3D Ultrastructural Study of Synapses in the Human Entorhinal Cortex

Abstract

The entorhinal cortex (EC) is a brain region that has been shown to be essential for memory functions and spatial navigation. However, detailed three-dimensional (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 to perform a 3D 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.

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

“…Moreover, the most frequent combinations were axodendritic AS (established on dendritic shafts) and axospinous AS (on dendritic spine heads). These findings were similar to the results of our previous analysis in control cases in the same EC layers (Domínguez-Álvaro et al ., 2020) and in layer II of the TEC (Domínguez-Álvaro et al ., 2019). However, the analysis of postsynaptic targets in AD samples in layer II of the TEC revealed a lower number of excitatory synapses on dendritic spine heads (Domínguez-Álvaro et al ., 2019).…”

“…Data regarding the synaptic organization of the human layers II and III from the EC in the four control subjects has been previously published and detailed information can be found therein (Domínguez-Álvaro et al ., 2020). What follows are the alterations of synapses in AD cases in comparison with control cases.…”

“…As previously described in control EC (Domínguez-Álvaro et al ., 2020), the most abundant type of synapse was AS on dendritic shafts (range 45–49%), closely followed by AS on spine heads (39–44%). SS on dendritic shafts were less common (6–9%), SS on spine heads were very uncommon (1%), and AS or SS on spine necks were very infrequent indeed (<1%; Fig.…”

“…In the present study, we performed 3D ultrastructural analysis of the EC using Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) and specific software that allows the segmentation of synapses in a reconstructed 3D volume (Morales et al, 2011). This technology greatly facilitates the analysis of possible alterations at the synaptic level in AD, as previously shown in the human transentorhinal cortex and the CA1 hippocampal region (Domínguez-Álvaro et al, 2019;Montero-Crespo et al, 2021). Our goal was to investigate the possible synaptic changes occurring in layers II and III of the EC related to AD, not only with regard to numbers, spatial distribution and types of synapses, but also regarding the morphological characteristics of each synapse (shape and size), as well as possible changes in their postsynaptic targets.…”

“…In total, 24 stacks of images of the neuropil from layer II and III of the EC from AD cases were obtained (three stacks for each of the 4 cases, with a total volume studied of 9,006 μm 3 , Table 1). The data obtained from these stacks of images were compared to the data obtained in the same regions of four control cases, previously described by Domínguez-Álvaro et al . (2020).…”