The micron-scale structural organization of hippocampal area CA1 neuropil

Various structures in the brain contain many important clues to the brain’s development and function. Among these, the organization of neuropil tissue at micron scales is of particular importance since such organization has a direct potential to affect the formation of synaptic connectivity between nearby axons and dendrites, thus, serving as an important factor contributing to the brain’s development and disorders. While the organization of the brain at large and intermediate scales had been well studied, the microscopic organization of neuropil tissue remains largely unknown. In particular, presently it is not known what specific structures exist in neuropil at micron scales, what effect such structures have on synaptic connectivity, and what processes shape the neuropil’s organization at micron scales. The present work performs an analysis of recent complete electron microscopy reconstructions of blocks of hippocampal CA1 neuropil tissue in rat to produce answers to these questions. We use a novel statistical approach to analyze the small-scale organization of neuropil systematically and to show that such organization can be well understood in terms of disordered, random arrangement of axonal and dendritic processes without significant local order. We also discuss several deviations from this simple picture observed in the distributions of glia and dendritic spines. Finally, we examine the question of the relationships between local neuropil’s organization and synaptic connectivity.

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