Bernhardt R R
Swiss Federal Institute of Technology, Hönggerberg, Zürich.
J Comp Neurol. 1994 Dec 1;350(1):122-32. doi: 10.1002/cne.903500109.
Early commissural axons in the zebrafish spinal cord extend along a pathway consisting of a ventrally directed ipsilateral, a contralateral diagonal, and a contralateral longitudinal segment. The midline floor plate cell is one important cue at the transition from the ipsilateral to the contralateral pathway segments. In order to identify additional guidance cues, the interactions between commissural growth cones and their substrates were examined at the electron microscopic level in the different pathway segments. The growth cones extended near the superficial margin of the spinal cord, within filopodial reach of three bilateral longitudinal axon pathways that were ignored irrespective of whether other axons were already present. Ultimately the commissural growth cones pioneered an additional independent longitudinal pathway in the dorsolateral spinal cord. Neuroepithelial cells were extensively contacted in the lateral marginal zone of the dorsal spinal cord and are thus in a position to contribute to the establishment of the longitudinal commissural pathway segment. The extent of contact with neuroepithelial cells in the ventral spinal cord was dependent on whether commissural growth cones had already crossed the ventral midline: ipsilateral, but not contralateral, growth cones showed extensive contacts with neuroepithelial processes and minor contacts with the basal lamina. In marked contrast, commissural growth cones that had already crossed the ventral midline and entered the diagonal pathway segment showed major appositions to the basal lamina. Extensive contact with the basal lamina was first established in the ventral midline region, where crossing growth cones always inserted between the basal lamina and the base of the midline floor plate cells. This indicates that a change occurs in the response characteristics of commissural growth cones as they cross the ventral midline of the spinal cord. Such a change could help to explain why the growth cones extend first toward but then away from the ventral midline.
斑马鱼脊髓中早期的连合轴突沿着一条由腹侧同侧、对侧对角和对侧纵向节段组成的路径延伸。中线底板细胞是同侧路径节段向对侧路径节段过渡时的一个重要线索。为了识别其他引导线索，在电子显微镜水平上研究了不同路径节段中连合生长锥与其底物之间的相互作用。生长锥在脊髓浅缘附近延伸，处于三条双侧纵向轴突路径的丝状伪足可及范围内，无论是否已有其他轴突存在，这些路径都被忽略。最终，连合生长锥在脊髓背外侧开创了一条额外的独立纵向路径。神经上皮细胞在脊髓背侧的外侧边缘区广泛接触，因此能够对纵向连合路径节段的建立做出贡献。脊髓腹侧与神经上皮细胞的接触程度取决于连合生长锥是否已经穿过腹侧中线：同侧而非对侧的生长锥与神经上皮突起有广泛接触，与基膜有少量接触。与之形成鲜明对比的是，已经穿过腹侧中线并进入对角路径节段的连合生长锥与基膜有主要的附着。与基膜的广泛接触首先在腹侧中线区域建立，穿过的生长锥总是插入基膜和中线底板细胞基部之间。这表明连合生长锥在穿过脊髓腹侧中线时其反应特性发生了变化。这样的变化有助于解释为什么生长锥首先朝着腹侧中线延伸，然后又远离腹侧中线。
