Synapse formation at embryonic Drosophila NMJs
Filopodia of growth cones (GC) are repelled by some muscles (muscle 2) attracted (stippled lines) by others (muscle 1) via CAMs (rectangles). Glutamate (little dots) is detectable in the terminal around the time of contact formation and vesicles (stippled circle) must form soon, as first transmission occurs within 30 min after neuromuscular contact.
Basement membrane (BM) forms and adheres to receptors (red ellipses). This may hold the presynaptic terminal against the muscle surface - which might also require the specific CAMs (connected rectangles). Basement membrane appears to be excluded from the synaptic cleft (white arrowheads) by a function downstream of the transcription factor mef2 (arrow from nucleus). Evenly distributed Glutamate receptors (Glu-recept.) cluster postsynaptically in response to presynaptic electrical activity (zigzag arrow) and depending on correct levels of presynaptic glutamate. Function of shot/kak mediates branching of the presynaptic terminal. Active zones (AZ) start assembling.
Specific CAMs fade from the NMJ or concentrate there (stippled rectangles). Fas2 (red zigzag) appears to manifest the cell junction. It is clustered together with Shaker channels by Discs large (Dlg). Dlg might localise to the NMJ via binding to synapse-specific cytoskeletal elements (three-pronged stars; purely hypothetical) or to synaptic transmembrane proteins (not shown). The amount of Glu-Rs is upregulated as a function of presynaptic electrical activity (arrow from nucleus)

An approximate timing of these processes is indicated in form of 17 standardised embryonic stages (>>> illustration) according to Campos-Ortega, J. A. & Hartenstein, V. The embryonic development of Drosophila melanogaster (Springer Verlag, Berlin, 1997).

Source: picture modified from Prokop, A. Integrating bits and pieces - synapse formation in Drosophila embryos. Cell Tissue Res. 297, 169-186 (1999).