Análisis de las sinapsis en regiones asociativas y primarias de la corteza cerebral humana mediante microscopía electrónica volumétrica

Abstract

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Medicina, Departamento de Anatomía, Histología y Neurociencia. Fecha de Lectura: 23-10-2024Esta tesis tiene embargado el acceso al texto completo hasta el 23-04-2026Functional and structural studies investigating large-scale connections in the human cerebral cortex suggest that higher-order associative cortical regions have more connections compared to primary regions. However, the way these regions are organized at the ultrastructural level is currently not well understood. Therefore, this doctoral thesis used a combination of light and three-dimensional electron microscopy techniques to investigate the synaptic organization of the human brain. First, we examined whether biopsy and autopsy human brain tissue samples were comparable for studying the organization of synapses, focusing on the neuropil of layer III of Brodmann area 21, using different methods to quantify the number of synapses. The next step was to investigate how synaptic organization differs between functionally and structurally distinct brain regions. To achieve this, associative Brodmann areas (21, 24, ventral and dorsal 38) and primary Brodmann areas (17, 3b, and 4), were analyzed from several individuals. This approach also allowed the variability of synaptic properties across individuals to be assessed. To achieve these goals, volume electron microscopy (focused ion beam/scanning electron microscopy) was used. This analysis involved examining 74 stacks of images, representing a total brain tissue volume of 35,481 μm3 of neuropil. Through this process, 13,903 synaptic junctions were reconstructed. Our analysis revealed no significant differences in the number of synapses quantified using different methods (stereological versus three-dimensional reconstructions) when applied to the same samples, as long as a relatively large number of images were analyzed. Additionally, we found that both biopsy and autopsy brain tissue sources are comparable when examining the synaptic characteristics. Furthermore, by comparing different brain regions, it was observed that some features of synaptic organization —such as the number of synapses per volume, the proportion of synapses according to the postsynaptic target, and the size of excitatory synapses— can vary depending on the region. Notably, associative regions seemed to have more synaptic connections compared to primary cortical regions. Interestingly, other aspects of the synaptic organization, like the proportion of excitatory and inhibitory synapses, their shapes, their spatial distribution, and a higher proportion of synapses located on dendritic spines, appeared to be consistent across all regions studied. These common features might represent fundamental principles of how synapses are organized in the human brain. Finally, the variability between individuals was linked to specific synaptic characteristics in certain cortical regions. This doctoral thesis sheds new light on the intricate connectivity and organization of the human cerebral cortex, paving the way for further advances in our understanding of the brainThis study was funded by grants from the spanish “Ministerio de Ciencia e Innovación” (grant PID2021-127924NB-I00 funded by MCIN/AEI/10.13039/501100011033), and CSIC Interdisciplinary Thematic Platform—Cajal Blue Brain. Research Fellowship (PRE2019- 089228) funded by MCIN/AEI/10.13039/501100011033 for Nicolás Cano Astorg