Development of reticulospinal neurons of the zebrafish

In many neuronal systems neurons that share similar features develop at similar times. I have studied identified reticulospinal neurons in the zebrafish (Brachydanio rerio) in order to learn how neuronal features including size, position, and axonal pathway are correlated with the time of cellular development. I hypothesized that time of development plays a role in the determination of these neuronal characteristics. The times of origin (birthdays) were determined by combining ('3)H-thymidine autoradiography with horseradish peroxidase (HRP) histochemistry. Individual reticulospinal neurons were born at distinct developmental times. Dorsally located cells always had earlier birthdays than ventral cells. Often, but not always, larger and more lateral neurons were born before smaller and more medial ones. There was no overall rostrocaudal gradient of neuronal generation and no overall correlation between time of origin and axonal pathway. The sequence of axonal outgrowth and the location of the cells were determined by filling the neurons retrogradely with HRP from a lesion of their growing axons. The lesions were located in the spinal cord and were made at a series of developmental stages. Individual neurons grew axons into the rostral spinal cord in a similar temporal sequence to that of their birthdays. Hindbrain cells developed in two distinct waves that were separated by about 10 hours. Some axons grew across the midline and others did not at nearly the same times and from origins at nearly the same locations in the brain. All somata were initially observed along the ventral surface of the brain and were subsequently displaced dorsally. The displacement could be a passive one, by growth of the ventral hindbrain, which would explain the observed dorsoventral differences in time of origin. Differences in the times of development do not always correlate with differences in axonal pathway, cell size, mediolateral or rostrocaudal position. However, the time when a reticulospinal cell develops may well determine its dorsoventral position in the brain.