Brainstem neurons with descending projections to the spinal cord of two elasmobranch fishes: Thornback guitarfish, Platyrhinoidis triseriata, and horn shark, Heterodontus francisci

We studied two cartilaginous fishes and described their brainstem supraspin al projections because most nuclei in the reticular formation can be identi fied that way. A retrogradely transported tracer, horseradish peroxidase or Fluoro-Gold, was injected into the spinal cord of Platyrhinoidis triseriat a (thornback guitarfish) or Heterodontus fransisci (horn shark). We describ ed labeled reticular cells by their position, morpohology, somatic orientat ion, dendritic processes, and laterality of spinal projections. Nineteen re ticular nuclei have spinal projections: reticularis (r.) dorsalis, r.ventra lis pars alpha and beta, r. gigantocellularis, r. magnocellularis, r. parvo cellularis, r. paragigantocellularis lateralis and dorsalis, r. pontis caud alis pars alpha and beta, r. pontis oralis pars medialis and lateralis, r. subcuneiformis, r. peduncularis pars compacta, r. subcoeruleus pars a, raph e obscurus, raphe pallidus, raphe magnus, and locus coeruleus. Twenty nonre ticular nuclei have spinal projections: descending trigeminal, retroambiguu s, solitarius, posterior octaval, descending octaval, magnocellular octaval , ruber, Edinger-Westphal, nucleus of the medial longitudinal fasciculus, i nterstitial nucleus of Cajal, latral mesencephalic complex, periventricular is pretectalis pars dorsalis, central pretectal, ventromedial thalamic, pos terior central thalamic, posterior dorsal thalamic, the posterior tuberculu m, and nuclei B, F, and J. The large number of distinct reticular nuclei wi th spinal projections corroborates the hypothesis that the reticular format ion of elasmobranches is complexly organized into many of the same nuclei t hat are found in frogs, reptiles, birds, and mammals. (C) 1999 Wiley-Liss, Inc.