Developing descending neurons of the early Xenopus tail spinal cord in the caudal spinal cord of early Xenopus.

The enzyme horseradish peroxidase (HRP) was used to describe and identify neurons, axons of which initiate the earliest descending pathways of the tail spinal cord of Xenopus embryos and larvae. Spinal cords were pierced at different rostrocaudal levels with fine insect pins coated with HRP. The resulting pattern of cellular labeling indicated that primitive sensory (Rohon-Beard) axons were at the lead of developing descending tracts followed by axons of primary motor neurons. Axons of these two neuron types travel in widely separated fascicles located dorso- and ventro-laterally, respectively. Subsequently, axons of several morphologically distinct intersegmental interneurons establish several additional fascicles positioned dorsal to the descending motor axons. Descending supraspinal axons appear only later. The distinctive morphological characteristics of each of the early descending cell types are illustrated along with some stages in their early differentiation. These observations establish the temporal pattern by which new axons are added to descending pathways beginning with the simplest level of the amphibian spinal cord and determine the identity of neurons to which axons at early stages in this sequence belong.