models, cell movement is encouraged by cell-cell contact, which may be manifested as nudging from behind (Davis and Trinkaus, 1981) or the detachment of cells at the front of a migratory cell sheet (Carmona-Fontaine et al., 2008) to create space. These cell behaviors alone may not produce directional movement of a multicellular stream, but when local inhibitory signals restrict cell movements the result can be long-distance directed cell movement.By contrast, other models suggest that cells respond to chemotactic signals that drive the directional migration of individual cells (Dormann and Weijer, 2003;Richardson and Lehmann, 2010;Tarbashevich and Raz, 2010;Roussos et al., 2011;Cai et al., 2012) or cell clusters (Valentin et al., 2007;Aman and Piotrowski, 2010;Streichan et al., 2011). In these models, cells may respond directly to a chemotactic signal or receive guidance from neighboring cells.As long-distance cell migration is a major aspect of embryonic development (Dormann and Weijer, 2003;Richardson and Lehmann, 2010;Tarbashevich and Raz, 2010;Kulesa and Gammill, 2010), adult morphogenesis (Hatten and Roussel, 2011), tissue repair (Burns and Steinberg, 2011) and cancer metastasis (Roussos et al., 2011;Friedl and Gilmour, 2009), the examination of this phenomenon could have significant implications for better understanding birth defects and disease. Yet, even with multiscale data collected from different model systems and emerging computational models, the cellular and molecular mechanisms of long distance cell migration are still unclear. This is due in part to a disconnect between theory and experiment that limits the testing of various hypotheses parametrised by biological data. Thus, what is needed is a fully integrative experimental-modeling approach that can reject certain hypotheses in favor of others and elucidate multiscale mechanisms of cell migration.Here, we examine how a subpopulation of embryonic cells travel long distances and respond to tissue growth to accurately reach a target. We study this question using the neural crest (NC) as our model experimental system. NC cells exit the dorsal neural tube (NT) and travel long distances throughout the developing embryo along stereotypical pathways rich in microenvironmental signals, mesoderm and extracellular matrix (Noden and Trainor, 2005;Perris and Perissinotto, 2000). The NC cell population is crucial for proper development of the face, heart and peripheral nervous systems, and is the cellular origin of the highly aggressive cancers, melanoma and neuroblastoma (Trainor, 2005;Sauka-Spengler and Bronner-Fraser, 2008;Gammill and Roffers-Agarwal, 2010;Kasemeier-Kulesa et al., 2008;Jiang et al., 2011 SUMMARYLong-distance cell migration is an important feature of embryonic development, adult morphogenesis and cancer, yet the mechanisms that drive subpopulations of cells to distinct targets are poorly understood. Here, we use the embryonic neural crest (NC) in tandem with theoretical studies to evaluate model mechanisms of long-distance cell migration....

Neural crest (NC) cell migration is crucial to the formation of peripheral tissues during vertebrate development. However, how NC cells respond to different microenvironments to maintain persistence of direction and cohesion in multicellular streams remains unclear. To address this, we profiled eight subregions of a typical cranial NC cell migratory stream. Hierarchical clustering showed significant differences in the expression profiles of the lead three subregions compared with newly emerged cells. Multiplexed imaging of mRNA expression using fluorescent hybridization chain reaction (HCR) quantitatively confirmed the expression profiles of lead cells. Computational modeling predicted that a small fraction of lead cells that detect directional information is optimal for successful stream migration. Single-cell profiling then revealed a unique molecular signature that is consistent and stable over time in a subset of lead cells within the most advanced portion of the migratory front, which we term trailblazers. Model simulations that forced a lead cell behavior in the trailing subpopulation predicted cell bunching near the migratory domain entrance. Misexpression of the trailblazer molecular signature by perturbation of two upstream transcription factors agreed with the in silico prediction and showed alterations to NC cell migration distance and stream shape. These data are the first to characterize the molecular diversity within an NC cell migratory stream and offer insights into how molecular patterns are transduced into cell behaviors.

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The neural crest serve as an excellent model to better understand mechanisms of embryonic cell migration. Cell tracing studies have shown that cranial neural crest cells (CNCCs) emerge from the dorsal neural tube in a rostrocaudal manner and are spatially distributed along stereotypical, long distance migratory routes to precise targets in the head and branchial arches. Although the CNCC migratory pattern is a beautifully choreographed and programmed invasion, the underlying orchestration of molecular events is not well known. For example, it is still unclear how single CNCCs react to signals that direct their choice of direction and how groups of CNCCs coordinate their interactions to arrive at a target in an ordered manner. In this review, we discuss recent cellular and molecular discoveries of the CNCC migratory pattern. We focus on events from the time when CNCCs encounter the tissue adjacent to the neural tube and their travel through different microenvironments and into the branchial arches. We describe the patterning of discrete cell migratory streams that emerge from the hindbrain, rhombomere (r) segments r1–r7, and the signals that coordinate directed migration. We propose a model that attempts to unify many complex events that establish the CNCC migratory pattern, and based on this model we integrate information between cranial and trunk neural crest development.

The neural crest is an excellent model to study embryonic cell migration, since cell behaviors can be studied in vivo with advanced optical imaging and molecular intervention. What is unclear is how molecular signals direct neural crest cell (NCC) migration through multiple microenvironments and into specific targets. Here, we tested the hypothesis that the invasion of cranial NCCs, specifically the rhombomere 4 (r4) migratory stream into branchial arch 2 (ba2), is due to chemoattraction through neuropilin-1-vascular endothelial growth factor (VEGF) interactions. We found that the spatio-temporal expression pattern of VEGF in the ectoderm correlated with the NCC migratory front. RT-PCR analysis of the r4 migratory stream showed that ba2 tissue expressed VEGF and r4 NCCs expressed VEGF receptor 2. When soluble VEGF receptor 1 (sVEGFR1) was injected distal to the r4 migratory front, to bind up endogenous VEGF, NCCs failed to completely invade ba2. Time-lapse imaging revealed that cranial NCCs were attracted to ba2 tissue or VEGF sources in vitro. VEGF-soaked beads or VEGF-expressing cells placed adjacent to the r4 migratory stream caused NCCs to divert from stereotypical pathways and move towards an ectopic VEGF source. Our results suggest a model in which NCC entry and invasion of ba2 is dependent on chemoattractive signaling through neuropilin-1-VEGF interactions.