Three-Dimensional Structure of Dendritic Spines Revealed by Volume Electron Microscopy Techniques

Abstract

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 have 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.

Scaffold modelling captures the structure-function-dynamics relationship in brain microcircuits

“…The number of pf -PC synapses may range up to ~100˙000 (Hoxha et al, 2016), many of which would be silent (Isope and Barbour, 2002). Based on spines density (Parajuli and Koike, 2021) and the total length of PC dendritic tree , the number of possible pf synapses was estimated to be 15˙000-20˙000, with a minimum of ~100 synapses needed to generate a simple spike (Walter and Khodakhah, 2006). Here, limited to our 200-μm pf length, fiber intersection identified 1˙500 pf synapses per PC.…”

An Optimized and Detailed Step-by-Step Protocol for the Analysis of Neuronal Morphology in Golgi-Stained Fetal Sheep Brain

“…These do not, however, offer the unique advantage of Golgi in staining the entire neuron structure including the axons, dendrites, and spines. Electron microscopy remains the gold standard for visualizing synaptic morphology by providing a complete 3D reconstruction of a synapse using immunogold labeling [31]. The challenges of overcoming low-throughput electron microscopy imaging has been addressed elsewhere [31], but here we describe an alternative method using conventional light microscopy for measuring neurite and spine morphology.…”

“…Electron microscopy remains the gold standard for visualizing synaptic morphology by providing a complete 3D reconstruction of a synapse using immunogold labeling [31]. The challenges of overcoming low-throughput electron microscopy imaging has been addressed elsewhere [31], but here we describe an alternative method using conventional light microscopy for measuring neurite and spine morphology. Further, great advances have been made in whole brain imaging, accelerating our ability to detect and quantify structural and functional connectivity in the human brain noninvasively as well as experimentally (including the organization of neural pathways and network architecture) [2].…”

Model simulations unveil the structure-function-dynamics relationship of the cerebellar cortical microcircuit

“…As a whole, the number of pf -PC synapses may range up to ~100˙000 35 , many of which would be silent 97 . Based on spines density 98 and the total length of a PC dendritic tree 18,20 , the number of possible pf synapses was estimated to be 15˙000-20˙000, with a minimum of ~100 synapses needed to generate a simple spike 36 . (ii) The number of aa-PC synapses is not known but there would be multiple synapses per aa-PC pair 36 .…”