F-spondin is required for accurate pathfinding of commissural axons at thefloor plate

The commissural axons project toward and across the floor plate. They then turn into the longitudinal axis, extending along the contralateral side of the floor plate. F-spondin, a protein produced and secreted by the floor pl ate, promotes adhesion and neurite extension of commissural neurons in vitr o. Injection of purified F-spondin protein into the lumen of the spinal cor d of chicken embryos in ovo resulted in longitudinal turning of commissural axons before reaching the floor plate, whereas neutralizing antibody (Ab) injections caused lateral turning at the contralateral floor plate boundary . These combined in vitro and in vivo results suggest that F-spondin is req uired to prevent the lateral drifting of the commissural axons after having crossed the floor plate.