CHEMOREPULSION OF DEVELOPING MOTOR AXONS BY THE FLOOR PLATE

In the developing nervous system, motor axons grow away from the ventr al midline floor plate, suggesting that the latter might be a source o f repulsive axonal guidance cues. In donor to host transplantation exp eriments, ectopic pieces of floor plate were positioned between chick hindbrain motor neurons and their exit points. Immunohistochemistry an d retrograde axonal labeling techniques demonstrated that motor axons diverted from their normal pathways to avoid grafted floor plate, ofte n traversing abnormally long circuitous trajectories to reach exit poi nts. When ventral explants of rat hindbrain and spinal cord were cocul tured at a distance from floor plate explants within collagen gel matr ices, the outgrowth of motor axons was dramatically reduced from expla nt borders that faced the floor plate. Thus, the floor plate secretes diffusible repulsive cues in vitro that may exclude motor axons from t he midline during development.