This image shows oligodendrocytes in the mouse neocortex, derived from neural progenitors in the embryonic dorsal forebrain. Neural progenitor cells were electroporated in utero at embryonic day 15.5 with plasmids encoding various fluorescent proteins. These progenitors give rise to a diverse array of morphologically and functionally distinct cells types, including neurons, astrocytes and oligodendrocytes. During embryonic development, Sonic hedgehog from multiple sources acts as a timing mechanism that promotes the transition from neurogenesis to oligodendrogenesis in the neocortical progenitor pool. For more information, see the article by Winkler et al. (pages 5237–5250).