PT  - JOURNAL ARTICLE
AU  - Nicolás Cano-Astorga
AU  - Javier DeFelipe
AU  - Lidia Alonso-Nanclares
TI  - Three-dimensional synaptic organization of layer III of the human temporal neocortex
AID  - 10.1101/2021.01.18.427131
DP  - 2021 Jan 01
TA  - bioRxiv
PG  - 2021.01.18.427131
4099  - http://biorxiv.org/content/early/2021/04/08/2021.01.18.427131.short
4100  - http://biorxiv.org/content/early/2021/04/08/2021.01.18.427131.full
AB  - In the present study, we have used Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) to perform a study of the synaptic organization of layer III of Brodmann’s area 21 in human tissue samples obtained from autopsies and biopsies. We analyzed the synaptic density, 3D spatial distribution, and type (asymmetric/symmetric), as well as the size and shape of each synaptic junction of 4945 synapses that were fully reconstructed in 3D. Significant differences in the mean synaptic density between autopsy and biopsy samples were found (0.49 and 0.66 synapses/μm3, respectively). However, in both types of samples (autopsy and biopsy), the asymmetric:symmetric ratio was similar (93:7) and most asymmetric synapses were established on dendritic spines (75%), while most symmetric synapses were established on dendritic shafts (85%). We also compared several electron microscopy methods and analysis tools to estimate the synaptic density in the same brain tissue. We have shown that FIB/SEM is much more reliable and robust than the majority of the other commonly used EM techniques. The present work constitutes a detailed description of the synaptic organization of cortical layer III. Further studies on the rest of the cortical layers are necessary to better understand the functional organization of this temporal cortical region.Competing Interest StatementThe authors have declared no competing interest.3Dthree-dimensionalASasymmetric synapsesCACornu Ammonis 1CFcounting frameCSRComplete Spatial RandomnessFIB/SEMfocused ion beam/scanning electron microscopyKSKolmogorov-SmirnovMWMann-WhitneyPBphosphate bufferPSDpostsynaptic densitySASsynaptic apposition surfaceSDstandard deviationSEMstandard error of the meanTEMtransmission electron microscopySSsymmetric synapses.