Colamarino S A, Tessier-Lavigne M
Howard Hughes Medical Institute, Department of Anatomy, University of California, San Francisco 94143-0452, USA.
Annu Rev Neurosci. 1995;18:497-529. doi: 10.1146/annurev.ne.18.030195.002433.
The data reviewed above have implicated the floor plate in directing axonal growth towards the midline, in directing the behavior of axons at the midline, and finally, in directing the longitudinal growth of axons alongside the midline. In the case of growth to the midline, there is clear evidence for the existence of two types of cues that collaborate to direct growth--short-range cues that can direct axons along the edge of the spinal cord and a long-range chemoattractant secreted by the floor plate cells whose main function may be to direct later-extending commissural axons that must migrate through the complex environment of the developing motor column. Determining the precise contribution of these cues will require identifying them and perturbing them in vivo. The cues that direct growth along the edge are unknown; their identification in the spinal cord would be of quite general significance, since the growth of axons parallel to (but not in contact with) the pial surface is a widespread feature of early axon growth at all axial levels of the neural tube. A strong candidate for the long-range chemoattractant is netrin-1, a homologue of the UNC-6 protein of C. elegans and a distant relative of laminin, which is expressed by floor plate cells and which can both promote and orient commissural axon outgrowth. Netrin-1 may also influence growth of other populations of neurons that exhibit stereotyped behaviors near the ventral midline. Much less is known about the exact role of the floor plate in directing axon growth at the midline, though it is clearly required for accurate guidance. In the absence of the floor plate, a range of errors has been found, the most prominent of which are aberrant midline crossings and errors in longitudinal growth near the ventral midline. The severity of these errors varies with species, which could result from either the variable importance of the floor plate in the different species or the fact that, so far, quite different manipulations of the ventral midline region have been performed in different species. The most specific perturbation of the ventral midline occurs in the zebrafish cyc-1 mutant, where the selective loss of the floor plate leads to stereotyped misrouting events. Perhaps surprisingly, virtually all axons that grow to the midline turn longitudinally (although sometimes in the wrong direction).(ABSTRACT TRUNCATED AT 400 WORDS)
上述回顾的数据表明，底板在引导轴突向中线生长、引导轴突在中线处的行为以及最终引导轴突沿中线纵向生长方面发挥着作用。在向中线生长的情况下，有明确证据表明存在两种协同引导生长的线索——短程线索，可引导轴突沿着脊髓边缘生长；以及由底板细胞分泌的长程化学引诱剂，其主要功能可能是引导后来延伸的连合轴突，这些轴突必须在发育中的运动柱的复杂环境中迁移。确定这些线索的确切作用需要在体内识别并干扰它们。引导沿边缘生长的线索尚不清楚；在脊髓中识别它们具有相当普遍的意义，因为轴突平行于（但不接触）软膜表面生长是神经管所有轴向水平早期轴突生长的一个普遍特征。长程化学引诱剂的一个有力候选者是netrin-1，它是秀丽隐杆线虫UNC-6蛋白的同源物，也是层粘连蛋白的远亲，由底板细胞表达，既能促进连合轴突生长又能使其定向。Netrin-1也可能影响在腹侧中线附近表现出定型行为的其他神经元群体的生长。关于底板在引导轴突在中线处生长的确切作用，人们了解得要少得多，尽管它显然是精确引导所必需的。在没有底板的情况下，已发现一系列错误，其中最突出的是异常的中线交叉和腹侧中线附近纵向生长的错误。这些错误的严重程度因物种而异，这可能是由于底板在不同物种中的重要性不同，或者是由于到目前为止，在不同物种中对腹侧中线区域进行了相当不同的操作。腹侧中线最特异性的扰动发生在斑马鱼cyc-1突变体中，其中底板的选择性缺失导致定型的错误布线事件。也许令人惊讶的是，几乎所有生长到中线的轴突都会纵向转向（尽管有时方向错误）。（摘要截选至400词）
