Electron microscopy (EM)-based synaptology is a fundamental discipline for achieving a complex wiring diagram of the brain. A quantitative understanding of synaptic ultrastructure also serves as a basis to estimate the relative magnitude of synaptic transmission across individual circuits in the brain. Although conventional light microscopic techniques have substantially contributed to our ever-increasing understanding of the morphological characteristics of the putative synaptic junctions, EM is the gold standard for systematic visualization of the synaptic morphology. Furthermore, a complete three-dimensional reconstruction of an individual synaptic profile is required for the precise quantitation of different parameters that shape synaptic transmission. While volumetric imaging of synapses can be routinely obtained from the transmission EM (TEM) imaging of ultrathin sections, it requires an unimaginable amount of effort and time to reconstruct very long segments of dendrites and their spines from the serial section TEM images. The challenges of low throughput EM imaging has been addressed to an appreciable degree by the development of automated EM imaging tools that allow imaging and reconstruction of dendritic segments in a realistic time-frame. Here, we review studies that have been instrumental in determining the three-dimensional ultrastructure of synapses. With a particular focus on dendritic spine synapses in the rodent brain, we discuss various key studies that have highlighted the structural diversity of spines, the principles of their organization in the dendrites, their presynaptic wiring patterns, and their activity-dependent structural remodeling.

中文翻译：

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体积电子显微镜技术揭示树突棘的三维结构

基于电子显微镜（EM）的突触学是实现大脑复杂接线图的基础学科。对突触超微结构的定量理解也可作为估算跨大脑单个回路的突触传递相对幅度的基础。尽管常规的光学显微镜技术对我们不断增长的对假定的突触连接处形态特征的理解作出了巨大贡献，但是EM是系统可视化突触形态的金标准。此外，为精确定量塑造突触传递的不同参数，需要对单个突触轮廓进行完整的三维重建。虽然可以通过超薄切片的透射EM（TEM）成像常规获得突触的体积成像，但要从连续切片TEM图像重建非常长的树突及其刺节，则需要大量的精力和时间。低通量EM成像的挑战已通过开发自动化EM成像工具得到了可观的解决，该工具可以在现实的时间范围内对树突节进行成像和重建。在这里，我们回顾了有助于确定突触的三维超微结构的研究。我们特别关注啮齿动物大脑中的树突棘突触，我们讨论了各种关键研究，这些研究突显了棘突的结构多样性，它们在树突中的组织原理，