Age-Related Morphological Changes of Cortical Thin Unmyelinated Axons in Mice Pubblico
Glodener, Daniel (Spring 2019)
Abstract
Cognition undergoes radical age-dependent change throughout life. Behind those cognitive changes are morphological ones. The brain loses weight, grey matter shrinks, and some populations of neurons decrease in number (Svennerholm 1997; Terribilli 2011; Terry 1987). In parallel, neurons change with age in the cortex, with lower firing rates in hippocampal neurons and neuronal pathology mirroring aging cells throughout the body (Geinisman 1986; Phillip 2017). The structures of hippocampal and neocortical neurons have been thoroughly studied, but data regarding features of the longest and most abundant component, the axon, remains sparse. The thin axons that project from these cells and comprise most of grey matter have a typical structure. Defining features of these typical unmyelinated axons (TUAs) are thin unmyelinated shafts, en passant boutons down the length of the shaft, and excitatory synapses with dendritic spines. The organization of these features, such as bouton spacing, influences the arrangement of neuronal connectivity. Although patterns of TUA structural organization have been observed they are poorly understood, and very little is known about the changes in these patterns. We hypothesized that normal aging would correlate with changes in commonly observed morphological features, including inter-bouton interval (IBI), the spatial distribution of boutons, and bouton size. We used fluorescent microscopy with DiI labelling to image TUAs in mice and measure these variables of morphology. Here we show that morphological change can be predictable in TUAs, depending on the location of the axon. Axons were similar in mean IBI and spatial variability but differed in bouton size. Hippocampal axons decreased in IBI and increased in bouton size with age, while neocortical axons showed no significant morphological change with age. We also explore various labelling methods to determine their advantages and disadvantages.
Table of Contents
Introduction……………………………………………...…………………………1
Methods and Materials…………………………………………………………......6
Animals……………………………………………………………………...6
Sample Extraction and Preparation………………………………………….6
Slicing…………………………………………………………………....…..7
Dyeing……………………………………………………………………….7
Mounting…………………………………………………………………….8
Imaging………………………………………………………………………9
Image Processing…………………………………………………………….9
Results………………………………………………………………………….....10
Axon IBI, CV, and Bouton Size…………………………………....………10
Preservation Methods and Microscope Choice.............................................11
Discussion................................................................................................................12
Significant Change in IBI, CV, and Bouton Size of Hippocampal Axons...13
Lack of Morphological Change with Age in the Neocortex.........................18
Methodological Considerations.....................................................................19
Future Directions...........................................................................................22
Graphs and Figures..................................................................................................23
References...............................................................................................................34
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