An integrated view of neurogenesis in Drosophila embryos
Within the ectodermal monolayer precursor cells (dark green, NB-PC) for about 30 different neuroblasts are singled out from cell clusters expressing proneural genes (green). The mesoderm invaginates as an epithelial layer at the ventral midline of the embryo (>>> other illustration).
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Neuroblasts (NB, light grey) segregate from proneural clusters, divide repeatedly (schematic metaphase). They give rise to ganglion mother cells (medium grey), which divide again to produce postmitotic neurons (dark grey, red). The mesoderm disintegrates into a mesenchymal cell mass. 42 different sensory neurons/organs (SO-PC) and 30 different muscles (M-PC) are singled out (dark green) from clusters of cells expressing proneural genes (green).
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Postmitotic neurons (dark grey IN interneurons, red MN motoneuron, green PN peptidergic neuron) associate into the densely packed ventral nerve cord (vNC). Muscle precursors develop into founder cells (FC), which express specific markers (red) and fuse with surrounding mesenchymal myoblasts. Sensory organ precursors (SOP2) remain in the epidermis and divide twice.
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Progeny of sensory organ precursors differentiate into sheath/support cells (light blue) and a sensory neuron (SN, dark blue). Starting at stage 12, neurons form axons by sending out growth cones (GC) that follow cell-specific paths to establish appropriate contacts. Tips of multinucleated muscles (M) migrate actively to specific insertion points on the epidermis (arrowhead).
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Neuronal contacts have differentiated into synapses (active zones with clustered vesicles): Motoneurons establish neuromuscular junctions (NMJ), interneurons and sensory neurons form central synapses in the neuropile (NP), peptidergic neurons contain dense cored vesicles (arrow) with neurohemal release sites (NH) in the neuropile or periphery. Basement membrane (BM, grey) surrounds all tissues, whereas the apical epidermal surface is covered by cuticle. Myofilaments (MF) differentiate, and epidermal muscle attachment cells (MA) become connectors to the cuticle.
An approximate timing of these processes is indicated in form of 17 standardised embryonic stages (st.) or as percent of embryonic development (100% = 21hr at 25°C). For comprehensive descriptions of Drosophila embryogenesis and description of embryonic stages see: Campos-Ortega, J. A. & Hartenstein, V. The embryonic development of Drosophila melanogaster (Springer Verlag, Berlin, 1997).
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Source: picture modified from Prokop, A. Integrating bits and pieces - synapse formation in Drosophila embryos. Cell Tissue Res. 297, 169-186 (1999).