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.

Near the floor plate of the embryonic neural tube there is a group of neuroepithelial precursor cells that are specialized for production of the oligodendrocyte lineage. We performed experiments to test whether specification of these neuroepithelial oligodendrocyte precursors, like other ventral neural cell types, depends on signals from the notochord and/or floor plate. We analyzed heterozygous Danforth's short tail (Sd/+) mutant mice, which lack a notochord and floor plate in caudal regions of the neural tube, and found that oligodendrocyte precursors did not appear at the ventricular surface where there was no floor plate. Moreover, oligodendrocytes did not develop in explant cultures of Sd/+ spinal cord in the absence of a floor plate. When a second notochord was grafted into an ectopic position dorsolateral to the endogenous notochord of a chicken embryo, an additional floor plate was induced along with an ectopic focus of oligodendrocyte precursors at the ventricular surface. Oligodendrocytes developed in explants of intermediate neural tube only when they were cocultured with fragments of notochord or in the presence of purified Sonic hedgehog (Shh) protein. Thus, signals from the notochord/floor plate, possibly involving Shh, are necessary and sufficient to induce the development of ventrally derived oligodendroglia. These signals appear to act by specifying the future fate(s) of neuroepithelial cells at the ventricular surface rather than by influencing the proliferation or differentiation of prespecified progenitor cells in the parenchyma of the cord.

Duane's retraction syndrome (DRS) is a complex congenital eye movement disorder caused by aberrant innervation of the extraocular muscles by axons of brainstem motor neurons. Studying families with a variant form of the disorder (DURS2-DRS), we have identified causative heterozygous missense mutations in CHN1 , a gene on chromosome 2q31 that encodes α2-chimaerin, a Rac guanosine triphosphatase–activating protein (RacGAP) signaling protein previously implicated in the pathfinding of corticospinal axons in mice. We found that these are gain-of-function mutations that increase α2-chimaerin RacGAP activity in vitro. Several of the mutations appeared to enhance α2-chimaerin translocation to the cell membrane or enhance its ability to self-associate. Expression of mutant α2-chimaerin constructs in chick embryos resulted in failure of oculomotor axons to innervate their target extraocular muscles. We conclude that α2-chimaerin has a critical developmental function in ocular motor axon pathfinding.

Antibodies to the major protein of rat liver gap junctions, molecular weight 27,000 (27K), have been microinjected into one identified cell of 8-cell stage Xenopus embryos. This treatment selectively disrupts both dye transfer and electrical coupling between the progeny cells. These results provide evidence that the 27K protein is an integral component of the cell-to-cell junctional channel. The disruption of junctional communication at early stages results in specific developmental defects, suggesting that blocking intercellular communication can have a pronounced influence on embryonic development.

During development, growing motor axons are excluded from the ventral midline of the neural tube by diffusible chemorepellents emanating from this region. Molecular candidates for this chemorepellent activity include semaphorin D and netrin-1; the latter is known to repel trochlear motor axons. Qualitatively or quantitatively different responses to these molecules might underlie the initial deflection from the midline and subsequent segregation of motor axon trajectories. To test this idea, we have cocultured cell aggregates secreting netrin-1 or semaphorin D at a distance from tissue explants containing different motor neuron subpopulations, in collagen gels. Cranial motor axons that project dorsally in vivo such as those of the trigeminal, facial, and glossopharyngeal nuclei were repelled by both netrin-1 and semaphorin D. By contrast, ventrally projecting spinal motor axons and abducens axons were not affected by netrin-1. Spinal and abducens motor neurons also responded to semaphorin D. The ventrally projecting axons of oculomotor neurons were not repelled by netrin-1 or semaphorin D. Differential responsiveness to netrin-1 and semaphorin D could thus contribute to the generation of dorsal and ventral motor axon pathways during development.