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Mishchenko Y, Hu T, Spacek J, Mendenhall J, Harris KM, Chklovskii DB. Ultrastructural analysis of hippocampal neuropil from the connectomics perspective. Neuron. 2010 Sep 23;67(6):1009-20. doi: 10.1016/j.neuron.2010.08.014.
SUMMARY
Complete reconstructions of vertebrate neuronal
circuits on the synaptic level require new approaches.
Here, serial section transmission electron
microscopy was automated to densely reconstruct
four volumes, totaling 670 mm3
, from the rat hippocampus as proving grounds to determine when
axo-dendritic proximities predict synapses. First, in
contrast with Peters’ rule, the density of axons within
reach of dendritic spines did not predict synaptic
density along dendrites because the fraction of
axons making synapses was variable. Second, an
axo-dendritic touch did not predict a synapse;
nevertheless, the density of synapses along a hippocampal
dendrite appeared to be a universal fraction, 0.2, of
the density of touches. Finally, the largest touch
between an axonal bouton and spine indicated the
site of actual synapses with about 80% precision
but would miss about half of all synapses. Thus, it
will be difficult to predict synaptic connectivity using
data sets missing ultrastructural details that distinguish
between axo-dendritic touches and bona fide
synapses.
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