The effects of aging on neuropil structure in mouse somatosensory cortex-A 3D electron microscopy analysis of layer 1

• Published in: PloS one 2018-07, Vol.13 (7), p.e0198131-e0198131
• Main Authors: Calì, Corrado, Wawrzyniak, Marta, Becker, Carlos, Maco, Bohumil, Cantoni, Marco, Jorstad, Anne, Nigro, Biagio, Grillo, Federico, De Paola, Vincenzo, Fua, Pascal, Knott, Graham William
• Format: Article
• Language: English
• Subjects: Age; Aging; Aging (Biology); Aging - pathology; Animal models; Animals; Axons; Axons - ultrastructure; Biology and Life Sciences; Brain; Cortex (somatosensory); Dendrites; Dendritic spines; Electron microscopy; Humans; Imaging, Three-Dimensional; Laboratories; Life sciences; Medicine and Health Sciences; Mice; Microscopy; Microscopy, Electron; Morphology; Nerves; Neural networks; Neural plasticity; Neurological research; Neurons; Neurons - pathology; Neurons - ultrastructure; Neuropil; Neuropil - pathology; Neuropil - ultrastructure; Neuroplasticity; Neurosciences; Physiological aspects; Rodents; Somatosensory cortex; Somatosensory Cortex - blood supply; Somatosensory Cortex - pathology; Somatosensory Cortex - ultrastructure; Spine; Synapses; Synapses - pathology; Synapses - ultrastructure; Ultrastructure
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0198131

This study has used dense reconstructions from serial EM images to compare the neuropil ultrastructure and connectivity of aged and adult mice. The analysis used models of axons, dendrites, and their synaptic connections, reconstructed from volumes of neuropil imaged in layer 1 of the somatosensory cortex. This shows the changes to neuropil structure that accompany a general loss of synapses in a well-defined brain region. The loss of excitatory synapses was balanced by an increase in their size such that the total amount of synaptic surface, per unit length of axon, and per unit volume of neuropil, stayed the same. There was also a greater reduction of inhibitory synapses than excitatory, particularly those found on dendritic spines, resulting in an increase in the excitatory/inhibitory balance. The close correlations, that exist in young and adult neurons, between spine volume, bouton volume, synaptic size, and docked vesicle numbers are all preserved during aging. These comparisons display features that indicate a reduced plasticity of cortical circuits, with fewer, more transient, connections, but nevertheless an enhancement of the remaining connectivity that compensates for a generalized synapse loss.