title: Doublecortin is expressed in trigeminal motoneurons that innervate the velar musculature of lampreys: considerations on the evolution and development of the trigeminal system.
abstract: Studies in lampreys have revealed interesting aspects of the evolution of the trigeminal system and the jaw. In the present study, we found a marker that distinguishes subpopulations of trigeminal motoneurons innervating two different kinds of oropharyngeal muscles. Immunofluorescence with an antibody against doublecortin (DCX; a neuron-specific phosphoprotein) enabled identification of the trigeminal motoneurons that innervate the velar musculature of larval and recently transformed sea lampreys. DCX-immunoreactive (-ir) motoneurons were observed in the rostro-lateral part of the trigeminal motor nucleus of these animals, but not in lampreys 1 month or more after metamorphosis. Combined double DCX/tubulin and serotonin/tubulin immunofluorescence and tract-tracing experiments with neurobiotin (NB) were also performed in larvae for further characterization of this system. Rich innervation by DCX-ir fibers was observed on the muscle fibers of the velum but not on the upper lip or lower lip muscles, which were innervated by tubulin-ir/DCX-negative fibers. No double-labelled DCX-ir motoneurons were observed in experiments in which the tracer NB was applied to the upper lip. Innervation of velar muscles by serotonergic fibers is also reported. The present results indicate that development of the trigeminal motoneurons innervating the velum differs from that of the trigeminal motoneurons innervating the lips, which is probably related to the dramatic regression of the velum during metamorphosis. The absence of data on a similar subsystem in the trigeminal motor nucleus of gnathostomes suggests that they may be lamprey-specific motoneurons. These results provide support for the "heterotopic theory" of jaw evolution and are inconsistent with the theories of a velar origin for the gnathostome jaw.
authors.affiliation: Department of Cell Biology and Ecology, CIBUS, Faculty of Biology, University of Santiago de Compostela, Santiago de Compostela, Spain.
authors.forename: Antón
authors.initials: A
authors.lastname: Barreiro-Iglesias
authors.forename: Daniel
authors.initials: D
authors.lastname: Romaus-Sanjurjo
authors.forename: Pablo
authors.initials: P
authors.lastname: Senra-Martínez
authors.forename: Ramón
authors.initials: R
authors.lastname: Anadón
authors.forename: María Celina
authors.initials: MC
authors.lastname: Rodicio
mesh_terms: D000818:Animals; D005075:Biological Evolution; D000089622:Doublecortin Domain Proteins; D007568:Jaw; D007798:Lampreys / Q000033:anatomy & histology* / Q000196:embryology / Q000235:genetics* / Q000502:physiology; D008869:Microtubule-Associated Proteins / Q000235:genetics* / Q000378:metabolism; D009046:Motor Neurons; D009479:Neuropeptides / Q000235:genetics* / Q000378:metabolism; D014278:Trigeminal Nuclei / Q000378:metabolism
chemical_list: D000089622:Doublecortin Domain Proteins; D008869:Microtubule-Associated Proteins; D009479:Neuropeptides