Fig. 6. Postcommissural axons do not re-enter the spinal cord gray matter. Transverse vibratome sections taken from E5.5 chick (A) and E13 mouse (B) embryos were labeled with anti-L1 antibodies to reveal the position of postcommissural axons within the M/L axis of the spinal cord. In both chick and mouse embryos, decussated commissural axons are excluded from the gray matter and are confined to fiber tracts within the marginal zone of the spinal cord (indicated by red arrowheads in A,B). (C) The contralateral trajectory of commissural axons in a schematic representation of a spinal cord cross-section. After exiting the floor plate, most commissural axons navigate along a complex pathway within the spinal cord white matter and assemble into an ascending tract within the dorsal (chick) or intermediate (mouse) marginal zone. In C, an axon is shown exiting the marginal zone (indicated by red arrow) to emphasize the observation that crossed commissural axons do not re-enter the gray matter. c, caudal; d, dorsal; fp, floor plate; r, rostral; v, ventral.