An Update on the Model of Connectivity of the Hippocampal Formation (I): The Perforant Pathway to the Dentate Gyrus.

The hippocampus participates in crucial functions such as memory consolidation, spatial processing and emotional regulation that require diverse input from multiple cortical areas that is funneled through the upper layers of the entorhinal cortex (EC), mostly from layer II to the dentate gyrus (DG). Traditional models of the hippocampal formation described 200,000 EC layer II neurons projecting to 1 million granule cells (GCs) in the rat, rendering low divergence (1:5), with each EC neuron establishing about 18,000 synapses with GCs and each GC receiving about 4000 synapses from EC neurons. In this manuscript, we update this model of connectivity incorporating new features described in the last three decades that include updated populations of EC layer II neurons obtained with design-based stereology, a revised definition of EC layer II based on molecular criteria and selecting reelin expressing neurons as the only layer II neurons projecting to the hippocampus. The updated model shows ~80,000 neurons from EC layer II projecting to the DG, ~45,000 from the medial entorhinal cortex (MEC) and ~35,000 from the lateral entorhinal cortex (LEC) with high divergence of 1:20 and 1:30. We also show that EC layer II neurons may establish ~90,000-115,000 synapses on GCs, while GCs receive about 8000 synapses from EC layer II neurons. We estimate a ~25% redundancy in the connectivity, so each EC neuron may contact ~68,000-86,000 GCs and each GC would be contacted by ~3000 neurons from MEC and 3000 from LEC. In addition, we quantitatively assess a potential projection of mossy cells to the middle molecular layer described in mice, which could have an impact on GC inhibition. Overall, we produced a detailed, complete, and updated quantitative model of EC projections to the DG that reveals a much more divergent and richer projection than previously described, with implications for functional models (e.g., pattern separation) and more widely for building realistic hippocampal models or establishing comparisons across species.