Pathway selection by embryonic chick motoneurons in an experimentally altered environment.

To characterize cues used by motoneuron axons to reach their appropriate targets, connectivity patterns within the embryonic chick hindlimb have been analysed after early experimental manipulations of the limb or spinal cord. The manipulations altered the anterior-posterior (a.-p.) relationship between motoneurons within the lumbosacral motor column and their specific targets in the limb. Primary emphasis was placed on analysing the pathways taken by embryonic motoneuron axons at stages 23-36 which had been orthogradely labelled by horseradish peroxidase (HRP) injection into the motor column. Motoneuron pool topography and functional patterns of connectivity were also identified by retrograde HRP labelling and spinal cord stimulation coupled with electromyographic recording. With small shifts in position, as in two or three segment a.-p. cord reversals or a.-p. limb shifts, motoneuron axons frequently entered the appropriate plexus but in an inappropriate spinal nerve sequence. Despite this, axons altered their course to innervate specifically and consistently their current target. When motoneuron axons entered an inappropriate plexus as the result of a greater positional shift (i.e. more extensive cord reversal or limb shift) or in experiments where posterior cord segments were replaced with anterior cord segments and supernumerary limbs were added, they behaved in one of two ways. They either formed inappropriate and largely unpatterned or unordered connections or they took totally aberrant paths within the limb to reach their appropriate target. We conclude that axons are capable of responding in a precise and specific manner to environmental cues when displaced up to a certain distance from their target or normal point of entry into the limb. Their failure to form patterned connections at more extreme distances suggests that the cues to which they are responding may be local, or that an axon's ability to respond to them is restricted to subclasses of the motoneuron population.