Nociceptive sensory neurons derive from contralaterally migrating, fate-restricted neural crest cells Lynn George 1 , Marta Chaverra 1 , Valerie Todd 1 , Rusty Lansford 2 & Frances Lefcort 1 Neural crest cells (NCCs) are a transient population of multipotent progenitors that give rise to numerous cell types in the embryo. An unresolved issue is the degree to which the fate of NCCs is specified prior to their emigration from the neural tube. In chick embryos, we identified a subpopulation of NCCs that, upon delamination, crossed the dorsal midline to colonize spatially discrete regions of the contralateral dorsal root ganglia (DRG), where they later gave rise to nearly half of the nociceptor sensory neuron population. Our data indicate that before emigration, this NCC subset is phenotypically distinct, with an intrinsic lineage potential that differs from its temporally synchronized, but ipsilaterally migrating, cohort. These findings not only identify a major source of progenitor cells for the pain- and temperature-sensing afferents, but also reveal a previously unknown migratory pathway for sensory-fated NCCs that requires the capacity to cross the embryonic midline. Cells specified to be neural crest undergo an epithelial-to-mesenchymal transition in the dorsal neural tube as they delaminate, and migrate either ventrally or dorsolaterally through the embryo to give rise to a wide variety of cell types, including the majority of neurons and glia in the peripheral nervous system, melanocytes, and cartilaginous and skeletal elements of the head 1 . The migratory pathways along which NCCs travel have been described in detail and an extensive literature exists documenting the fate restrictions of cells that migrate along each of these specific trajectories 1–7 . One of the major structures that derives from ventrally migrating trunk NCCs is the chain of DRG that line each side of the spinal cord 1,4,7 . The mature DRG consist of a heterogeneous population of sensory neurons whose phenotypes can be distinguished on the basis of the sensory modality that they subserve, their neuro- trophic factor dependency and corresponding high-affinity neuro- trophic factor–receptor expression (for example, TrkA, TrkB, TrkC and ret), central and peripheral innervation targets, and neurotrans- mitter synthesis 8–10 . The DRG also contain two types of glial cells, satellite and Schwann cells, which are produced in a Notch-mediated gliogenic wave that occurs subsequent to the completion of neurogen- esis 11 . As a result of the considerable molecular and functional diversity among cell types in the DRG, this system has proven to be an excellent one for studying the mechanisms by which a multipotent stem cell–like population, such as the neural crest, gives rise to such a varied spectrum of cellular derivatives. In the developing DRG, sensory neurons are generated in two distinct, but temporally overlapping, waves. The first wave consists primarily of large-diameter, TrkC + and TrkB + neurons that mediate transmission of proprioceptive and mechanoceptive information, respectively, whereas the second wave generates the majority of small-diameter, TrkA + nociceptive neurons that mediate pain sensation 10,12,13 . An unresolved question has been the source of the progenitors for neurons born during this second wave. Although boundary cap cells (BCCs), a population of late-emigrating NCCs that coalesce at the boundary between the spinal cord and dorsal root, have been demonstrated to give rise to 5% of TrkA + DRG neurons 14 , the identity of the progenitors for the remaining TrkA + neuronal population is unknown, and of interest given that they function as the mediators of noxious stimuli and constitute 70–80% of the neurons in the mature DRG 9,14 . The development of all DRG neurons requires one of two basic-helix-loop-helix transcription factor genes, Neuro- genin1 (Ngn1) or Neurogenin2 (Ngn2), with Ngn2 mediating the first wave of neurogenesis, and Ngn1 controlling the second 15 . Wnt, via activation of b-catenin 16,17 , acts upstream of Ngn1 and Ngn2 to specify sensory fate for NCCs, and recently the Runx family of transcription factors has been shown to regulate the differentiation of subtypes of sensory neurons in the DRG 18–21 . In spite of these considerable molecular advances, our understanding of the cellular mechanisms that mediate the migration of NCCs and their differentiation into distinct cell types in the DRG remains sparse. To examine this issue, we sought to track the origin, lineage and migratory behavior of the NCCs that give rise to cells in the DRG. RESULTS NCCs colonize discrete regions of the contralateral DRG To label NCCs, the enhanced green fluorescent protein (EGFP) plasmid pCAX was injected in ovo into the neural tube of chick embryos before neural crest emigration, at Hamburger/Hamilton Stage (St) 11 (ref. 22), followed by electroporation along the rostral/caudal axis of the embryo. This induces DNA entry into cells along one side of the neural tube (the ipsilateral side), whereas the other side (the Received 27 March; accepted 16 July; published online 2 September 2007; doi:10.1038/nn1962 1 Department of Cell Biology and Neuroscience, Montana State University, Leon Johnson Hall, Rm 512, Bozeman, Montana 59717, USA. 2 Beckman Imaging Center, California Institute of Technology, MC 139-74, Pasadena, California 91125, USA. Correspondence should be addressed to F.L. (lefcort@montana.edu). NATURE NEUROSCIENCE VOLUME 10 [ NUMBER 10 [ OCTOBER 2007 1287 ARTICLES © 2007 Nature Publishing Group http://www.nature.com/natureneuroscience