REVIEWS INDUCTION OF THE NEURAL CREST: A MULTIGENE PROCESS Anne K. Knecht and Marianne Bronner-Fraser In the embryo, the neural crest is an important population of cells that gives rise to diverse derivatives, including the peripheral nervous system and the craniofacial skeleton. Evolutionarily, the neural crest is of interest as an important innovation in vertebrates. Experimentally, it represents an excellent system for studying fundamental developmental processes, such as tissue induction. Classical embryologists have identified interactions between tissues that lead to neural crest formation. More recently, geneticists and molecular biologists have identified the genes that are involved in these interactions; this recent work has revealed that induction of the neural crest is a complex multistep process that involves many genes. NEURAL TUBE A central question in developmental biology is how a sin- ancestral vertebrates, such as lampreys (BOX 1). The cells A cylindrical structure that runs gle cell — the egg — can give rise to many tissues. A prin- that form neural crest are induced at the border between through the midline of the cipal mechanism for generating such complexity is the neural plate, which forms the central nervous sys- embryo; it expands in the head induction, whereby one set of cells influences another, tem, and the non-neural ectoderm, which forms the to form the brain and in the trunk to form the spinal cord. causing those cells to adopt a different fate. Although epidermis (FIG. 1). As the neural plate folds over itself to induction is usually represented by a single arrow in dia- form the NEURAL TUBE, border regions (NEURAL FOLDS) from NEURAL FOLDS grams, recent advances in the molecular genetics of devel- opposite sides of the ectoderm come together and later Tips of invaginating ectoderm opment have shown that induction involves several sig- fuse. In this way, neural crest progenitors come to lie in, that will close to form the dorsal nals. These include inducers, which activate a new genetic and/or immediately adjacent to, the dorsal neural tube1. portion of the neural tube. programme that leads to conversion to a different cell During or after neural tube closure (depending on the type; competence factors, which control the time at which species), neural crest cells leave the neural tube and cells can respond to these inducers; maintenance factors, migrate throughout the body, where they differentiate which maintain the induced developmental programme, into neurons, cartilage, melanocytes and many other perhaps through many intermediate stages; and cell- types of cell (BOX 2). This review focuses only on the survival or proliferation signals, which regulate growth. early formation of neural crest progenitors; neural crest Likewise, recent research has identified several genes that migration and differentiation are discussed elsewhere1. are involved in the induction of the neural crest, includ- Although neural crest progenitors lie in the neural ing, in particular, secreted growth factors of the bone plate border, this region does not give rise exclusively morphogenetic protein (BMP), fibroblast growth factor to neural crest. If a single cell in the neural folds is (FGF) and Wingless/INT-related (WNT) families. labelled before neural tube closure, then the labelled However, the precise functions of these genes remain derivatives can later be found in the neural crest, 2 Division of Biology 139-74, unclear. This review describes recent findings about the neural tube and epidermis . Even after neural tube clo- California Institute of molecular basis of neural crest induction, with the goal of sure, cells in the dorsal neural tube in the chick2 and in Technology, Pasadena, elucidating the steps and signals that are involved. the frog3 can generate both neural tube cells and neural California 91125, USA. crest cells. After emigration from the neural tube, Correspondence to M.B.-F. Early development of neural crest e-mail: neural crest cells do not normally contribute to the [email protected] The neural crest is a transient, migratory population of neural tube, but if these cells are injected into the ven- doi:10.1038/nrg819 cells found in all vertebrate embryos, including the tral neural tube, they can adopt the fates of their NATURE REVIEWS | GENETICS VOLUME 3 | JUNE 2002 | 453 REVIEWS Box 1 | Evolution of the neural crest regions of the neural folds that have the potential to form neural crest5,6. For simplicity, this review refers Vertebrate evolution has been intimately linked to the evolution of two embryonic cell to the induction of Slug as neural crest induction, but populations — the neural crest and the cranial ectodermal PLACODES. These cell with the understanding that this might actually repre- populations together give rise to many of the defining characteristics of vertebrates, sent an early step in an ongoing process. including a well-defined head with teeth and paired sensory organs. Both neural crest and placodes are migratory populations that form at the border between neural plate Models of neural crest induction and epidermis. They generate some of the same cell types, such as sensory neurons. Neural induction. An obvious first step in the forma- Although they share many characteristics, there are also some differences between them; tion of the neural-plate border is the formation of the for example, placodes are confined to the head, whereas neural crest cells arise from most of the anteroposterior axis. Unlike placodes, neural crest cells form melanocytes neural plate. During GASTRULATION, ectoderm is induced to form neural tissue by signals from a specialized and AUTONOMIC NEURONS, and produce mineralized matrices like bone. These differences might reflect differences in their evolutionary origin74. region of mesoderm known as ‘the organizer’. Neural crest evolved soon after the split of cephalochordates (amphioxus) and A model of the molecular basis of neural induction, vertebrates. Amphioxus, the closest living invertebrate relative of the vertebrates, shares which is referred to as the neural default model some characteristics with vertebrates, such as segmented muscles, but lacks definitive (reviewed in REF. 7), has been developed during the neural crest75,76. By contrast, structures that are derived from the neural crest, such as past decade on the basis of experiments carried out in PHARYNGEAL DENTICLES, are present in the earliest vertebrate fossils, and the most basal the frog Xenopus laevis. According to this model, the extant vertebrates, hagfish and lampreys, have well-developed structures that are default fate of ectoderm is to form neural tissue; how- derived from the neural crest. Lampreys are jawless fish that represent the most ever, before neural induction, all of the ectoderm pro- primitive extant vertebrates for which it is feasible to obtain embryos, and these clearly duces the growth factor BMP4, which suppresses the have neural crest cells77. formation of neural tissue and promotes the forma- Because of the genome-wide duplications associated with vertebrate evolution, one tion of epidermis. To overcome this suppression and, possible way to explain the evolution of neural crest is that, in the vertebrate lineage, new therefore, to generate the neural plate, the organizer genes were formed by duplication and this facilitated diversification of gene function, secretes BMP antagonists, such as noggin, chordin which led to the origin of a new cell type. However, many homologues of vertebrate neural and follistatin, which bind to BMPs and prevent crest markers have been cloned in both amphioxus and lampreys, which indicates that the signalling through their receptors7. same complement of genes exists in both species, even though the former lacks a definitive Although this model is well supported by experi- neural crest77–79.Although the functions of these genes have not been studied sufficiently ments in Xenopus, it remains unclear how well it applies for us to be certain that they act the same way in different species, in general, it seems more to other vertebrates, and whether other factors might be likely that the evolution of the neural crest was accompanied by the use of old genes in new involved. Mouse knockouts of BMPs and BMP antago- ways rather than by the invention of new genes for a new cell population. Further 8 molecular characterization and embryological analysis are necessary to gain a better nists were largely uninformative , probably owing to understanding of the evolutionary origin of neural crest cells and placode cells. functional redundancy between genes. In the chick, Streit et al.9 questioned whether BMP antagonism is necessary for neural induction for the following reasons. First, they showed that BMP4 and BMP7 expression in neighbours and form floor plate and motor neurons4. the chick disappears from the ectoderm before gastrula- Such results show that, although induction at the tion (HAMBURGER–HAMILTON STAGE 2)9.However,Faure et PLACODES neural plate border leads to the formation of neural al.10 found that BMP signalling, which is manifested by Thickenings in the vertebrate cranial ectoderm that invaginate crest cells, there are many steps in between, in which activation of the downstream signalling molecule and form parts of cranial cells show remarkable flexibility in their cell-fate deter- SMAD1, is reactivated throughout the ectoderm at stage sensory ganglia and paired mination. 3 of chick development and is subsequently downregu- sensory organs. Because of this, it is difficult to apply traditional lated in the prospective neural plate at stage 4. So, as in developmental terms, such as ‘commitment’ or ‘specifi- Xenopus, reduction of BMP signalling correlates with AUTONOMIC NEURONS Nerve cells of the peripheral cation’,to neural crest. A tissue is operationally defined neural induction. nervous system that innervate as specified for a certain fate if it continues to adopt Second, Streit et al. also found that the addition of the viscera, smooth muscles and that fate when explanted away from other external sig- BMPs to prospective neural plate in stage-3 chick exocrine glands.