Myelination and Synaptogenesis in Olfactory System White Matter Tracts

The olfactory forebrain is interconnected through two large white matter tracts:
the lateral olfactory tract (LOT) transfers information from the olfactory bulb to the
ipsilateral olfactory cortex while the anterior commissure (AC) interconnects the left and right olfactory cortices. Chapter 2 provides a developmental characterization of myelination and oligodendrocyte maturation within these two tracts. Results demonstrate that the LOT develops significantly earlier than the AC, exhibits key structural differences in axonal composition and myelination, and responds differently to sensory deprivation. Chapter 3 examines the susceptibility of the LOT and AC to experimental demyelination and whether they might have the capacity to remyelinate, as suggested by their proximity to the subventricular zone. Lysolecithin-induced demyelination and remyelination was examined at multiple post-injection times with electron microscopy. Significant demyelination was seen 7 days post-injection (dpi) in both tracts and evidence of remyelination was observed earlier in the LOT than AC. These findings indicate that the olfactory system could be an important model for studies of myelin regulation and suggest important differences in the capacity to recover from damage between the two tracts. Finally, Chapter 4 provides a detailed analysis of the terminations of axons coursing through the AC and synapsing in the contralateral AON. Axons were visualized by anterograde tracer injections into the AON and synapses were analyzed using electron microscopy. Results demonstrate that synapse formation within regions receiving input from the LOT and AC occurs prior to myelination of the tracts. Together, these studies indicate significant developmental and structural differences between the two tracts that have functional implications.