An Integrated Micro- and Macroarchitectural Analysis of the Drosophila Brain by Computer-Assisted Serial Section Electron Microscopy
(A–C) Generic types of neurites (globular/varicose, axiform, dendritiform) encountered in all regions of the neuropile. Upper panels of columns A, B, C show schematic representations of these neurites; lower panels show 3D digital models of representative neurite segments for corresponding classes, segmented from VNC microvolume (G1, globular neurite; V1, V2, varicose neurites; A1-3, axiform neurites; D1-3, dendritiform neurites). Elements are shown in lateral view; arrows indicate orientation (ant, anterior; dor, dorsal) relative to body axes. Red dots indicate position of presynaptic sites. (D) 3D digital model of all neurite segments shown in (A–C) as they are situated in VNC microvolume. Volume is shown in dorso-posterior view; arrows indicate axes (ant, anterior; dor, dorsal; lat, lateral). (E) Correlation between frequency of presynaptic and postsynaptic sites and neurite diameter. Presynaptic sites (blue; right) are predominantly found on large diameter profiles, which correspond to thick segments of varicose and globular neurites. Postsynaptic profiles (yellow, left) almost exclusively belong to thin dendritiform neurites. (F, G) Sections of two typical polyadic synapses. Green dots indicate presynaptic profile, characterized by T-bar (synaptic ribbon) and synaptic vesicles. Presynapses are contacted by multiple, thin branches of dendritiform processes. (H–J) Representative EM sections of VNC microvolume at three different levels along the antero-posterior-axis; levels are indicated by gray lines in panel A. Profiles of neurites shown in (A–D) are shaded in the corresponding colors and identified by the corresponding annotations (G1, V1-2, A1-3, D1-3). Scale bars: 0.1 µm (F, G); 1 µm (H–J).