Early organization of neurons in the zebrafish CNS

Studying the first invertebrate neurons to developing has revealed organizational patterns, and possible determinants of neuronal development. Zebrafish have small embryos, with a simple CNS containing many individually identifiable neurons that can be similarly studied. Immunohistochemistry using the monoclonal antibody zn-1 (labels neurons at axogenesis) demonstrates the early distribution of developing neurons. By 24 hours post fertilization (h), clusters of zn-1 immunoreactive neurons are present throughout the neuraxis. These are primary neurons. They are establishing the first long projection pathways and large clusters of neurons. Ventral clusters of primary neurons are in the midbrain, diencephalon, and telencephalon. Dorsal clusters are in the diencephalon and telencephalon. A special hindbrain and spinal cord transitional region contains the early developing "ipsilateral caudal" (ic) cells, whose distribution when quantified indicates segmental continuity. The hindbrain is organized into neuromeres; swellings of the neuroepithelium repeated along the axis. Antibodies labeled an extensive and unsuspected internal structure in each of the seven rostral hindbrain neuromeres. These internal features show a neuromere as an alternation of two distinct regions, a neuromeric center (nc) and an interneuromeric zone (inz). The nc's are evident in the early neuromere swellings, that contain of the first hindbrain neurons. Next, neuron groups, fiber tracts, neuropil areas, and radial glia develop, indicating the inz's. The inz's contain mediolateral rows of dorsoventral running glial fibers in close contact with transversely oriented fiber tracts that originate from lateral clusters of neurons in the hindbrain's inz's. This juxtaposition indicates that neuronal growth cones of the lateral clusters may use the glia as migrational cues or substrates. Dendrites of individually identifiable reticulospinal neurons are restricted to nc's, where segmentally arranged immunoreactive at the time of dendritogenesis. An antigen or a co-localized molecule may provide cues or a substrate for dendritic growth cones. It seems likely the segmental pattern of primary neurons and other neural components is determined by developmental algorithms repeated in each neuromere. These may have continuing effects on both cytoarchitecture and connectivity of developed neurons.