Patterns and distribution of presynaptic and postsynaptic elements within serial electron microscopic reconstructions of neuronal arbors from the medicinal leech Hirudo verbana

Microscale connectomics involves the large‐scale acquisition of high‐resolution serial electron micrographs from which neuronal arbors can be reconstructed and synapses can be detected. In addition to connectivity information, these data sets are also rich with structural information, including vesicle types, number of postsynaptic partners at a given presynaptic site, and spatial distribution of synaptic inputs and outputs. This study uses serial block‐face scanning electron microscopy (EM) to collect two volumes of serial EM data from ganglia of the medicinal leech. For the first volume, we sampled a small fraction of the neuropil belonging to an adult ganglion. From this data set we measured the proportion of arbors that contained vesicles and the types of vesicles contained and developed criteria to identify synapses and to measure the number of apparent postsynaptic partners in apposition to presynaptic boutons. For the second data set, we sampled an entire juvenile ganglion, which included the somata and arbors of all the neurons. We used this data set to placd our findings from mature tissue in the context of fully reconstructed arbors and to explore the spatial distribution of synaptic inputs and outputs on these arbors. We observed that some neurons segregated their arbors into input only and mixed input/output zones, that other neurons contained exclusively mixed input/output zones, and that still others contained only input zones. These results provide the groundwork for future behavioral studies. J. Comp. Neurol. 524:3677–3695, 2016. © 2016 Wiley Periodicals, Inc.