Development of early brainstem projections to the tail spinal cord of Xenopus.

Horseradish peroxidase (HRP) was used to determine the sequence in which axons from different brain neurons reach the tail spinal cord during embryonic and early larval development of Xenopus laevis. Brainstem cells of several classes project to the tail at these stages: mesencephalic reticulospinal neurons of the nucleus of the medial longitudinal fasciculus, a variety of other reticulospinal neurons, vestibulospinal neurons, and a group of median basal cells which may be raphe neurons. Among the reticulospinal neurons the paired Mauthner cells are the most prominent. They and caudally situated reticular neurons are the first to label with HRP applied to the tail spinal cord (stage 37). Vestibulospinal and other reticular neurons begin to label next (stage 39), followed by mesencephalic and then median basal neurons (stage 41). Except for the Mauthner cells, the number of labeled cells belonging to each neuron class increases gradually as development proceeds.