Development of reticulospinal neurons of the zebrafish. II. Early axonal outgrowth and cell body position.

The sequence of axonal outgrowth and the early cell body positions of identifiable types of reticulospinal (RS) neurons of the zebrafish (Brachydanio rerio) were determined in order to learn if differences in neuronal characteristics among specific cell types correlate with differences in the times at which the cells develop. The time of axonal outgrowth and the location of the cells in the developing embryo were determined by filling RS neurons retrogradely with horseradish peroxidase from their growing axons over a series of developmental stages. Hindbrain cells developed in two distinct waves separated by about 10 hours. In the first wave, axons initiating growth at nearly the same time and from nearly the some location in the hindbrain diverged in their choice of an ipsilateral or contralateral pathway into the spinal cord. Individual types of RS neurons grew axons into the rostral spinal cord in a similar temporal sequence to that of their birthdays although the lag time between birthday and time of axonal outgrowth was variable (RS neuronal birthdays were reported in the previous paper, Mendelson: J. Comp. Neurol., 251:160-171 1986). All RS somata were initially observed along the ventral surface of the brain and were subsequently displaced dorsally. The displacement could be passive, due to growth of the ventral hindbrain, and could explain the previously observed dorsoventral differences in RS neuronal time of origin.