Fig. 4.

Drosophila neuropile ultrastructure. A-C: Types of neurite profiles. A shows 3D digital model of several short neurites segmented from one micro-volume. B: representative EM section of microvolume in which profiles of neurites modeled in A are shaded in the corresponding colors. C: Schematic depiction of types of neurites. Axiform neurites (light blue) are straight, unbranched processes of intermediate (0.2–0.4µm) diameter. Globular neurites and varicose neurites (dark blue and green) have alternating segments of intermediate (0.2–0.4µm) and large diameter (0.5–1.5µm for varicose; 1–3µm for globular neurites). Dendritiform neurites (yellow, brown, orange) are highly branched and thin (< 0.2µm). D: Section of two typical polyadic synapses. Green arrow points at presynaptic specialization, consisting of the T-bar and synaptic vesicles. Presynapses are contacted by multiple, thin branches of dendritiform processes. Numbers 1–5 in upper panel denote profiles of thin, postsynaptic profiles (dendritiform neurites). #2–5 have clear, direct contact to presynaptic zone; #1 is located right adjacent to the presynaptic site, and represents a case that may or may not represent a postsynaptic neurite of this synapse. E: Correlation between frequency of presynaptic and postsynaptic sites and neurite diameter. Presynaptic sites (blue; top) are predominantly found on large diameter profiles, which correspond to thick segments of varicose and globular neurites. These neurites represent terminal axonal branches. Postsynaptic profiles (yellow, bottom) almost exclusively belong to thin dendritiform neurites; they represent terminal dendritic branches. F-H: Distribution of axonal and dendritic branches. F: Polarized vertebrate neuron with dendrite/soma compartment carrying postsynaptic sites, and axon compartment with presynaptic sites. G: Drosophila type PD neuron which comes close to the vertebrate pattern, with postsynaptic dendritiform processes concentrated proximally and presynaptic varicose/globular processes distally. H: In Drosophila type C and D neurons, terminal axons and dendrites are intermingled.

Bars: 0.5µm (B); 0.2µm (D)