REVIEWS Patterning and axon guidance of cranial motor neurons Sarah Guthrie Abstract | The cranial motor nerves control muscles involved in eye, head and neck movements, feeding, speech and facial expression. The generic and specific properties of cranial motor neurons depend on a matrix of rostrocaudal and dorsoventral patterning information. Repertoires of transcription factors, including Hox genes, confer generic and specific properties on motor neurons, and endow subpopulations at various axial levels with the ability to navigate to their targets. Cranial motor axon projections are guided by diffusible cues and aided by guideposts, such as nerve exit points, glial cells and muscle primordia. The recent identification of genes that are mutated in human cranial dysinnervation disorders is now shedding light on the functional consequences of perturbations of cranial motor neuron development. Neural tube In humans, the cell bodies of cranial motor neurons lie experiments. The specificity of cranial motor neuron The primordium of the nervous in the brainstem, and their axons extend through the projections is governed by rostrocaudal and dorsoven- system. cranial nerves to control muscles in the head and neck. tral patterning mechanisms that produce a diversity of Other vertebrates (including fish, chicks and mice) show motor neuron subpopulations with distinct differentia- Floor plate The ventral midline structure of a high degree of conservation in both the arrangement tion programmes. Some of the guidance molecules that the CNS. It has a role in of brainstem motor neurons and the muscles they inner- are involved in elaborating axon projections have also patterning and axon guidance. vate. Developing motor axons perform a spectacular feat, been characterized. However, many important ques- navigating over long distances from the CNS to their tions remain. The unique features of the differentiation Branchial arches targets in the periphery. programmes of each of the cranial nerves are only partly Repeated bars of mesenchymal tissue that Early in development, the neural tube acquires a series of characterized. In particular, we know little about how contribute to the lower jaw and swellings at its rostral end, presaging the development patterning genes dictate the repertoires of receptors on neck; each contains a of the forebrain, the midbrain and the hindbrain. axons, or how these receptors determine axon pathfind- cartilaginous component, a Caudally, the neural tube remains narrow and elongates ing behaviour to particular muscle targets. Deciphering muscular component, a nerve and an artery. to form the spinal cord. Motor neurons differentiate these molecular mechanisms is a major challenge in ventrally, on either side of a midline structure, the floor developmental neurobiology. plate. Cranial motor neurons reside in the midbrain and In this context, it is fascinating that cranial dysinner- the hindbrain (which together constitute the brainstem), vation disorders, which reflect abnormalities of one or where they are partitioned into a series of nuclei. By more cranial nerves, are starting to be genetically charac- contrast, spinal motor neurons form a number of dis- terized in humans6. Clinical studies, together with studies continuous columns along the length of the cord (for in animal models, are now providing fresh impetus to reviews, see REFS 1,2). Cranial motor axons follow dorsal understand how normal and abnormal cranial nerve wir- or ventral pathways from the brainstem; the axial posi- ing develops. In this Review, I describe the latest findings tion of this site of exit in turn dictates their peripheral in cranial motor neuron patterning and axon guidance, paths to muscles of the eye, tongue, branchial arches or to focusing mainly on mouse and chick studies (as zebrafish parasympathetic ganglia3 (FIG. 1; TABLE 1). studies have been reviewed elsewhere: see REF. 5). MRC Centre for Surprisingly, despite the functional significance of Developmental Neurobiology, cranial motor nerves, an understanding of the molecular Motor nuclei form at distinct axial levels King’s College, Guy’s Campus, mechanisms that underlie their development is only just Cranial motor neurons comprise three subsets: bran- London, SE1 1UL, UK. 4,5 e-mail: starting to emerge . Exciting progress has been made in chiomotor (BM), visceral motor (VM) and somatic [email protected] understanding cranial motor neuron development, par- motor (SM) neurons (FIG. 2; TABLE 1). Early in develop- doi:10.1038/nrn2254 ticularly from gene gain-of-function and loss-of-function ment, these neurons arise in longitudinal progenitor NATUre revieWS | NEUROSCIENCE voLUme 8 | november 2007 | 859 © 2007 Nature Publishing Group REVIEWS r1, r2 and r3 (in mice) or r2 and r3 (in chicks), the facial nucleus (nucleus VII; BM and VM neurons) lies in r4 and r5, the glossopharyngeal nucleus (nucleus IX; BM MB and VM neurons) lies in r6 (in mice) or r6 and r7 (in chicks), and the vagus nucleus (nucleus X; BM and VM neurons) and cranial accessory nucleus (XI; BM FB III neurons) occupy r7 and r8 (REF.10) (FIG. 2). In the cau- dal hindbrain, the abducens nucleus (nucleus VI, SM IV neurons) occupies r5 in mice and r5 and r6 in chicks, with the extended hypoglossal nucleus (nucleus XII, HB SM neurons) found in r8. In both mice and chicks, the facial motor neurons of the BM subtype are segregated V in r4, and those of the VM subtype are segregated in r5 (REFS 11,12). In all except avian species, the facial bran- VII/VII chiomotor (FBM) neurons are born in r4 and then undertake a striking caudal migration to r6 (REFS 10,13), BAs VI OV unlike most BM and VM neuron somata, which migrate dorsally14. Rhombomere 4 also contains a population of IX vestibuloacoustic neurons, which are efferent to the hair XII cells of the inner ear; a subset of these neurons (contral- ateral vestibuloacoustic neurons) translocate their cell X/XI bodies across the midline15. Following their exit into the periphery, cranial motor axons converge to form components of the cranial nerves (FIG. 1). BM axons travel, through the trigeminal, facial, glossopharyngeal, vagus and cranial accessory nerves, towards branchial arches 1, 2, 3, 4 and 6, respectively, where they innervate muscles of Figure 1 | Cranial nerves in the chick embryo. A lateral the jaw and muscles that control facial expression, as view of cranial nerves in the chickNatur embryoe Revie wsat embryonic| Neuroscienc e well as the pharynx and the larynx. VM axons project day four, showing the pathways from the hindbrain (HB), on the right, into the branchial arches (BAs) and other head towards parasympathetic ganglia, the neurons of structures (the midbrain (MB) and the forebrain (FB)), on the which supply salivary and lacrimal glands, smooth left. Roman numerals denote the nerves: III, oculomotor; IV, muscle and visceral organs. Oculomotor, trochlear trochlear; V, trigeminal; VI, abducens; VII/VIII, facial/ and abducens SM neurons innervate the six eye mus- vestibuloacoustic; IX, glossopharyngeal; X, vagus; XI, cles, with an additional oculomotor VM component cranial accessory; XII, hypoglossal. OV, otic vesicle. Figure synapsing at the ciliary ganglion. Hypoglossal neurons modified, with permission, from REF. 3 (1990) Wiley-Liss. project rostrally through the floor of the pharynx to the tongue muscles. Cranial motor nuclei conform to a theme, sharing common features, such as morphology domains in the hindbrain basal plate: BM and VM and initial axon trajectory, but nevertheless possess- neuronal somata migrate dorsally into the alar plate, ing distinct positional identity, synaptic targets and whereas SM somata remain ventral (in the basal functions. plate). BM and VM axons extend dorsally through the neuroepithelium to large common exit points, whereas Rostrocaudal patterning of the brainstem Basal plate The ventral half of the SM axons leave the neuroepithelium ventrally in small The midbrain is divided into a series of ‘arcs’, which neuroepithelium. groups (FIG. 2c), with the exception of trochlear SM have been proposed to underlie the differentiation of axons, which grow dorsally and cross the dorsal mid- nuclei and are distinguished by the expression of vari- Alar plate line at the midbrain–hindbrain boundary to project ous homeobox genes and other molecular markers16–18. The dorsal half of the contralaterally. The most medial arc contains oculomotor neurons, neuroepithelium. Individual motor nuclei can contain one or more and fibroblast growth factor 8 (FGF8), produced by Neuroepithelium of BM, VM and SM neuron subsets. The oculomotor the midbrain–hindbrain boundary, has been proposed A part of the early nervous nucleus (nucleus III), which contains SM and VM to dictate the rostrocaudal position of the oculomo- system that consists of dividing neurons, lies most rostrally in the midbrain. Along the tor nucleus, because misexpression of FGF8 shifts the progenitors arranged in a 17 columnar epithelium. rostrocaudal axis, the hindbrain is divided into rhom- nucleus rostrally . Differentiating oculomotor neurons bomeres, segmental entities that contain repeating sets express the homeobox gene paired-like homeobox 2a Homeobox of neurons with distinct differentiation programmes (Phox2a)16, which is an important determinant of ocu- A conserved 180 base pair at different axial levels7–9. Motor nuclei differentiate lomotor identity, as oculomotor neurons (as well as sequence that encodes in individual rhombomeres or pairs of rhombomeres. trochlear