Pathway selection by chick lumbosacral motoneurons during normal development

Pathways taken by motoneuron axons from the lumbosacral lateral motor column to individual hindlimb muscles have been characterized throughout the normal period of outgrowth and the establishment of specific functional connections in the chick embryo. Axon pathways from individual cord segments were identified after injections of horseradish peroxidase (HRP) directly into the cord. Labelled motoneuron axons were then traced through the plexus and major nerve trunks to termination sites within the limb. At stages 23-24 labelled axons within spinal nerves have just reached the base of the limb and have begun to converge and form the crural and the ischiadic plexus. Even at this early stage, before periods of muscle cleavage, motoneuron cell death and muscle nerve formation, axons show no evidence of widespread random distribution within the limb. Rather, they generally maintain their anterior-posterior position as far as the base of the limb. At stages 27-30, although axons to individual muscles were found to course in discrete tracts within the plexus and nerve trunks they also changed their topographical position with respect to other axons. Axon pathways to single muscles were characterized by tracing retrogradely labelled axons back to the cord after injections of HRP into specific muscle nerves. Axons destined for a single muscle are intermingled with other axons in the spinal nerves and proximal plexus but by the distal plexus have converged to form a discrete tract which then diverges as an individual muscle nerve at more distal levels. These observations exclude models for the establishment of specific connections in which there is widespread testing of the environment with removal of projection errors by cell death and/or axon retraction. They also exclude models that require axons to maintain their topographical position with respect to each other throughout their course.