The adult organization and development of the rubrospinal tract. An experimental study using the orthograde transport of WGA-HRP in the North-American opossum.

We have employed the orthograde transport of wheat germ agglutinin conjugated to horseradish peroxidase to study the organization of rubrospinal connections in adult and pouch young opossums. Our results suggest that: in the adult opossum rubrospinal axons are distributed more widely than suggested by previous studies; rubrospinal projections are formed postnatally in the opossum, but much earlier than corticospinal connections; rubrospinal axons do not grow synchronously, as a massive bundle following a few leading axons, but by addition of axons over a protracted period of time; and the growth of rubral axons into the spinal gray matter follows a predictable rostral to caudal gradient as well as a proximal to distal one relative to the tract. Rubrospinal development is discussed in light of the growth of cerebellar and cortical axons into the red nucleus and the development of motor function.