Many neural and non-neural cell types in vertebrates are produced by the neural crest, a transient embryonic structure that emerges during neurulation in the dorsal part of the neural tube (Le Douarin

INTRODUCTION Many neural and non-neural cell types in vertebrates are produced by the neural crest, a transient embryonic structure that emerges during neurulation in the dorsal part of the neural tube (Le Douarin et al., 2008). Among the neural crest derivatives are the pigment cells in the skin that are reported to originate either directly from neural crest cells migrating from the neural tube on a dorsolateral pathway or from nerves innervating the skin (Sommer, 2011). Melanocyte lineage specification of dorsolaterally migrating cells is thought to occur at an early stage of neural crest development. Cells fated for the melanocyte lineage stall in the migration staging area (MSA) adjacent to the neural tube before continuing their dorsolateral migration (Weston, 1991). In the MSA, melanocyte specification is indicated by expression of the microphthalmiaassociated transcription factor (Mitf), which controls melanoblast development and survival (Opdecamp et al., 1997). Both in cell culture and in vivo, expression of Mitf is regulated by canonical Wnt signaling, indicating a central role of this signal transduction pathway in melanocyte development (Dorsky et al., 2000; Takeda et al., 2000; Hari et al., 2002; Widlund et al., 2002). Canonical Wnt signaling involves the intracellular signaling component -catenin, which, upon activation, translocates to the nucleus and induces specific transcriptional responses (Gordon and Nusse, 2006). In cultures of mouse and quail neural crest cells, treatment with Wnt or activation of -catenin enhances melanoblast proliferation and differentiation (Dunn et al., 2000; Jin et al., 2001). More strikingly, constitutive activation of -catenin in zebrafish in vivo promotes the formation of melanocytes while repressing neural cell lineages (Dorsky et al., 1998). By contrast, upon inhibition of Wnt signaling, neural crest cells adopt a neural rather than a pigment cell fate. Similarly, in the mouse, conditional deletion of -catenin (Ctnnb1) in premigratory neural crest cells also prevents the generation of melanocytes (Hari et al., 2002). However, unlike in zebrafish, inactivation of Ctnnb1 in the mouse neural crest not only prevents melanocyte formation but also sensory neurogenesis (Hari et al., 2002). Moreover, in contrast to zebrafish, Ctnnb1 signal activation in the mouse neural crest in vivo does not enhance melanocyte formation, but rather promotes the widespread formation of sensory neurons at the expense of virtually all other possible neural crest cell fates, including melanocytes (Lee et al., 2004). In the present study we offer an explanation for these inconsistent findings, suggesting that in mouse embryos Wnt/catenin signaling controls sensory fate acquisition before regulating melanocyte lineage formation. To analyze the temporal control of neural crest lineage generation by Wnt/-catenin signaling, we genetically manipulated Ctnnb1 at different stages of neural crest and early melanocyte development. Our results are in agreement with the idea that sensory neuronal and melanocyte lineages are successively generated from neural crest cells by sequential catenin signaling.