Our ability to repeatedly image identified neuronal processes such as axons and dendrites in vivo over time have allowed for the documentation of structural changes related to plasticity. These microarchitectural changes have profound impact, both qualitatively and quantitatively, on neuronal functions. In this thesis, we examined dendritic and axonal structural features through high-resolution imaging by means of correlative light and electron microscopy (CLEM).
Chapter I will introduce readers to the general concept of somatosensation (in mice) through the following topics: i) inputs and outputs of the Thalamus involved in somatosensation; ii) thalamocortical (TC) and cortico-cortical (CC) inputs to the primary somatosensory cortex (S1), part of this will direct readers to a book chapter: Thalamocortical Synapses in the upcoming book: “The Cerebral Cortex and Thalamus” ; and iii) how they fit into the general landscape in the superficial layer (L) 1 of S1. I will also cover a non-exhaustive list of techniques that have been essential in examining the players involved in synaptic transmission. Finally, I round off Chapter I by specifying the various aims of this thesis.
Chapter II presents a proof-of-concept CLEM workflow that was crucial in the characterization of a subtype of vaso-intestinal peptide positive (VIP+) interneurons (INs) with spiny dendrites. Here, we describe a subpopulation of VIP+ INs with dendrites of high spine densities extending to the superficial L1. This work led to a published manuscript (Georgiou et al., 2022).
Chapter III represents the core of my thesis. Here, I characterize both the TC input arising from the posterior medial complex (POm) of the thalamus as well as the CC inputs from the primary motor cortex (M1) that arborizes in the layer 1 of S1. First, we describe and compare the axonal varicosities of both populations as observed in LM. I meticulously traced axons and attributed detected varicosities with bouton probabilities. With the bouton probabilities, I was also able to describe the structural plasticity (e.g., bouton addition, elimination, potentiation, and depression) over subsequent imaging sessions. This structural index will allow future studies to investigate bouton turnover on targeted axonal population. Further, with the ultrastructural data, we describe the difference between the two axonal population both qualitatively and quantitatively.
In Chapter IV, we investigated the elusive serotonergic (SERT) fibers also known to arborise in the superficial L1. We found that SERT fibers have an unusually high degree of tortuosity and do not form “traditional” synapses. I also highlighted work from other collaborators to allow for meaningful comparisons with other neuromodulatory afferents, as well as TC and CC axons presented in the previous chapter.
In Chapter V, we round up the thesis and summarise the findings from Chapter II – IV. Here, we discussed how the work and findings from my thesis contribute towards the general understanding of the insofar inscrutable superficial L1 of the murine somatosensory cortex, and shed light on the interchange of axonal and dendritic morphology, axon targeting strategies and non-conventional neurotransmission.
