THE RELATIONSHIP BETWEEN RHOMBOMERES AND VESTIBULAR NEURON POPULATIONS AS ASSESSED IN QUAIL-CHICKEN CHIMERAS

The aim of this study was to evaluate the role segmentation plays in t he determination of neuronal identity in the hindbrain. We focused on two specific sets of hindbrain neurons, namely, the vestibulospinal an d vestibule-ocular neurons, which comprise distinct groups that can be identified and distinguished by virtue of their axonal projection pat hways. The relationship between rhombomeres and the vestibular neuron groups was assessed by a combination of quail-chicken chimeric graftin g and selective retrograde axonal tracing. Individual quail hemirhombo meres were transplanted homotopically and isochronically into a chicke n embryo host. Subsequently, vestibulospinal and vestibule-ocular neur ons with specific axon trajectories were labeled retrogradely with bio tin-conjugated dextran-amines. The relationship between the spatial do mains of the vestibular neuron groups and rhombomere-derived domains h ad the following features: (1) some groups were derived from single rh ombomeres; (2) some groups were derived from multiple contiguous rhomb omeres; (3) two groups occupied domains that could not be defined in t erms of whole rhombomere lengths; (4) some groups spanning multiple rh ombomeres exhibited an internal cytoarchitectonic organization that re lated to individual rhombomeres; and (5) some groups exhibited limited boundary violation. These results support the notion that positional information within defined domains of the neural tube provides a groun dplan for the regional determination of neuronal identity and axon pat hfinding, and that hindbrain segmentation contributes to this process. But they also indicate that segmentation is not the only mechanism th at defines the rostrocaudal domains of neuron types. Moreover, they em phasize that the relationship between rhombomeres and neuronal determi nation cannot be couched simply in terms of segmental iteration or of bimeric (paired rule) specification. (C) 1998 Academic Press.