Ultrastructure and quantification of synapses in the insect nervous system

Standard EM methods can be successfully used to reveal the various organelles of synaptic junctions in different insect species. The individual junctions of a synaptic class exhibit a high level of morphological stereotypy, but the study of serial sections is generally necessary to understand the different appearances of a junction's profiles when it is cut in different planes. Most synaptic profiles seen in single sections may then be attributed to one or a few morphological classes, not to many. Probably most central synapses are of the multiple-contact type, containing a number of postsynaptic elements, with the diversity of the combinations of these providing the major difference between particular synaptic junctions. The different profiles of a synapse when cut serially in oblique, non-canonical section planes provide the investigator with search images, prior knowledge of which is needed for a comprehensive identification of synaptic sites in single sections. The latter can be used to describe the synaptic organization of an unknown neuropile from the variety of synaptic contacts that form between different neurons. This requires that continuity be established between a postsynaptic dendrite and its parent axon, and that the position of the axon can then be used to identify the neuron of origin. Tracing between dendrite and axon can be undertaken either systematically in serial sections of a restricted region or by protracted searches of single sections. The number of synaptic profiles in a single section can be used to estimate the number of synaptic contacts, either in relative terms, as the number of profiles per section in different cells, or as the absolute number of synapses per cell. The latter requires use of correction formulae, taking into account the influence of section thickness and of the mean size of the synaptic junction on the number of synaptic profiles recorded in a particular section.