Neurogenesis and Migration

Neural progenitor cells of the neural tube proper are located and undergo mitosis near the inner (luminal) surface of the tube in a layer called the ventricular zone. As postmitotic neurons are generated, they migrate radially toward the outer (pial) surface, establishing a mantle zone where they begin to differentiate and aggregate into nuclei. Most neurons take up residence somewhere along this radial trajectory, but some turn to migrate circumferentially, either toward the floor plate or toward the roof plate, or longitudinally. Neurons generated in the alar plate, particularly the rhombic lip region, have a particular predilection for circumferential migration. Indeed, several nuclei, including the raphe nuclei and the inferior olive, are established within the basal plate through immigration from the alar plate (Fig. 2B).

By and large, the alar and basal plate domains maintain a coherent relationship to the sensory and motor divisions of the cranial nerve nuclei, respectively (Fig. 2), with the sulcus limitans persisting in the mature hindbrain as a landmark of this division. In general, motoneurons are generated within the basal plate, whereas the neurons of the sensory nuclei are generated within the alar plate. Some hindbrain cranial nerve nuclei, however, undergo circumferential migration, such as the trigeminal motoneurons, which migrate away from the floor plate, and the cochlear efferent neurons, which migrate toward the floor plate and cross the midline. The disposition of neuron groups in the mature hindbrain therefore does not necessarily accurately indicate their embryonic origins.

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