Endocrine Abstracts

Synapses are the key elements for signal transduction and plasticity in the brain. Despite a relatively large number of assumptions that have been made on the structure of mature cortical synapses in many other animal species, little is known about these structures in human. Hence, synapses in cortical layer 5 (L5), the main recipient layer of thalamocortical efferents, which representing the first station of cortical information processing were investigated structurally and quantitatively using serial ultrathin sections and digital electron microscopic images (DEMI) followed by three dimensional (3D) reconstruction and immunohistochemistry against glutamine synthetase. The quantitative 3D-reconstructions of cortical synapses will allow to directly comparing structural and functional aspects of synaptic transmission and plasticity thus leading to a better understanding of the function of cortical networks in the human neocortex.

The structural parameters such as the size, number, and distribution of active zones as well as the size and organisation of the pools of synaptic vesicles were the most critical factors investigated not only for the induction but also for the maintenance of synaptic transmission and plasticity in the neocortex. The DEMI examination followed by 3D-reconstruction of cortical L5 showed clearly a large pyramidal neurons (85%); GABAergic interneurons and astrocytes, multiple innervations on either the same or different dendrites, multivesicular bodies (MVBs) and mitochondria (two to eight) in the pre-synaptic element and perforations exist in the pre and post-synaptic densities. Besides, in 85% of the spines have a spine apparatus and a specialized form of endoplasmic reticulum. For the quantitative geometry study, the results showed a total pool of synaptic vesicles of 1671.57±391.55 (ranging from 88 to 5841) with a mean diameter of 31.99±0.87 nm, and a dense-core vesicles of 66.2±11.27 nm, while, the volume of mitochondria and synaptic boutons and their surface were 0.05±0.01 μm³, 0.63±0.09 μm³, and 8.05±0.27 μm² respectively. We can conclude that the morphological parameters observed in this study will help to better understand the mechanisms underlying synaptic transmission and plasticity in the adult cortical synapses.