Wizenmann A, Thanos S, von Boxberg Y, Bonhoeffer F
MPI für Entwicklungsbiologie, Tübingen, Germany.
Development. 1993 Feb;117(2):725-35. doi: 10.1242/dev.117.2.725.
In the rat, a small subpopulation of retinal ganglion cell axons forms a persistent projection to the ipsilateral half of the brain. These fibres originate almost exclusively from the ventrotemporal margin of the retina. In contrast to all other retinal axons they seem to be deflected from the midline of the optic chiasm and thereby led into the ipsilateral optic tract. In order to analyse the interactions between growing fibres and chiasm midline, we have developed the following in vitro model. Axons of the embryonic rat retina are grown on a carpet of tectal cell membranes used as a general growth-permissive substratum. At a certain distance from the explant (200-450 microns), the advancing fibres are confronted with two stripes of cell membranes prepared from the chiasm midline. Such chiasm membranes are shown to act as a barrier for the presumptive non-crossing axons, while they do not influence growth of fibres originating from any other regions of the retina, including the dorsotemporal part. The repulsion of non-crossing fibres by chiasm membranes is observed in vitro only when retinal explants from embryonic day (E) 17/18 and chiasm preparations from E14/15 are used. Fibres and tissue from different regions of the brain as well as from different developmental ages, and even from different species, can be combined in this assay system. In a first attempt to characterize the molecular basis of the repulsive effect of chiasm membranes on ventrotemporal fibres, similar assays were performed with membranes derived from other regions of the central nervous system midline, some of which are known to have repulsive properties against certain axon populations. Since these cell membranes did not act as a barrier for the ventrotemporal retinal axons, we suggest that the guidance cues at the chiasm are very specific. Our results are consistent with the hypothesis that certain cells at the chiasm midline (very likely radial glial cells) express 'repulsive or inhibitory' molecules, which act in a specific way on ipsilaterally projecting axons.
在大鼠中，一小部分视网膜神经节细胞轴突会形成一条持续投射至同侧脑半球的通路。这些纤维几乎完全起源于视网膜的颞腹侧边缘。与所有其他视网膜轴突不同，它们似乎会从视交叉中线处转向，从而进入同侧视束。为了分析生长中的纤维与视交叉中线之间的相互作用，我们开发了以下体外模型。胚胎大鼠视网膜的轴突生长在一层用作通用生长许可基质的顶盖细胞膜上。在距外植体一定距离（200 - 450微米）处，前进的纤维会遇到两条由视交叉中线制备的细胞膜条带。结果表明，这种视交叉细胞膜对推测的不交叉轴突起到了屏障作用，而对源自视网膜其他区域（包括颞背侧部分）的纤维生长没有影响。仅当使用胚胎第17/18天的视网膜外植体和第14/15天的视交叉制剂时，才能在体外观察到视交叉细胞膜对不交叉纤维的排斥作用。在这个检测系统中，可以将来自大脑不同区域、不同发育年龄甚至不同物种的纤维和组织进行组合。在首次尝试表征视交叉细胞膜对颞腹侧纤维排斥作用的分子基础时，我们用源自中枢神经系统中线其他区域的细胞膜进行了类似检测，其中一些区域已知对某些轴突群体具有排斥特性。由于这些细胞膜对颞腹侧视网膜轴突没有起到屏障作用，我们认为视交叉处的导向线索非常特异。我们的结果与以下假设一致，即视交叉中线处的某些细胞（很可能是放射状胶质细胞）表达“排斥性或抑制性”分子，这些分子以特定方式作用于同侧投射的轴突。
