentiate appropriately, and become integrated into functional circuits. Previously, the fate of ES cells that were transplanted into the brain could only be assessed retrospectively, through the examination of post-mortem tissue. However, Hoehn et al. have now developed a magnetic resonance imaging (MRI) technique that allows them to follow the movements of grafted cells in the living brain.

In a study reported in the Proceedings of the National Academy of Sciences, the authors induced ischaemia in the right cerebral hemisphere of the rat brain by transiently blocking the middle cerebral artery. They labelled an ES cell line with an MRI contrast agent, grafted these cells into the intact left hemisphere, and tracked their migratory behaviour using MRI. They found that they could detect clusters containing as few as 40 labelled cells — a higher resolution than has previously been achieved with this type of approach.

It has long been suspected that stem cells are preferentially targeted to damaged tissue, and the experiments of Hoehn et al. provide a striking confirmation of this phenomenon. In spite of the relatively large distance between the graft site and the site of brain injury, a large proportion of the grafted ES cells migrated directly across the corpus callosum, which bridges the two hemispheres, and headed straight for the lesioned area. This raises the possibility that ischaemic tissue might release a long-range chemoattractant that directs ES cell migration.

So, Hoehn et al. have developed a high-resolution imaging method for tracking the movements of grafted ES cells in the brain. This should prove to be extremely valuable for gauging the success of stem cell transplantation in experimental studies, and perhaps even in the clinic. Also, studying the migratory behaviour of grafted cells might provide some important clues to the nature of the endogenous signals that influence their migration, making it possible to develop more effective transplantation protocols.