Mechanism for the formation of synaptic projections in the arthropod visual system
Mechanism for the formation of synaptic projections in the arthropod visual system
复制标题
节肢动物视觉系统突触投射的形成机制
作者：
E. Macagno
节肢动物的感光器轴突以精确有序的方式从视网膜投射到椎板。由于视网膜和层板由结构单元的二维阵列（分别是小眼和墨盒）组成，因此可以为每个轴突唯一地定义特定的起源单元和特定的目标单元。在许多物种中，例如水蚤1、螃蟹2和蝗虫3，所有源自小眼的轴突都终止于同一个盒中，在那里它们在层状神经元上形成突触。当网状轴突在胚胎发生过程中长成层状原基时，如何选择特定的靶盒？成人的投射模式可能以多种方式出现。例如，每个小眼和每个盒可以具有随其在各自域中的位置而变化或根据其在各自域中的位置而变化的独特化学标记。然后可以通过化学亲和机制来匹配输入和目标，例如 Sperry4 最初提出的用于形成脊椎动物视网膜顶盖连接的机制。第二种可能性是网状纤维不识别层状细胞中的任何特定标签，而是以适当的时空顺序生长到该区域，以顺序填充预形成的层状阵列中的可用位点。在第二种机制的变体中，在网状轴突到达时，层板将是非结构化的，并且轴突将通过顺序募集未成熟的层状神经元以形成规则的盒阵列而在层板上施加图案。这些可能性已经通过在大型溞（一种小型鳃足甲壳类动物，共有 22 个小眼和 22 个墨盒）特定发育阶段删除小眼进行了测试。这里报告的观察结果支持最后一个假设，即网状轴突向内生长的顺序决定了该生物体中的视网膜层状投影。
THE axons of photoreceptors in arthropods project in a precisely ordered manner from the retina to the lamina. As retina and lamina consist of two-dimensional arrays of structural units (the ommatidia and the cartridges, respectively), a particular unit of origin and a specific target unit can be defined uniquely for each axon. In a number of species, such as water fleas1, crabs2 and locusts3, all axons originating from an ommatidium terminate in the same cartridge, where they form synapses onto laminar neurones. How is a particular target cartridge selected by the retinular axons as they grow into the laminar anlage during embryogenesis? The adult pattern of projections could arise in several ways. For example, each ommatidium and each cartridge could have unique chemical labels varying with or according to their positions in their respective domains. Input and target could then be matched by a chemoaffinity mechanism such as that originally proposed by Sperry4 for the formation of vertebrate retinotectal connections. A second possibility is that retinular fibres do not recognise any specific labels in the laminar cells, but grow into this region in the appropriate spatio–temporal order to sequentially fill available sites in a pre-formed laminar array. In a variation of this second mechanism the lamina would be unstructured at the time of arrival of retinular axons and the axons would impose a pattern on the lamina by sequentially recruiting immature laminar neurones to form the regular array of cartridges. These possibilities have been tested by deleting ommatidia at specified developmental stages in Daphnia magna, a small branchiopod crustacean with a total of 22 ommatidia and 22 cartridges1. The observations reported here support the last hypothesis, that the order of retinular axon ingrowth determines the retino–laminar projections in this organism.