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DEVELOPMENTAL DYNAMICS 229:5–13, 2004

REVIEWS–A PEER REVIEWED FORUM

Significance of the Cranial

Anthony Graham,* Jo Begbie, and Imelda McGonnell

The 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, , and , but additionally forms skeletal derivatives (bones, cartilage, and teeth, as well as and connective ), 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 of the pharyngeal arches is largely driven by the pharyngeal . Furthermore, it is now apparent that 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 by the cranial sensory ganglia. Thus, in the past few years, our views of the significance of the cranial neural crest for 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 , 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 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 into a periphery that is devoid the second —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- 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 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 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 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 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 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 ganglia. The later mi- crest depletion of rhombomeres 2 nopus (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 , 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 , while by restricting the expression of the (Niederlander and Lumsden, 1996). the later migrating cells stay closer to Bmp-4 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). but migrate anterior and posterior. shown that one mechanism that In the chick, however, cranial neural Furthermore, work in has helps enforce the segregation of the crest cells only begin migrating before also suggested that interactions be- crest is the focal depletion of crest neural tube closure at mesence- tween the ephrins and their recep- cells from the two territories that lie phalic levels, with most cranial crest tors play a role in streaming the crest between the streams, rhombomeres cells migrating after tube closure in the periphery (Smith et al., 1997). 3 and 5. These two rhombomeres do (Tosney, 1982). Another difference be- Given these results, it seems likely produce some neural crest cells. tween chick and is that it that the segregation of the neural However, these cells do not migrate has long been established that, in crest into streams is in part due to laterally but rather move anteriorly mammals, the cranial crest do not mi- events in the hindbrain, including, in and posteriorly to join the streams of grate in a strict rostrocaudal se- chick, the induction of focal neural crest migrating from the adjacent quence. Indeed, there is also varia- crest in rhombomeres 3 even-number rhombomeres (Sechrist tion in the timings and patterns of and 5, as well the action of subse- et al., 1993; Birgbauer et al., 1995; migration between mammalian em- quent cues in the periphery, which Kulesa and Fraser, 2000). The majority bryos (Tan and Morris-Kay, 1985). Re- act to reinforce and stabilise this of the neural crest cells produced by cent studies have also suggested that segregation. rhombomeres 3 and 5, however, are the mechanisms underlying the lost by means of apoptosis, which is streaming of the cranial crest in the MORPHOGENETIC ROLE FOR induced in these crest cells by signals mouse differ significantly from those THE CRANIAL NEURAL CREST from the neighbouring segments uncovered in the chick, although the (Graham et al., 1993). One of the precise nature of these mechanisms It has in the past been argued that main consequences of this inductive has yet to be elucidated (Trainor et the segregation of the cranial neural interaction is to promote the expres- al., 2002). Similarly, we know little crest into these streams was essential sion of the signalling molecules Bmp-4 about the mechanism by which cra- for ensuring correct morphogene- 8 GRAHAM ET AL. sis—creating skeletal elements of populations from different axial lev- mation of endodermal pharyngeal the correct size, shape, and orienta- els. Furthermore, this even holds true pouches is the first indication of pha- tion in the appropriate location. A if one of the attachment points is on ryngeal arch development (Veitch key piece of evidence supporting the mesodermally derived basal et al., 1999). Furthermore, the post- this viewpoint comes from studies in cranial capsule; these attachment otic crest that fills the posterior which neural crest cells were forced points were shown to derive from arches is split by the formation of the to populate an inappropriate pha- discrete clusters of crest cells em- pharyngeal pouches. In keeping ryngeal arch (Noden, 1983). Thus, if bedded on the skull. with this view, it has been shown presumptive first arch crest is grafted In many ways, the hypothesis that that, if the neural crest is ablated, in place of presumptive second it is the neural crest cells that pattern through removal of the neural tube arch crest, then the grafted crest the pharyngeal arches is very attrac- before crest production, pharyngeal cells fill the second arch, rather than tive. First, although this patterning arches do form and are regionalised the first, and, furthermore, although mechanism is seemingly different (Veitch et al., 1999). In normal pha- these transposed crest cells gener- from that observed in the rest of the ryngeal arches, Bmp-7 is expressed ate skeletal derivatives, they differ- embryo, there is a commonality in at the posterior endodermal margin, entiate as components of the lower that, in all cases, the skeletogenic FGF-8 in the anterior endoderm, and jaw, i.e., first arch, rather than sec- tissue is the repository of the pattern- Pax-1 in the most dorsal endoderm ond arch elements. These trans- ing information. In the trunk, the of the pharyngeal pouches, and posed neural crest cells also caused paraxial -derived somites, these expression domains are un- the musculature of the second arch which generate the vertebra, direct changed in arches that are devoid to form skeletal attachments typical patterning in the ad- of neural crest cells (Veitch et al., of the first arch (Noden, 1983). The jacent (Ensini et al., 1999; Fig. 2). Also, these crest-free conclusion that emerged from these 1998). The , arches have a sense of individual studies was that it was the neural which forms the appendicular skele- identity. Normally, is crest cells that were acting to organ- ton, acts to attract muscle precur- a prominent early feature of the pos- ise the development of the pharyn- sors from adjacent somites and sub- terior endoderm of the second arch, geal arches. Furthermore, as these sequently directs their patterning following later in the posterior grafts were of neural tube origin be- into specific muscle types (Cheval- endoderm of the third arch, and fore the emergence of the neural lier and Kieny, 1982; Francis-West et again in the absence of crest the crest, it was also suggested that the al., 2003). In the head, the neural spatial and temporal dynamics of neural crest cells acquired their crest is the prime source of skeletal sonic hedgehog is unchanged in sense of identity when they were tissue, and here these cells are the absence of crest (Veitch et al., within the neural primordium. Thus, thought to pattern the arches. Sec- 1999; Fig. 2). Thus, the endoderm the segregation of the cranial crest ond, if the neural crest cells do pat- can form a segmented series that is into stream would act to ensure that tern the arches, then this also pro- appropriately regionalized in the ab- first arch crest, carrying cues for jaw vides an explanation of how the sence of neural crest. development, did not intermingle different tissues of the developing That pharyngeal segmentation is with hyoid crest, carrying cues for head can be coordinated. For ex- not dependent upon the crest is also the patterning of the second arch; ample, the neural crest cells that significant, because it reflects the the streaming of the crest would act populate and organise the first arch evolutionary history of the pharyn- to guarantee the faithful transfer of arise from the same axial level as the geal arches. Endodermal pharyn- patterning information from the neu- trigeminal motor neurons that inner- geal segmentation is a feature of all ral primordium to the periphery. vate that arch, and thus one can (Schaeffer, 1987) and, The idea that the segregation of imagine how a correspondence is thus, originated before the evolution the cranial crest into streams was im- achieved between sensorimotor in- of the neural crest. The regionalisa- portant for the morphogenesis of the nervation, centered on the hind- tion of the pharyngeal endoderm head was further enforced by a brain, and the musculoskeletal ef- also preceded the emergence of long-term fate map of the cranial fectors in the periphery. the neural crest, and it has been crest, which revealed a highly con- shown in Amphioxus that the Pax-1/9 strained pattern of skeletomuscular ENDODERM PLAYS A KEY ROLE gene is expressed within the pharyn- connectivity in the head (Kontges IN PHARYNGEAL geal pouches in a similar manner to and Lumsden, 1996). In this study, it (Holland and Holland, MORPHOGENESIS was found that head muscle con- 1996). Thus, the evolution of the ver- nective tissues derived from neural Several studies, however, have now tebrate must have involved crest from a particular axial level suggested that this crest-centric an integration between the neural were always exclusively anchored view of pharyngeal patterning crest cells, which generate the pha- to skeletal domains derived from the needs to be reassessed. More spe- ryngeal skeleton, and a more an- same origin. This finding occurred cifically, it is now apparent that the cient segmentally organised pha- even within skeletal elements, such pharyngeal endoderm plays a key ryngeal endoderm. as the jaw or the skeleton, role in organising the development Further evidence for a role for which are compounds of crest cells of the pharyngeal arches. The for- endoderm in directing pharyngeal CRANIAL NEURAL CREST 9 arch development has come from neural crest streams expresses differ- enchyme only expresses Dlx-1 and the zebrafish mutant vgo in which ent Hox genes (Hunt et al., 1991). The -2, thus these cells behave as if the pharyngeal pouches fail to form trigeminal stream is Hox negative, they were maxillary. Thus, the pat- (Piotrowski and Nusslein-Volhard, the hyoid expresses Hox-a2, and the terning of the pharyngeal arches is 2000). In this mutant, arch develop- post-otic crest express members of a consensual process in which pat- ment is severely perturbed and the third paralogous Hox group—Hox- terning cues emanating from the neural crest-derived pharyngeal a3, Hoxb3, Hox-d3. Of interest, in endoderm, or other pharyngeal tis- cartilages are disorganised, often mice lacking Hox-a2, the second sues, are differentially interpreted fusing with each other, or in the arch exhibits a transformation, and by the crest cells of each of the more posterior arches, failing to jaw elements form within it (Gend- arches, and this process is depen- form altogether. This failure is not ron-Maguire et al., 1993; Rijli et al., dent upon the transcription factors due to defects in the crest but re- 1993). These duplicated jaw ele- they express. sults from the failure of pouch for- ments form as a mirror image to the mation. Other mutants that display first arch elements, which would be IMPORTANCE OF THE endoderm defects, such as bon consistent with these cells also re- STREAMING OF THE CRANIAL and cas which do not produce sponding to cues emanating from endoderm, also show defects in the first pharyngeal pouch. How- NEURAL CREST the formation of the pharyngeal ever, if Hox-a2 expression is forced in The demonstration that it is the arches, and in these , the the neural crest cells of the first arch, endoderm that is the prime mover in pharyngeal cartilages fail to form then jaw development is inhibited organising the development of the at all. It has also now been demon- and an ectopic hyoid forms, and pharyngeal arches diminishes the strated that the neural crest cells again this is mirror imaged (Gram- idea that the key role of crest are induced to form cartilage by matopoulos et al., 2000; Pasqualetti streaming is for the patterning the means of the action of FGF-3 em- et al., 2000). That Hox genes allow arches, segregating the first stream anating from the endoderm (David neural crest cells to respond differ- carrying cues for the morphogenesis et al., 2002). Furthermore, it has been entially to endoderm cues is also of the jaw from the second for the definitively demonstrated, in chick supported by the observation that, if hyoid arch. This idea has become embryos, that neural crest cells re- Hox-positive crest were exposed to even less attractive with the finding spond to patterning cues produced an ectopic strip of rostral pharyn- that lamprey embryos also display by the pharyngeal endoderm to gen- geal endoderm, then, unlike the ros- three obvious crest streams, even erate distinct skeletal components tral crest which do not express Hox though these animals lack a jaw (Couly et al., 2002). The ablation of genes and form a jaw in this situa- and hyoid (Horigome et al., 1999). specific domains of the pharyngeal tion, these cells do not form a jaw We have demonstrated recently endoderm at early stages of develop- (Couly et al., 2002; Creuzet et al., that the streaming of cranial neural ment results in the specific loss of par- 2002). crest, in fact, is necessary for a com- ticular neural crest-derived skeletal el- It has become clear that other pletely different role: the organisa- ements. Moreover, if rostral neural transcription factors, in addition to tion of the afferent innervation of the crest cells are exposed to an ectopic Hox genes, can similarly act to mod- hindbrain. piece of pharyngeal endoderm, then ulate the way in which crest cells The majority of the neurons of the they will generate a supernumerary interpret the peripheral patterning cranial sensory ganglia derive from jaw. Of interest, the orientation of the cues. In particular, the Dlx family, neurogenic placodes; focal thicken- ectopic strip of endoderm determines which display nested expression do- ings of the embryonic ectoderm that the orientation of the skeletal ele- mains along the proximodistal axis of generate neuronal cells (Graham ments of the additional jaw. the arches seems to function in this and Begbie, 2000). The neurogenic way (Simeone et al., 1994; Qiu et al., placodes form in stereotypical posi- PHARYNGEAL PATTERNING IS 1997a). Normally, the crest-derived tions in all vertebrates. The trigeminal of the maxillary pri- placodes emerge opposite the ante- CONSENSUAL mordia (forming the upper jaw) and rior hindbrain, the otic placodes at the These studies clearly highlight that the proximal mandibular primordia middle hindbrain level, and the the endoderm plays a pre-eminent only express Dlx-1 and -2. The rest of epibranchial placodes—geniculate, role in the patterning of the pharyn- the mandibular mesenchyme ex- petrosal, and nodose—close to the geal arches; however, the neural presses these genes and also Dlx-5 tips of the clefts between the pharyn- crest cells are not completely pas- and -6. Additionally, the distal-most geal arches. The neuronal cells that sive in this process. Rather, several mesenchyme expresses Dlx-7 and -3. are formed within these placodes studies suggest that the response of In mice mutant for both Dlx-5 and -6, then move internally to the site of different populations of the neural several mandibular primordia skele- ganglion formation wherein they dif- crest to endodermal cues is depen- tal elements are missing and in their ferentiate and send out axons toward dent upon the transcription factors place skeletal elements characteris- their central and peripheral targets. that they express, most notably their tic of the maxillary primordia form Importantly, these ganglia innervate repertoire. During normal (Depew et al., 2002). In these mu- the hindbrain at specific axial levels development, each of the cranial tants, most of the mandibular mes- (Lumsden and Keynes, 1989). The ax- 10 GRAHAM ET AL. ons of the trigeminal ganglion enter be appreciated that these defects of the oro-pharyngeal region. Fur- the hindbrain at rhombomere 2, those are likely to be a secondary conse- thermore, it has also been shown of the geniculate and vestibuloac- quence of alteration in the behav- that the cranial crest generates the coustic, which is derived from the otic iour of the neuroglial neural crest connective tissues of the muscles, placode, at rhombomere 4, and the cells. Further insights into this process the of the head, and the axons of the petrosal and the nodose can be gained from another mouse smooth muscle cells associated with ganglia at rhombomeres 6 and 7. mutant in which ␤-catenin is mu- the feathers and the aortic arches Rhombomeres 3 and 5 do not receive tated at the sites of wnt-1 expression (Le Douarin and Kalcheim, 1999). any afferent innervation. There is, thus, and which, thus, lacks ␤-catenin in its Similar fate maps of the chick trunk a clear relationship between the migratory crest cells (Brault et al., crest have suggested that these rhombomeres that receive sensory in- 2001). In this , the neural crest cells do not generate bone and car- puts and those that generate the cells migrate normally but are subse- tilage. Although studies in zebrafish crest streams (Fig. 3). quently eliminated by means of apo- and Xenopus have shown that trunk The neural crest streams that ex- ptosis, and again the epibranchial neural crest cells do contribute to fin tend from the hindbrain into the pe- ganglia fail to connect to the hind- connective tissue (Collazo et al., riphery are generated in advance of brain. Thus, the epibranchial neuro- 1993; Smith et al., 1994), these stud- the production of any of the pla- nal cells are dependent upon the ies did not find trunk crest forming codal neuronal cells. Importantly, crest cells themselves, rather than skeletal derivatives. the neuronal cells that delaminate some common earlier pathway that The different fates of the cranial from the placodes migrate internally is used by both the crest and the and the trunk neural crest cells were along the neural crest streams (Beg- placodal neuronal cells. also thought to represent differ- bie and Graham, 2001). For exam- This evidence suggests that the ences in potential. Thus, while cra- ple, the cells of the geniculate pla- prime function of the cranial crest nial crest can form all ectomesen- code, which is associated with the streaming is to organise the afferent chymal derivatives, trunk crest is only second arch, move inward along innervation of the hindbrain. The real capable of forming connective tis- the hyoid crest stream (Fig. 3). If the clue to this comes with the fact that sues and smooth muscle, but not hindbrain is ablated before the pro- crest streaming occurs in lampreys, cartilage or bone. The supporting duction of the neural crest, the cells which lack a jaw but have basically evidence for this comes from both in leaving the placode enter a crest- the same relationship between their vitro and in vivo studies. Clonal anal- free environment and do not mi- cranial sensory ganglia and the ysis of cranial crest cells found that grate internally. Instead, they form hindbrain as all other vertebrates. these would generate cartilage in disconnected subectodermal gan- That is not to say, however, that with addition to neurons, glia, and mela- glia with aberrant projections that the evolution of the gnathostomes, nocytes when cultured in rich me- do not reach the hindbrain and of- existing crest streaming has been dium on a 3T3 feeder layer, which ten extend toward other ganglia co-opted to help pattern the jaws offered support for a range of differ- (Begbie and Graham, 2001). It could and jaw-support, and indeed to entiation options (Baroffio et al., be argued that this failure in the mi- help organise the constrained pat- 1991). Yet, trunk crest cultured under gration of the placodal cells is due tern of skeletomuscular connectivity the same conditions were only ever to the absence of the neural tube in the head. Rather, it is likely that observed to produce, neurons, glia, and not specifically the neural crest. these functions of crest streaming and melanocytes (Baroffio et al., However, if the hindbrain is removed are built upon a more ancestral 1988). Correspondingly, it was found after the production of the crest, the function of helping to organise the that when trunk neural tube was placodal cells do move inward and sensory innervation of the hindbrain. grafted into the head, the neural establish appropriate projections crest cells produced by this graft (Begbie and Graham, 2001). Thus, CRANIAL NEURAL CREST HAS never contributed to cranial skeletal the presence of a neural crest elements (Nakamura and Ayer-le SKELETOGENIC POTENTIAL stream is essential for the projection Lievre, 1982). Rather, these trans- of sensory axons to the correct loca- Besides any morphogenetic role, the posed cells tended to stay close to tion in the developing hindbrain. major difference between the cra- the neural tube and differentiate as This integrative role for the neural nial and the trunk neural crest lies in neurons and Schwann cells. It was crest further clarifies the phenotypic the fact that the cranial crest gener- found that these cells could form alterations seen in several mutant ates ectomesenchymal derivatives: connective tissue and muscle, but mice in which one or more of the bone, cartilage, dentine, muscle, they were never found to contribute epibranchial ganglia fail to connect and connective tissue. Indeed, fate to cranial skeletal elements (Naka- to the hindbrain, even though none maps of the cranial crest con- mura and Ayer-le Lievre, 1982). of the genes in question are ex- structed in chick have suggested Although these experimental ap- pressed in the epibranchial ganglia: that the majority of the head skele- proaches yielded results that were Sox-10 (Britsch et al., 2001), COUP-TFI ton is crest derived (Noden, 1988; consistent with trunk neural crest (Qiu et al., 1997b), ErbB4 (Golding et Couly et al., 1993). These elements cells lacking the potential to gener- al., 2000), CRKL (Guris et al., 2001), include dermal bones of the skull ate skeletal derivatives, they did not AP-2 (Zhang et al., 1996). It can now and all of the cartilages and bones directly test its ability to do so. There- CRANIAL NEURAL CREST 11

Fig. 4. Trunk neural crest has skeletogenic potential. Neural crest fates are restricted along the anteroposterior axis of the embryo. Crest-derived parasympathetic (green) and sensory neurons (blue) are differentially dis- tributed along the whole embryo, while sym- pathetic neurons (grey) are restricted to the trunk and (red) to the cra- nial region. Other fates not shown, such as cardiac and enteric crest, are similarly re- stricted. However, all neural crest has the po- tential to make the full range of possible crest derivatives, indicating that fate decisions are driven by the environment that crest migrates through and differentiates in.

Fig. 5. Evolution of the trunk skeleton. Fos- sil evidence shows that early vertebrates, such as the Heterostracan, possessed ex- tensive postcranial dermal exoskeleton (red), likely derived from the trunk neural crest. Trunk exoskeleton became much re- duced with the appearance of the - derived endoskeleton. The zebrafish has a predominantly endoskeletal trunk support but possesses a remnant of exoskeleton in the dermal fin rays, which is also likely de- rived from the trunk neural crest. Higher ver- tebrates, such as the chick, have an exclu- sively endoskeletal trunk support derived solely from the sclerotome of the somite. fore, we explicitly assessed the skel- drocytes and osteocytes. The ability grafted trunk neural crest cells di- etogenic potential of trunk crest by of trunk crest to contribute to cranial rectly into the facial primordia culturing both chick cranial and skeletal elements was also tested in (McGonnell and Graham, 2002). trunk crest cells in media commonly vivo. As described, previous studies These cells dispersed, and as ex- used for growing bone and carti- had found that grafted trunk crest pected formed neurons, Schwann lage cells (McGonnell and Graham, cells did not contribute to skeletal cells, and melanocytes; however, 2002). As expected, bone and carti- elements, yet this of course could they also contributed to the forming lage differentiation occurred in cul- have resulted from the failure of skeletal elements, such as Meckel’s tures of cranial crest. Importantly, these cells to migrate from the trans- cartilage. Another much older study this was also found to be true of trunk posed piece of neural tissue to the has also shown that mammalian trunk crest cultures, which under these sites of skeletal differentiation. To cir- neural crest cells can participate in conditions generated both chon- cumvent this potential difficulty, we formation when recombined 12 GRAHAM ET AL. with oral ectoderm (Lumsden, 1988). crest into cranial versus truncal pop- crest cells revealed by intravital micros- Thus, trunk crest cells do have skeleto- ulations is likely to be a derived fea- copy. Development 121:935–945. genic potential. ture, with what we term cranial crest Brault V, Moore R, Kutsch S, Ishibashi M, Rowitch DH, McMahon AP, Sommer L, In the past, the generation of dif- representing the ancestral state. In- Boussadia O, Kemler R. 2001. Inactiva- ferent derivatives by the neural crest deed, in many ways it’s not that the tion of the (␤)-catenin gene by Wnt1- of distinct axial levels was thought development of the head is distinct Cre-mediated results in dra- not to reflect a lack of potential, but because of the role played by the matic brain malformation and failure of craniofacial development. Devel- to be a consequence of environ- crest, but rather it’s the trunk that opment 128:1253–1264. mental cues restricting these fates. differs, because in this region of the Britsch S, Goerich DE, Riethmacher D, The only exception to this scenario embryo, the mesoderm has be- Peirano RI, Rossner M, Nave KA, Birch- was thought to apply to skeleto- come dominant. meier C, Wegner M. 2001. The tran- genic potential, which was believed scription factor Sox10 is a key regulator of peripheral glial development. to be intrinsic to the cranial crest (Le CONCLUDING REMARKS Genes Dev 15:66–78. Douarin and Kalcheim, 1999). Our Chevallier A, Kieny M. 1982. On the role results demonstrate that, in fact, Five years ago, many people, our- of connective tissue in the patterning there is no difference in potential selves included, would have argued of the chick limb musculature. Wilhelm Roux Arch Dev Biol 191:277–280. between the cranial and the trunk strongly that the cranial neural crest played a pivotal role in the develop- Collazo A, Bronner-Fraser M, Fraser SE. neural crest; both can generate the 1993. Vital dye labelling of Xenopus full repertoire of crest derivatives ment of the vertebrate head. The cra- laevis trunk neural crest reveals multi- (Fig. 4). Thus, with regard to poten- nial crest generates the head skele- potency and novel pathways of migra- tial, there is nothing special about ton, and these cells were believed to tion. Development 118:363–376. coordinate the developed of this re- Couly GF, Coltey PM, Le Douarin NM. the cranial crest. 1993. The triple origin of skull in higher During normal development the gion. Now, however, it is apparent vertebrates: a study in quail- chick chi- skeletogenic potential of trunk crest that this view must be reassessed. The meras. Development 117:409–429. must be suppressed. However, the ability of the cranial crest to generate Couly G, Creuzet S, Bennaceur S, Vincent skeletogenic potential of the trunk skeletal derivatives is not unique to this C, Le Douarin NM. 2002. Interactions axial level, but is shared by all crest between Hox-negative cephalic neu- crest was likely realized in early ver- ral crest cells and the tebrates (Fig. 5). There are many fos- cells. Rather, as with all other crest po- endoderm in patterning the facial skel- sil fish that display extensive postcra- tentials, skeletal potential is restricted eton in the vertebrate head. Develop- nial exoskeletal coverings of dermal by environmental cues. Furthermore, ment 129:1061–1073. Creuzet S, Couly G, Vincent C, Le Doua- bone and dentine, two cells types it is now clear that the crest does not play the key role in patterning the rin NM. 2002. Negative effect of Hox that derive from neural crest cells gene expression on the development (Maisey, 1988; Smith and Hall, 1990; head and that, for the pharyngeal of the neural crest-derived facial skel- Donoghue and Sansom, 2002). The arches, this patterning is fulfilled by the eton. Development 129:4301–4313. skeletogenic potential of trunk crest pharyngeal endoderm. However, it is David NB, Saint-Etienne L, Tsang M, Schill- now evident that the streaming of the ing TF, Rosa FM. 2002. Requirement for is also expressed, albeit to a limited endoderm and FGF3 in ventral head extent, in some extant vertebrates. A cranial neural crest is fundamental to skeleton formation. Development 129: study in zebrafish has suggested that the organisation of the afferent inner- 4457–4468. the lepidotrichia, the distal miner- vation of the hindbrain. Thus, today, Depew MJ, Lufkin T, Rubenstein JL. 2002. alised portions of the fins rays, are our views of the significance of the Specification of jaw subdivisions by Dlx cranial neural crest have changed. genes. Science 298:381–385. neural crest derived (Smith et al., Donoghue PC, Sansom IJ. 2002. Origin and 1994). Thus, it would seem that, dur- early evolution of vertebrate skeletoniza- ing vertebrate evolution, the sup- REFERENCES tion. Microsc Res Tech 59:352–372. pression in the skeletogenic poten- Eickholt BJ, Mackenzie SL, Graham A, tial of the trunk crest mirrors the Baker CV, Bronner-Fraser M, Le Douarin Walsh FS, Doherty P. 1999. Evidence for NM, Teillet MA. 1997. Early- and late- collapsin-1 functioning in the control of reduction in the extent of the trunk migrating cranial neural crest cell pop- neural crest migration in both trunk and exoskeleton. Contrastingly, during ulations have equivalent developmen- hindbrain regions. Development 126: this process, the endoskeleton has tal potential in vivo. Development 124: 2181–2189. emerged to be the major axial skel- 3077–3087. Ensini M, Tsuchida TN, Belting HG, Jessell Baroffio A, Dupin E, Le Douarin NM. 1988. TM. 1998. The control of rostrocaudal etal support. This has undoubtedly Clone-forming ability and differentiation pattern in the developing spinal cord: involved an enhancement of the potential of migratory neural crest cells. specification of motor neuron subtype role of the somites, and in particular Proc Natl Acad SciUSA85:5325–5329. identity is initiated by signals from of the sclerotome. It is likely, there- Baroffio A, Dupin E, Le Douarin NM. 1991. . Development 125: fore, that the environmental cues Common precursors for neural and 969–982. mesectodermal derivatives in the ce- Erickson CA, Duong TD, Tosney KW. 1992. that suppress trunk neural crest skel- phalic neural crest. Development 112: Descriptive and experimental analysis etal differentiation, in amniotes, are 301–305. of the dispersion of neural crest cells either expressed in or related to the Begbie J, Graham A. 2001. Integration be- along the dorsolateral path and their formation of the sclerotome. tween the epibranchial placodes and entry into ectoderm in the chick em- the hindbrain. Science 294:595–598. bryo. Dev Biol 151:251–272. It is also important to appreciate Birgbauer E, Sechrist J, Bronner-Fraser M, Farlie PG, Kerr R, Thomas P, Symes T, that, from an evolutionary perspec- Fraser S. 1995. Rhombomeric origin and Minichiello J, Hearn CJ, Newgreen D. tive, the separation of the neural rostrocaudal reassortment of neural 1999. A paraxial exclusion zone creates CRANIAL NEURAL CREST 13

patterned cranial neural crest cell out- Lumsden AG. 1988. Spatial organization tral branchial region of the head by growth adjacent to rhombomeres 3 of the and the role of neural disruption of Hoxa-2, which acts as a and 5. Dev Biol 213:70–84. crest cells in the initiation of the mam- selector gene. Cell 75:1333–1349. Francis-West PH, Antoni L, Anakwe K. malian tooth germ. Development 103: Schaeffer B. 1987. Deuterostome mono- 2003. Regulation of myogenic differen- 155–169. phyly and phylogeny. Evol Biol 21:179– tiation in the developing limb bud. J Lumsden A, Keynes R. 1989. Segmental 234. Anat 202:69–81. patterns of neuronal development in the Schilling TF, Kimmel CB. 1994. Segment Gendron-Maguire M, Mallo M, Zhang M, chick hindbrain. Nature 337:424–428. and lineage restrictions during Gridley T. 1993. Hoxa-2 mutant mice Lumsden A, Sprawson N, Graham A. pharyngeal arch development in the exhibit homeotic transformation of 1991. Segmental origin and migration zebrafish embryo. Development 120: skeletal elements derived from cranial of neural crest cells in the hindbrain 483–494. neural crest. Cell 75:1317–1331. region of the chick embryo. Develop- Sechrist J, Serbedzija G, Scherson T, Fraser Golding JP, Trainor P, Krumlauf R, Gass- ment 113:1281–1291. S, Bronner-Fraser M. 1993. Segmental mann M. 2000. Defects in pathfinding Maisey JG. 1988. Phylogeny of early ver- migration of the hindbrain neural crest by cranial neural crest cells in mice tebrate skeletal induction and ossifica- does not arise from its segmental gen- lacking the neuregulin ErbB4. tion patterns. New York: Plenum Pub- eration. Development 118:691–703. Nat Cell Biol 2:103–109. lishing Corporation. p 1–36. Serbedzija GN, Fraser SE, Bronner-Fraser Graham A, Begbie J. 2000. Neurogenic McGonnell IM, Graham A. 2002. Trunk M. 1990. Pathways of trunk neural crest placodes: a common front. Trends neural crest has skeletogenic potential. in the mouse embryo as Neurosci 23:313–316. Curr Biol 12:767–771. revealed by vital dye labelling. Devel- Graham A, Heyman I, Lumsden A. 1993. Morriss-Kay G, Tucket F. 1991. Early events opment 108:605–612. Even-numbered rhombomeres control in mammalian craniofacial morpho- the apoptotic elimination of neural genesis. J Craniofac Genet Dev Biol 11: Simeone A, Acampora D, Pannese M, crest cells from odd-numbered rhom- 181–191. D’Esposito M, Stornaiuolo A, Gulisano bomeres in the chick hindbrain. Devel- Nakamura H, Ayer-le Lievre CS. 1982. Me- M, Mallamaci A, Kastury K, Druck T, opment 119:233–245. sectodermal capabilities of the trunk Huebner K, Boncinelli E. 1994. Cloning Graham A, Francis-West P, Brickell P, neural crest of . J Embryol Exp Mor- and Characterization of the Members Lumsden A. 1994. The signalling mole- phol 70:1–18. of the Vertebrate Dlx Gene Family. cule BMP4 mediates apoptosis in the Niederlander C, Lumsden A. 1996. Late Proc Natl Acad SciUSA91:2250–2254. rhombencephalic neural crest. Nature emigrating neural crest cells migrate Smith A, Graham A. 2001. Restricting 372:684–686. specifically to the exit points of cra- Bmp-4 mediated apoptosis in hindbrain Grammatopoulos GA, Bell E, Toole L, nial branchiomotor . Develop- neural crest. Dev Dyn 220:276–283. Lumsden A, Tucker AS. 2000. Homeotic ment 122:2367–2374. Smith MM, Hall BK. 1990. Development transformation of branchial arch iden- Noden DM. 1983. The role of the neural and evolutionary origins of vertebrate tity after Hoxa2 overexpression. Devel- crest in patterning of avian cranial skel- skeletogenic and odontogenic tissues. opment 127:5355–5365. etal, connective, and muscle tissues. Biol Rev Camb Philos Soc 65:277–373. Guris DL, Fantes J, Tara D, Druker BJ, Dev Biol 96:144–165. Smith M, Hickman A, Amanze D, Lums- Imamoto A. 2001. Mice lacking the homo- Noden DM. 1988. Interactions and fates den A, Thorogood P. 1994. Trunk neural logue of the 22q11.2 gene CRKL of avian craniofacial mesenchyme. crest origin of caudal fin mesenchyme phenocopy neurocristopathies of Di- Development 103:121–140. in the zebrafish Brachydanio rerio. Proc George syndrome. Nat Genet 27:293–298. Pasqualetti M, Ori M, Nardi I, Rijli FM. 2000. R Soc Lond B 256:137–145. Hensey C, Gautier J. 1998. Programmed Ectopic Hoxa2 induction after neural Smith A, Robinson V, Patel K, Wilkinson cell death during Xenopus develop- crest migration results in homeosis of DG. 1997. The EphA4 and EphB1 recep- ment: a spatio-temporal analysis. Dev jaw elements in Xenopus. Develop- tor tyrosine kinases and ephrin-B2 li- Biol 203:36–48. ment 127:5367–5378. gand regulate targeted migration of Holland LZ, Holland ND. 1996. Expression of Piotrowski T, Nusslein-Volhard C. 2000. The branchial neural crest cells. Curr Biol AmphiHox-1 and AmphiPax-1 in am- endoderm plays an important role in 7:561–570. phioxus embryos treated with retinoic patterning the segmented pharyngeal Tan SS, Morris-Kay GM. 1985. The devel- acid: insights into evolution and pattern- region in zebrafish (Danio rerio). Dev opment and distribution of the cranial ing of the cord and Biol 225:339–356. neural crest in the rat embryo. Cell Tis- pharynx. Development 122:1829–1838. Qiu M, Bulfone A, Ghattas I, Meneses JJ, sue Res 240:403–416. Horigome N, Myojin M, Ueki T, Hirano S, Christensen L, Sharpe PT, Presley R, Ped- Tosney KW. 1982. The segregation and early Aizawa S, Kuratani S. 1999. Develop- ersen RA, Rubenstein JL. 1997a. Role of migration of cranial neural crest cells in the ment of cephalic neural crest cells in the Dlx homeobox genes in proximodis- avian embryo. Dev Biol 89:13–24. embryos of Lampetra japonica, with tal patterning of the branchial arches: special reference to the evolution of of Dlx-1, Dlx-2, and Dlx-1 and Trainor PA, Sobieszczuk D, Wilkinson D, the jaw. Dev Biol 207:287–308. -2 alter morphogenesis of proximal skel- Krumlauf R. 2002. Signalling between Hunt P, Gulisano M, Cook M, Sham MH, etal and soft tissue structures derived the hindbrain and paraxial tissues dic- Faiella A, Wilkinson D, Boncinelli E, from the first and second arches. Dev tates neural crest migration pathways. Krumlauf R. 1991. A distinct Hox code Biol 185:165–184. Development 129:433–442. for the branchial region of the verte- Qiu Y, Pereira FA, DeMayo FJ, Lydon JP, Veitch E, Begbie J, Schilling TF, Smith MM, brate head. Nature 353:861–864. Tsai SY, Tsai MJ. 1997b. Null of Graham A. 1999. Pharyngeal arch pat- Kontges G, Lumsden A. 1996. Rhomben- mCOUP-TFI results in defects in mor- terning in the absence of neural crest. cephalic neural crest segmentation is phogenesis of the glossopharyngeal Curr Biol 9:1481–1484. preserved throughout craniofacial on- ganglion, axonal projection, and ar- Weston JA, Butler SL. 1966. Temporal fac- togeny. Development 122:3229–3242. borization. Genes Dev 11:1925–1937. tors affecting localization of neural Kulesa PM, Fraser SE. 2000. In ovo time- Rickmann M, Fawcett JW, Keynes RJ. 1985. crest cells in tbe embryo. Dev lapse analysis of chick hindbrain neural The migration of neural crest cells and Biol 14:246–266. crest cell migration shows cell interac- the growth of motor axons through the Zhang J, Hagopian-Donaldson S, Ser- tions during migration to the branchial rostral half of the chick somite. J Embryol bedzija G, Elsemore J, Plehn-Dujowich arches. Development 127:1161–1172. Exp Morphol 90:437–455. D, McMahon AP, Flavell RA, Williams T. Le Douarin NM, Kalcheim C. 1999. The Rijli FM, Mark M, Lakkaraju S, Dierich A, 1996. Neural tube, skeletal and body neural crest. New York: Cambridge Uni- Dolle P, Chambon P. 1993. A homeotic wall defects in mice lacking transcrip- versity Press. transformation is generated in the ros- tion factor AP-2. Nature 381:238–241.