Charting the neuropile
Making synapse formation in the Drosophila CNS accessible to investigation is pivotal since only in the CNS postsynaptic neuronal mechanisms and a larger variety of synapse types can be analysed. This endeavour is rather challenging due to the high synaptic density in the synaptic neuropile of the CNS. In order to facilitate this approach reproducible landmarks can be used to describe spatial arrangments of neuronal projections and synapses. Such landmarks are the ascending/descending fiber tracts labelled by the N-CAM-homologue Fasciclin2 (Fas2):

The image shows a schematic transverse section through the larval ventral nerve cord which is composed of the synaptic neuropile (orange) and the cell body area (cortex; brown; >>> illustration 1; >>> illustration 2). Right: The neuropile displays a reproducible pattern of ascending/descending Fas2-positive fiber tracts (blue), which have been named individually (D, dorsal; V, ventral; C, central; M, median; I, intermediate; L, lateral). Left: Making use of these landmarks, different areas of the neuropile can be mapped, as shown for the sensory output areas (red) and the motor-input areas (green) which do not overlap.

Example of identified neurons (green) mapped relative to Fas2-positive fiber tracts (magenta, nomenclature see above). 3 consecutive segments of a late embryonic ventral nerve cord of Drosophila are shown in dorsal view (anterior points up; >>> illustration). Presentation scrolls through subsequent horizontal focal planes from dorsal to ventral. Left: The aCC and RP2 motorneurons (green in left image) have dorsal cell bodies (black asterisk) and their arborisations (curved arrows) are restricted to areas dorsal of the CI-fascicle (compare green area in picture above). Right: Neurons expressing the neurotransmitter/-modulator Serotonin (green in right image) display ventral cell bodies (white asterisks), transverse projections ventral of the VM-fascicle (arrow heads), and segmentally organised projections in all dorsoventral layers of the neuropile. Spatial arrangements of these identified neurons can clearly be described and charted on the basis of Fas2 landmarks.

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Source: both pictures modified from Landgraf, M., Sánchez-Soriano, N., Technau, G. M., Urban, J., and Prokop, A. (2003). Charting the Drosophila neuropile: a strategy for the standardised characterisation of genetically amenable neurites. Dev Biol 260, 207-225.