The entorhinal cortex is the main gateway between the hippocampal formation and the
neocortex. The entorhinal cortex mainly projects to the hippocampal formation via layers II
and III. While the various functions of entorhinal cortex layer II has been extensively studied
in recent years, the connectivity of entorhinal cortex layer III has remained relatively
unstudied. Here we seek to take advantage of novel transgenic tools and viral tracers in order
to trace the up‐ and downstream connections of layer III in the medial entorhinal cortex. The
combination of methods we use enable us to target a group of cells with unprecedented
specificity, allowing accurate identification of local and global projections.
Previous work has shown that layer III of the entorhinal cortex projects to CA1 and the
subiculum, both ipsilaterally and contralaterally, while the main inputs to layer III are the
presubiculum, postrhinal cortex and layer V of the entorhinal cortex. Using cell type specific
tracing methods, I find largely similar afferent and efferent connections, but for the first time
with layer specificity in mice. I also find that a recently described input from CA2 to layer II of
the medial entorhinal cortex could also project to layer III. Additionally, I find support for the
more recently described parasubicular input to layer III of the medial entorhinal cortex. Finally,
these data show a larger input to MEC LIII from the CA1 than what was expected from
traditional tracing methods. These novel findings may be the result of technical artifacts,
species differences or previously under‐reported connections. Overall, this thesis shows the
structural connectivity to and from MEC LIII in the mouse, and could lead to future functional
connectivity studies in the area.