From identified precursors to identified neurons
Embryos at the early gastrula stage represent a monolayer of cells (dark, middle, light green, areas giving rise to CNS in head, thorax and abdomen, respectively; vertical lines, segment borders; dEpi, dorsal epidermis; M, mesoderm). About 30 different neural precursor cells (numbered circles) per thoracic/abdominal segment segregate on each side of the body (>>> illustration); they can be individually named (numbers; MNB, midline neuroblast) with respect to position, time of formation, and combination of markers they express. Each precursor gives rise to a reproducible lineage with respect to number and type of neurons and/or glia cells. Single neurons within these lineages can be identified (see below; here example of a motorneuron in blue) and individually monitored or manipulated (>>> orientation of image; brown, cortex; orange, neuropile; green, muscle; NMJ, neuromuscular junction).
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Source: picture modified from Doe, C. Q. & Technau, G. M. Identification and cell lineage of individual neural precursors in the Drosophila CNS. Trends Neurosci. 16, 510-514 (1993).

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What is an identified neuron? Many neurons in each body half of the embryonic CNS of Drosophila occur as single copies per segment (segmental units indicated by asterisks), derived from one particular neural lineage, respectively (see above). Such neurons can be labelled with different techniques, e.g. by applying lipophilic Dye I (left image) or through Gal4-mediated marker expression (right image; >>> spatial orientation). Neurons are identifiable due to reproducible features of their morphology: position of cell bodies (S), paths of their projections, the points where they form contacts (white arrow heads) or spatial arrangements of their arborisations (curved arrows). Subtle changes of these features can be spotted and used as sensitive readouts in genetic or experimental studies.
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Source: right picture modified from Landgraf, M., Sánchez-Soriano, N., Technau, G. M., Urban, J. & Prokop, A. Charting the Drosophila neuropile: a strategy for the standardised characterisation of genetically amenable neurites. Dev. Biol. 260, 207–225 (2003).

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