A zebrafish mosaic assay to study mammalian stem cells in real time in vivo.

The differentiation potentials of stem cells have been evaluated by various in vivo and in vitro assays. However, these assays have different limitations hindering efficient study of mammalian stem cells. Here we describe a rapid and powerful mosaic assay to study the differentiation potentials of stem cells in real time in vivo by using zebrafish embryo. We transplanted mouse neural stem cells into zebrafish embryos at different developmental stages and found that they mainly formed neural tissues while occasionally trans-differentiated into mesoderm- and endoderm-derived tissues. Because zebrafish embryo is transparent, the behaviors of transplanted mouse stem cells can be easily tracked in a real-time manner and at single-cell resolution. We expect that this assay may be widely applied to explore the in vivo behaviors of any stem cells available.