Biology subjects

Alonso-Nanclares, L.

Publications and source records attributed to Alonso-Nanclares, L..

9 recordsLinked to original sources

bioRxiv2024

Volume Electron Microscopy Analysis of Synapses in Associative and Primary Regions of the Human Cerebral Cortex

Functional and structural studies investigating macroscopic connectivity in the human cerebral cortex suggest that high-order associative regions exhibit greater connectivity compared to primary ones. However, the synaptic organization of these brain regions remains unexplored due to the difficulties involved in studying the human brain at the nanoscopic level. In the present work, we conducted volume electron microscopy to investigate the synaptic organization of the human brain obtained at autopsy. Specifically, we examined layer III of Brodmann areas 17, 3b, and 4, as representative areas of primary visual, somatosensorial, and motor cortex. Additionally, we conducted comparative analyses with our previous synaptic datasets of layer III from temporopolar and anterior cingulate associative cortical regions (Brodmann areas 24, 38, and 21). 9,690 synaptic junctions were 3D reconstructed, showing that certain synaptic characteristics appeared to be specific to particular cortical regions. The number of synapses per volume, the proportion of the postsynaptic targets, and the synaptic size may distinguish one region from another, regardless of whether they are associative or primary cortex. By contrast, other synaptic characteristics were common to all analyzed regions, such as the proportion of excitatory and inhibitory synapses, their shapes, their spatial distribution, and a higher proportion of synapses located on dendritic spines. These observations may be included within the general rules of synaptic organization of the human cerebral cortex. The present results on nanoscopic characteristics of synapses provide further insights into the structural design of the human cerebral cortex. Significance statementStructural and functional studies investigating macroscopic connectivity in the human cerebral cortex have suggested that high-order associative regions exhibit greater connectivity compared to primary ones. However, the synaptic organization of these brain regions remains unexplored. Here, thousands of synaptic junctions were 3D reconstructed in associative and primary cortical regions. We found that certain synaptic characteristics appeared to be specific to particular cortical regions --regardless of whether they are associative or primary cortex-- whereas others were common to all analyzed regions. The present results provide further insights into the structural design of the human cerebral cortex.

neuroscience↗

bioRxiv2023

Volume Electron Microscopy Reveals Unique Laminar Synaptic Characteristics in the Human Entorhinal Cortex

The entorhinal cortex (EC) plays a pivotal role in memory function and spatial navigation, connecting the hippocampus with the neocortex. The EC integrates a wide range of cortical and subcortical inputs, but its synaptic organization in the human brain is largely unknown. We used volume electron microscopy to perform a 3D analysis of the microanatomical features of synapses in all layers of the medial EC (MEC) from the human brain. Using this technology, 12,974 synapses were fully 3D reconstructed at the ultrastructural level. The MEC presented a distinct set of synaptic features, differentiating this region from other human cortical areas. Furthermore, ultrastructural synaptic characteristics within the MEC was predominantly similar, although layers I and VI exhibited several synaptic characteristics that were distinct from other layers. The present study constitutes an extensive description of the synaptic characteristics of the neuropil of all layers of the EC, a crucial step to better understand the connectivity of this cortical region, in both health and disease.

neuroscience↗

bioRxiv2023

3D Synaptic Organization of Layer III of the Human Anterior Cingulate and Temporopolar Cortex

The human anterior cingulate and temporopolar cortices have been proposed as highly connected nodes involved in high-order cognitive functions, but their synaptic organization is still basically unknown due to the difficulties involved in studying the human brain. Using Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) to study the synaptic organization of the human brain obtained with a short post-mortem delay allows excellent results to be obtained. We have used this technology to analyze the neuropil (where the vast majority of synapses are found) of layer III of the anterior cingulate cortex (Brodmanns area 24) and the temporopolar cortex, including the temporal pole (Brodmanns area 38 ventral and dorsal) and anterior middle temporal gyrus (Brodmanns area 21). Our results, based on 6695 synapses fully reconstructed in 3D, revealed that Brodmanns areas 24, 21 and ventral area 38 showed similar synaptic density and synaptic size, whereas dorsal area 38 displayed the highest synaptic density and the smallest synaptic size. However, the proportion of the different types of synapses (excitatory and inhibitory), the postsynaptic targets and the shapes of excitatory and inhibitory synapses were similar, regardless of the region examined. These observations indicate that certain aspects of the synaptic organization are rather homogeneous, whereas others show specific variations across cortical regions. Since not all data obtained in a given cortical region can be extrapolated to other cortical regions, further studies on the other cortical regions and layers are necessary to better understand the functional organization of the human cerebral cortex.

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bioRxiv2022

Cortical synapses of the world's smallest mammal: an FIB/SEM study in the Etruscan shrew

The main aim of the present study was to determine if synapses from the exceptionally small brain of the Etruscan shrew show any peculiarities compared to the much larger human brain. This study constitutes the first description of the Etruscan shrew synaptic characteristics using Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM). We analyzed the synaptic density and a variety of structural characteristics of 7,239 3D reconstructed synapses, obtaining the following major results: (i) cortical synaptic density was very high, particularly in layer I; (ii) the vast majority of synapses were excitatory, with the highest proportion found in layer I; (iii) excitatory synapses were larger than inhibitory synapses in all layers except in layer VI; and (iv) synapses were either randomly distributed in space or showed a slight tendency to be organised in a regular arrangement. Some of these general synaptic characteristics are remarkably similar to those found in the human cerebral cortex. However, the cortical volume of the human brain is about 50,000 times larger than the cortical volume of the Etruscan shrew, while the total number of cortical synapses in human is only 20,000 times the number of synapses in the shrew, and synaptic junctions are 35% smaller in the Etruscan shrew. Thus, the differences in the number and size of synapses cannot be attributed to a brain size scaling effect but rather to adaptations of synaptic circuits to particular functions. The present work provides a quantitative dataset from the Etruscan shrew synapses -- not only contributing to the knowledge of the ultrastructure of the mammalian cortex, but also identifying common and differing principles of synaptic organization.

neuroscience↗

bioRxiv2021

Three-dimensional synaptic organization of layer III of the human temporal neocortex

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 Brodmanns 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/m3, 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.

neuroscience↗

bioRxiv2020

3D analysis of the synaptic organization in the Entorhinal cortex in Alzheimer's disease

The entorhinal cortex (EC) is especially vulnerable in the early stages of Alzheimers disease (AD). In particular, cognitive deficits have been linked to alterations in the upper layers of EC. In the present report, we examined layers II and III from eight human brain autopsies (four subjects with no recorded neurological alterations and four AD cases). We used stereological methods to assess cortical atrophy of the EC, and possible changes in the volume occupied by different cortical elements (neuronal and glial cell bodies; blood vessels; and neuropil). We performed 3D ultrastructural analyses of synapses using Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) to examine possible alterations related to AD. At the light microscope level, we found a significantly lower volume fraction occupied by neuronal bodies in layer III and a higher volume fraction occupied by glial cell bodies in layer II in AD cases. At the ultrastructural level we observed that (i) there was a significantly lower synaptic density in both layers in AD cases; (ii) synapses were larger and more complex in layer II in AD cases; and (iii) there was a greater proportion of small and simple synapses in layer III in AD cases than in control individuals. These structural differences may play a role in the anatomical basis for the impairment of cognitive functions in AD. Significant StatementAnalysis of the synaptic characteristics provides critical data on synaptic organization. Using 3D electron microscopy, the present study shows the synaptic organization of the neuropil of the human entorhinal cortex (EC) at the ultrastructural level. The EC is especially vulnerable in the early stages of Alzheimers disease (AD). Our present results show structural differences that may contribute as anatomical basis for the impairment of cognitive functions in AD. Thus, these results may help to understand the relationship between alterations of the synaptic circuits and the cognitive deterioration in AD.

neuroscience↗

bioRxiv2020

3D ultrastructural study of synapses in the human entorhinal cortex

The entorhinal cortex (EC) is a brain region that has been shown to be essential for memory functions and spatial navigation. However, detailed 3D synaptic morphology analysis and identification of postsynaptic targets at the ultrastructural level have not been performed before in the human EC. In the present study, we used Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) to perform a three-dimensional analysis of the synapses in the neuropil of medial EC in layers II and III from human brain autopsies. Specifically, we studied synaptic structural parameters of 3561 synapses, which were fully reconstructed in 3D. We analyzed the synaptic density, 3D spatial distribution, and type (excitatory and inhibitory), as well as the shape and size of each synaptic junction. Moreover, the postsynaptic targets of synapses could be clearly determined. The present work constitutes a detailed description of the synaptic organization of the human EC, which is a necessary step to better understand the functional organization of this region in both health and disease. Significance StatementThe present study represents the first attempt to unveil the detailed synaptic organization of the neuropil of the human entorhinal cortex -- a brain region that is essential for memory function and spatial navigation. Using 3D electron microscopy, we have characterized the synaptic morphology and identified the postsynaptic targets of thousands of synapses. The results provide a new, large, quantitative ultrastructure dataset of the synaptic organization of the human entorhinal cortex. These data provide critical information to better understand synaptic functionality in the human brain. HighlightEstimation of the number of synapses, as well as determination of their type, shapes, sizes and postsynaptic targets, provides critical data to better understand synaptic functionality. This study provides a new, large, quantitative ultrastructure dataset of the synaptic organization of the human entorhinal cortex using 3D electron microscopy.

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bioRxiv2020

Three-dimensional synaptic organization of the human hippocampal CA1 field

The hippocampal CA1 field integrates a wide variety of subcortical and cortical inputs, but its synaptic organization in humans is still unknown due to the difficulties involved studying the human brain via electron microscope techniques. However, we have shown that the 3D reconstruction method using Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) can be applied to study in detail the synaptic organization of the human brain obtained from autopsies, yielding excellent results. Using this technology, 24,752 synapses were fully reconstructed in CA1, revealing that most of them were excitatory, targeting dendritic spines and displaying a macular shape, regardless of the layer examined. However, remarkable differences were observed between layers. These data constitute the first extensive description of the synaptic organization of the neuropil of the human CA1 region.

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bioRxiv2019

Area-specific synapse structure in branched axons reveals a subcellular level of complexity in thalamocortical networks

Thalamocortical Posterior nucleus (Po) axons innervating the somatosensory (S1) and motor (MC) vibrissal cortices are key links in the brain neuronal network that allows rodents to explore the environment whisking with their motile vibrissae. Here, using high-end 3D electron microscopy, we demonstrate massive differences between MC vs. S1 Po synapses in a) bouton and active zone size; b) neurotransmitter vesicle pool size; c) mitochondria distribution near synapses; and d) proportion of non-spinous dendrite contacts. These differences are as large, or bigger, than those between Po and ventroposterior thalamic nucleus synapses in S1. Moreover, using single-axon transfection labeling, we show that the structure of boutons in the MC vs. S1 branches of individual Po axons is different. These structural differences parallel striking, recently-discovered divergences in functional efficacy and plasticity between S1 and MC Po synapses, and overall reveal a new, subcellular level of thalamocortical circuit complexity, unaccounted for in current models.

neuroscience↗