We are interested in characterizing the specific cell lineages involved in the development of the mammalian brain.  The mammalian forebrain contains neural stem cells that can proliferate to produce cells with multiple cellular and spatial phenotypes and that are capable of long term self-renewal (over the lifetime of the animal).  In studying the earliest origin of these neural stem cells in the embryo, we have been led by our data to test the hypothesis that the default state for mouse embryonic stem cells is to become neural stem cells.  Mouse embryonic stem (ES) cells are lines of pluripotent early blastocyst cells that can give rise in vivo to entire and completely viable mice.  In vitro in the presence of serum these ES cells will form aggregates called embryoid bodies containing cells that differentiate into blood, neural, muscle and other cell types  In contrast to these differentiating ES cell aggregates (embryoid bodies), however, we found that when cultured under serum-free, low cell density conditions, a small (but quantitatively reliable) percentage (0.2%) of single ES cells proliferate in the presence of leukemia inhibitory factor (LIF) to form floating sphere colonies of cells that express the neuroepithelial marker nestin, but no longer express the ES cell markers  Oct4 and SSEA1.