Ultrastructural details of Drosophila neuromuscular junctions
Images show details of neuromuscular synapses (>>> context) analysed with conventional transmission electron microscopy in Drosophila (A,B,D) or freeze-fracture technique in Manduca (C,E). A) Schematic representation of an attachment between bouton (bo) and muscle (mu) opened up on two sides towards the front. The muscle exhibits myofilaments (mf), the bouton contains mitochondria (mi), vesicles (v) and T-bars (white arrowhead), and both are covered by basement membrane (bm). Pre- and postsynaptic membranes are partially unfolded, and external (e) and protoplasmic (p) membrane leaflets are shown. Positions of the plates (B-E) are indicated in the scheme; chosen symbols indicate the same distinct structures throughout the figure. B) Transverse section through a larval (i) or late embryonic (ii) contact between neuronal bouton (bo, blue) and muscle (green, bottom), forming synapses with similar dimensions and features (width of the synapse defined by a dashed line of material in the synaptic cleft; between bent arrows). Presynaptic T-bars (with a roof and a stem; white arrow heads) surrounded by synaptic vesicles (asteriscs, fusion events of vesicles at the presynaptic membrane). Obliquely sectioned T-bars (iii, larval example) reveal the pronged nature of their horizontal roof. C) Freeze-fracture images of comparable synapses in Manduca show concentrations of membrane particles (black arrow) on the protoplasmic membrane leaflet (p) believed to be accumulations of calcium channels at the base of T-bars. D) Oblique sections through the dashed material in the synaptic cleft of the late larva or late embryo (inset) reveal honeycomb-like patterning of the material (between curved arrows). E) Freeze-fracture images of the external leaflet of the postsynaptic membrane reveal oval particle fields (black arrowhead), suggesting that the extracellular material may be anchored in the postsynaptic membrane. Scale bar 300 nm .

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