Application of the radial distribution function for quantitative analysis of neuropil microstructure in stratum radiatum of CA1 region in hippocampus.

Various structures in the brain contain many important clues to the brain?s development and function. Among these, the microscopic organization of neural tissue is of particular interest since such organization has direct potential to affect the formation of local synaptic connectivity between axons and dendrites, serving as an important factor affecting the brain?s development. While the organization of the brain at large and intermediate scales had been well studied, the organization of neural tissue at micrometer scales remains largely unknown. In particular, at present it is not known what specific structures exist in neuropil at micrometer scales, what effect such structures have on formation of synaptic connectivity, and what processes shape the micrometer-scale organization of neuropil. In this work, we present an analysis of recent electron microscopy reconstructions of blocks of neuropil tissue from rat s. radiatum of hippocampal CA1 to provide insights into these questions. We propose a new statistical method for systematically analyzing the small-scale organization of neuropil based on an adaptation of the approach of radial distribution functions from statistical physics. Our results show that the micrometer-scale organization of hippocampal CA1 neuropil can be viewed as a disordered arrangement of axonal and dendritic processes without significant small-scale positional coordinations. We observe several deviations from this picture in the distributions of glia and dendritic spines. Finally, we study the relationship between local synaptic connectivity and the small-scale organization of neuropil.