Identifiable reticulospinal neurons of the adult zebrafish, Brachydanio rerio.

Reticulospinal neurons of the larval zebrafish Brachydanio rerio have been categorized into 27 different types (Kimmel et al.: Journal of Comparative Neurology 205:112-127, 1982; Metcalfe et al.: Journal of Comparative Neurology 251:147-159, 1986). Nineteen of these occur as bilateral pairs which are individually identifiable. Since considerable remolding of brain structures (e.g., cell death and modifications of neuronal architecture) occurs during development, we ask if these cells are preserved in the adult zebrafish and the extent to which neuronal morphology of the larva is conserved during ontogeny. In our analysis, we studied reticular neurons from 84 brains retrogradely labelled from the spinal cord with HRP. We show that all reticulospinal types of the larva are retained without considerable change in morphology in the adult. Many neurons, including the Mauthner cell and two of its serial homologues, MiD2cm and MiD3cm, can be individually and unambiguously identified. In addition, the appearance of later developing (tertiary) neurons leads to an increase in the numbers of some neuron types. Although tertiary neurons are often isomorphic with neighboring cells, they can have unique morphologies of their own and, therefore, are also individually identifiable. We suggest that the appearance of tertiary neurons may serve to extend the behavioural repertoire of the embryo. Moreover, morphological repetitions in adjacent segments of the otic region (level of VIIIth nerve entry) may represent the replication of a functional motif, perhaps involving the C-type escape response which is known to involve the Mauthner cell.