Visual pathways from the compound eye of an insect relay to four neuropils, successively the lamina, medulla, lobula, and lobula plate in the underlying optic lobe. Among these neuropils, the medulla, lobula, and lobula plate are interconnected by the complex second optic chiasm, through which the anteroposterior axis undergoes an inversion between the medulla and lobula. Given their complex structure, the projection patterns through the second optic chiasm have so far lacked critical analysis. By densely reconstructing axon trajectories using a volumetric scanning electron microscopy (SEM) technique, we reveal the three-dimensional structure of the second optic chiasm of Drosophila melanogaster, which comprises interleaving bundles and sheets of axons insulated from each other by glial sheaths. These axon bundles invert their horizontal sequence in passing between the medulla and lobula. Axons connecting the medulla and lobula plate are also bundled together with them but do not decussate the sequence of their horizontal positions. They interleave with sheets of projection neuron axons between the lobula and lobula plate, which also lack decussations. We estimate that approximately 19,500 cells per hemisphere, about two thirds of the optic lobe neurons, contribute to the second chiasm, most being Tm cells, with an estimated additional 2,780 T4 and T5 cells each. The chiasm mostly comprises axons and cell body fibers, but also a few synaptic elements. Based on our anatomical findings, we propose that a chiasmal structure between the neuropils is potentially advantageous for processing complex visual information in parallel. The EM reconstruction shows not only the structure of the chiasm in the adult brain, the previously unreported main topic of our study, but also suggest that the projection patterns of the neurons comprising the chiasm may be determined by the proliferation centers from which the neurons develop. Such a complex wiring pattern could, we suggest, only have arisen in several evolutionary steps.

中文翻译：

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果蝇眼裂的第二视交叉的组织。

从昆虫的复眼中继到四个神经柱的视觉路径，依次是底层视神经叶中的椎板，延髓，小叶和小叶板。在这些神经纤维中，延髓，小叶和小叶板通过复杂的第二视交叉连接在一起，后轴使髓后叶与小叶反转。鉴于其复杂的结构，到目前为止，通过第二次光学眼疾的投影模式尚缺乏严格的分析。通过使用体积扫描电子显微镜（SEM）技术密集地重建轴突轨迹，我们揭示了果蝇第二视神经chi的三维结构，该结构包括交错的束和通过胶质鞘相互绝缘的轴突片。这些轴突束在髓质和小叶之间通过时颠倒了其水平顺序。连接髓质和小叶板的轴突也与它们捆绑在一起，但不讨论其水平位置的顺序。它们与小叶和小叶板之间的投射神经元轴突片交织，后者也没有赘肉。我们估计每个半球大约19,500个细胞，约占视神经元的三分之二，有助于第二个正向性交叉症，大多数是Tm细胞，每个估计还有2,780个T4和T5细胞。胫骨主要包括轴突和细胞体纤维，但也包含一些突触元件。根据我们的解剖学发现，我们建议神经纤维之间的正向结构对于并行处理复杂的视觉信息可能具有优势。EM重建不仅显示了成年大脑中前交叉的结构，这是我们研究的先前未报道的主要话题，而且还表明，构成前交叉的神经元的投射模式可能取决于神经元从中发育的增殖中心。 。我们建议，这种复杂的布线方式只能在几个演进步骤中出现。