Three-dimensional spatial distribution of synapses in the neocortex: A dual-beam electron microscopy study. Cereb Cortex 2013 Jan 30. [Epub ahead of print] PMID

In the cerebral cortex, most synapses are found in the neuropil, but relatively little is known about their 3-dimensional organization. Using an automated dual-beam electron microscope that combines focused ion beam milling and scanning electron microscopy, we have been able to obtain 10 three-dimensional samples with an average volume of 180 µm3 from the neuropil of layer III of the young rat somatosen-sory cortex (hindlimb representation). We have used specific soft-ware tools to fully reconstruct 1695 synaptic junctions present in these samples and to accurately quantify the number of synapses per unit volume. These tools also allowed us to determine synapse pos-ition and to analyze their spatial distribution using spatial statistical methods. Our results indicate that the distribution of synaptic junc-tions in the neuropil is nearly random, only constrained by the fact that synapses cannot overlap in space. A theoretical model based on random sequential absorption, which closely reproduces the actual distribution of synapses, is also presented