Evolutionary Patterns of Cranial Nerve Efferent Nuclei in Vertebrates

Abstract

All vertebrates have a similar series of rhombomeric hindbrain segments within which cranial nerve efferent nuclei are distributed in a similar rostrocaudal sequence. The registration between these two morphological patterns is reviewed here to highlight the conserved vs. variable aspects of hindbrain organization contributing to diversification of efferent sub-nuclei. Recent studies of segmental origins and migrations of branchiomotor, visceromotor and octavolateral efferent neurons revealed more segmental similarities than differences among vertebrates. Nonetheless, discrete variations exist in the origins of trigeminal, abducens and glossopharyngeal efferent nuclei. Segmental variation of the abducens nucleus remains the sole example of efferent neuronal homeosis during vertebrate hindbrain evolution. Comparison of cranial efferent segmental variations with surrounding intrinsic neurons will distinguish evolutionary changes in segment identity from lesser transformations in expression of unique neuronal types. The diversification of motoneuronal subgroups serving new muscles and functions appears to occur primarily by elaboration within and migration from already established segmental efferent pools rather than by de novo specification in different segmental locations. Identifying subtle variations in segment-specific neuronal phenotypes requires studies of cranial efferent organization within highly diverse groups such as teleosts and mammals.

Neuroepithelial cells require fucosylated glycans to guide the migration of vagus motor neuron progenitors in the developing zebrafish hindbrain

“…Like mammals (Gilland and Baker, 2005;Taylor et al, 1999), zebrafish form two vagus motor nuclei in r8, the most caudal part of the hindbrain. One is the dorsolateral motor nucleus of the vagus (dlX), whereas the other is the medial motor nucleus of the vagus (mmX) (Fig.…”

“…It controls various body functions, including heart beat and gastrointestinal movement (Gilland and Baker, 2005;Taylor et al, 1999). Correct arrangement of vagus motor nuclei following neural migration and accumulation may be crucial for the normal functioning of vagus motor neurons.…”

The assembly of developing motor neurons depends on an interplay between spontaneous activity, type II cadherins and gap junctions

“…The activity we describe might also regulate neuronal maturation and the acquisition of afferent input, which may include descending input from early longitudinal tracts and the vestibular and raphe spinal systems (Gilland and Baker, 2005;Hughes et al, 2009). The link between motor nucleogenesis and brainstem neuronal circuit formation remains to be characterised.…”

Shared developmental and evolutionary origins for neural basis of vocal–acoustic and pectoral–gestural signaling

“…Vertebrates have two functional series of hindbrain motor nuclei, somatic and branchiomeric (1,2), that were likely present in the earliest, pregnathostome vertebrates (5). Somatic nuclei innervate head muscle derived from unsegmented (i.e., prechordal plate) and segmented paraxial mesoderm (i.e., occipital somites); branchiomeric nuclei target derivatives of paraxial mesoderm that migrate into the pharyngeal arches (1,6,7).…”

“…Somatic nuclei innervate head muscle derived from unsegmented (i.e., prechordal plate) and segmented paraxial mesoderm (i.e., occipital somites); branchiomeric nuclei target derivatives of paraxial mesoderm that migrate into the pharyngeal arches (1,6,7). Comparative studies delineate a conserved pattern of hindbrain somatic and branchiomeric motor nuclei spatially segregated along the rostral-caudal axis across eight rhs (1,8). Most nuclei originate in one or two rhs with little variation in extent or location across taxa (1).…”