Fig. 1. Chick commissural axons follow a complex pathway on the contralateral side of the floor plate. (A) Small crystals of DiI were iontophoretically applied to dorsal spinal cord regions containing commissural neuron cell bodies in fixed open-book spinal cord preparations obtained from E4-E5.5 chick embryos. The microelectrode used to deliver the dye is shown in the lower left corner of the micrograph. (B) At E4, commissural axons anterogradely labeled with DiI extend across the floor plate (indicated by the white lines) in the transverse plane. At the contralateral floor plate margin, axons turn rostrally and extend for approximately 100 µm alongside the floor plate in the longitudinal plane. (C,D) Labeled axons extending on only the contralateral side of the spinal cord (indicated by the white bracket in A; dorsal is upwards and rostral is towards the right). (C) At E5, axons turn away from the floor plate (after extending for 100 µm adjacent to this structure) and project obliquely into more dorsal regions of the white matter. (D) At E5.5, commissural axons execute a final rostral turn and grow longitudinally within the dorsal marginal zone (the white arrowheads indicate decussated axons labeled by an adjacent DiI application). A small number of commissural axons are shown at the bottom of this panel traveling adjacent to the floor plate. The dorsal limit of the explant shown in this panel extends beyond the top edge of this micrograph. Scale bars: in A, 500 µm for A; in D, 50 µm for B-D. contra, contralateral; fp, floor plate; ipsi, ipsilateral.
