FLOOR PLATE CHEMOATTRACTS CROSSED AXONS AND CHEMOREPELS UNCROSSED AXONS IN THE VERTEBRATE BRAIN

In the bilaterally symmetrical vertebrate CNS, all developing axons mu st choose between remaining on the same side of the midline or growing across it. The mechanism underlying this axonal pathfinding is, howev er, poorly understood. Here we demonstrate that the ventral midline fl oor plate (FP) chemorepels two types of ipsilaterally projecting axons , one from the alar plate and another from the basal plate in the mese ncephalon. We further demonstrate that the FP chemoattracts contralate rally projecting myelencephalic as well as metencephalic axons. The FP at all axial levels displayed both chemoattractive and chemorepellent activities, suggesting that FP chemoattraction and chemorepulsion may be at work throughout the neuraxis. Chemotropic guidance by the FP ma y therefore play a key role in the establishment of neuronal projectio n laterality.