DEVELOPMENTAL DYNAMICS 229:5–13, 2004 REVIEWS–A PEER REVIEWED FORUM Significance of the Cranial Neural Crest Anthony Graham,* Jo Begbie, and Imelda McGonnell The cranial neural crest has long been viewed as being of particular significance. First, it has been held that the cranial neural crest has a morphogenetic role, acting to coordinate the development of the pharyngeal arches. By contrast, the trunk crest seems to play a more subservient role in terms of embryonic patterning. Second, the cranial crest not only generates neurons, glia, and melanocytes, but additionally forms skeletal derivatives (bones, cartilage, and teeth, as well as smooth muscle and connective tissue), and this potential was thought to be a unique feature of the cranial crest. Recently, however, several studies have suggested that the cranial neural crest may not be so influential in terms of patterning, nor so exceptional in the derivatives that it makes. It is now becoming clear that the morphogenesis of the pharyngeal arches is largely driven by the pharyngeal endoderm. Furthermore, it is now apparent that trunk neural crest cells have skeletal potential. However, it has now been demonstrated that a key role for the cranial neural crest streams is to organise the innervation of the hindbrain by the cranial sensory ganglia. Thus, in the past few years, our views of the significance of the cranial neural crest for head development have been altered. Developmental Dynamics 229:5–13, 2004. © 2003 Wiley-Liss, Inc. Key words: neural crest; cranial; endoderm; pharyngeal; sensory Received 22 July 2003; Accepted 22 August 2003 INTRODUCTION cells are subjected to the alternating et al., 1991; Schilling and Kimmel, permissive and inhibitory halves of 1994). The second crest stream, the The established view of the cranial somites, which result in their segmen- hyoid, arises primarily from rhom- neural crest was that it played a piv- tal organisation (Rickmann et al., bomere 4 of the hindbrain and forms otal role in vertebrate head devel- 1985). In the head, the neural crest neurons of the proximal facial gan- opment. It was thought that the cra- nial neural crest carried the cues for cells pour out from the developing glion as well as the constituents of organising the morphogenesis of the brain into a periphery that is devoid the second pharyngeal arch—the head and, uniquely, was the source of somites. Yet, the cranial crest cells hyoid skeleton (Lumsden et al., 1991; of much of the head skeleton. How- do not migrate as a single mass but Schilling and Kimmel, 1994). Finally, ever, it is now becoming apparent are organised into streams, three of the post-otic crest is generated by that we must reassess our views of which can be identified in the devel- rhombomeres 6 and 7 of the hind- the significance of the cranial neural oping head of all vertebrate embry- brain and forms the neurons of the crest, and it is with this reassessment os: trigeminal, hyoid, and post-otic proximal and jugular ganglia, and that this review is concerned. (Fig. 1). The trigeminal crest arises the skeletal components of the pos- from the midbrain and rhom- terior pharyngeal arches (Lumsden bomeres 1 and 2 of the hindbrain et al., 1991; Schilling and Kimmel, MIGRATION OF THE CRANIAL and forms neurons within the trigem- 1994). NEURAL CREST inal ganglion and the components In the trunk, the first migrating neu- The migration of the cranial neural of the orofacial prominences and ral crest cells move ventrally from crest is quite distinct from that of the mandibular arch—the skeleton of the neural tube and migrate trunk. In the trunk, the neural crest the lower and upper jaw (Lumsden through the anterior half sclerotome. MRC Centre for Developmental Neurobiology, New Hunts House, Guys Campus, Kings College London, London, United Kingdom Grant sponsor: The Medical Research Council (UK); Grant sponsor: The Wellcome Trust. *Correspondence to: Anthony Graham, MRC Centre for Developmental Neurobiology, New Hunts House, Guys Campus, Kings College London, London SE1 1UL, UK. E-mail: [email protected] DOI 10.1002/dvdy.10442 © 2003 Wiley-Liss, Inc. 6 GRAHAM ET AL. Fig. 1. Cranial neural crest migrates in three streams. The trigeminal stream (red) emanates from the midbrain (mb), rhombomeres (r) 1 and 2. It migrates into the upper jaw primordia, underneath the eye, and into the first pharyngeal arch (PA1), which forms the lower jaw. The hyoid stream (green), from rhombomere 4, migrates into the second pharyn- geal arch, forming the jaw support. The post-otic stream (purple) emerges from rhombomeres 6 and 7. It populates the caudal pharyn- geal arches, of which there are three in the chick embryo depicted here. Fig. 2. Regionalisation of the pharyn- geal endoderm. Endodermal pharyngeal pouches (pp) sit between the pharyngeal arches and express a range of markers that demonstrate distinct regionalisation. Pax-1 (green) is expressed at the dorsal tip of each pouch. BMP-7 (blue) is ex- pressed in the posterior endodermal mar- gin, while Fgf8 (red) is expressed in the anterior endodermal margin. Shh (yel- low) is expressed at the posterior margin of pouches 2 and 3 only. D, dorsal; V, ventral; P, posterior; A, anterior; mb, mid- brain; r, rhombomere. Fig. 3. Segregation of cranial neural crest establishes correct afferent innervation in the vertebrate head. Whole-mount chick embryo labeled with neurofilament antibody to visualise axonal projections. The trigeminal axons (T) extend toward the hindbrain along the first or trigeminal crest stream (red), entering at rhom- bomere (r) 2. Axons of the geniculate (G) extend along the hyoid crest stream (green), into rhombomere 4. Axons from the petrosal (P) and nodose (N) extend along the post-otic crest stream (pur- ple) into rhombomeres 6 and 7, respectively. Mb, midbrain. CRANIAL NEURAL CREST 7 These cells have a neuronal fate. in the crest primordia of rhombomeres nial neural crest streaming is main- Those that move furthest ventrally 3 and 5, which in turn acts to sponsor tained in other species where focal form the sympathetic ganglia, while the apoptotic elimination of these cell death in rhombomeres 3 and 5 is the remainder cease migration in cells (Graham et al., 1994). In vitro ap- not observed, such as in zebrafish the anterior sclerotome and form plication of Bmp-4 can also promote (Schilling and Kimmel, 1994) and Xe- the dorsal root ganglia. The later mi- crest depletion of rhombomeres 2 nopus embryos (Hensey and Gautier, grating neural crest cells, however, and 6, which correlates with the find- 1998). move dorsolaterally between the ing that much of the hindbrain neural Other work has shown that there dermamyotome and the overlying crest expresses the necessary recep- are additional mechanisms that ectoderm, and these cells will form tors and intracellular effectors to re- could contribute to the streaming of the melanocytes (Weston and But- spond to this factor (Farlie et al., 1999; the neural crest. Several studies ler, 1966; Serbedzija et al., 1990; Smith and Graham, 2001). Bmp-4 will seem to suggest that there are inhib- Erickson et al., 1992). In the head, not induce, however, cell death in itory influences emanating from there is also a correlation between rhombomere 4 neural crest, which rhombomeres 3 and 5, which restrict the timing of crest migration and the corresponds with the finding that the movement of cells across these fates the crest cells follow, but it is these crest cells express the Bmp-4 an- segments. For example, if the neural significantly different. Here, the early tagonist Noggin in addition to the crest primordium of rhombomere 4 is migratory cells populate the pharyn- Bmp receptors and intracellular trans- grafted dorsally in rhombomere 3, geal arches and facial prominences, ducers (Smith and Graham, 2001). even though many neural crest cells and these groups will generate ecto- Thus, the neural crest cell death is lo- are now produced they are de- mesenchymal derivatives: bone, car- calised to rhombomeres 3 and 5 both flected anteriorly and posteriorly tilage, and connective tissue, while by restricting the expression of the (Niederlander and Lumsden, 1996). the later migrating cells stay closer to Bmp-4 ligand to these segments as a This effect could be due to the ex- the developing central nervous sys- result of an inductive interaction from pression in both rhombomeres 3 and tem and generate neurons and glia the flanking segments and through 5ofSema3A, a molecule which is of the cranial ganglia (Baker et al., the expression of Noggin in the neural known to inhibit neural crest migra- 1997). crest cells of rhombomere 4, which tion (Eickholt et al., 1999). Similarly, it are juxtaposed to sites of Bmp-4 ex- has been suggested that the mes- SEGREGATION OF THE CRANIAL pression. enchyme opposite these two seg- Studies in other species have sug- ments is inhibitory to neural crest mi- CREST STREAMS gested that the establishment of the gration (Farlie et al., 1999) and that The segregation of the cranial crest cranial neural crest streams may be this finding is due to the function of is evident as soon as they are gen- somewhat different to that observed ErbB4 in the hindbrain (Golding et erated by, and emerge from, the in chick. In mammalian embryos, cra- al., 2000). These studies help explain neural tube, and it is events within nial neural crest migration com- why the few crest cells generated by the developing hindbrain that are mences before neural tube closure rhombomeres 3 and 5 do not move responsible for establishing this seg- and occurs over a protracted period laterally from these rhombomeres regation. Studies in chick have of time (Morriss-Kay and Tucket, 1991).