Ultrastructure of somatic active zones.

<p><b>A</b>. Electron micrograph of a Retzius neuron in the ganglion after stimulation with 1 Hz trains, a stimulation frequency subthreshold to evoke secretion. In these conditions, the vesicle clusters (vc) remained at a distance from the plasma membrane (pm), although they were indirectly bound to it through bundles of microtubules and were near to mitochondria (m) and endoplasmic reticulum (er). The cytoplasmic space between the plasma membrane and the vesicles contained smooth endoplasmic reticulum layers. More internally, there was another population of vesicle clusters and multivesicular bodies (mvb). Retzius neurons were surrounded by layers of a giant glial cell (g). Scale bar = 500 nm. <b>B</b>. After stimulation with 20 Hz trains, the vesicle clusters were apposed to the plasma membrane (arrows) and flanked by endoplasmic reticulum and mitochondria. Scale bar = 1 µm. <b>C</b>. Higher magnification of a pseudo-colored active zone after 1 Hz stimulation. Vesicles (blue) were at a distance from the plasma membrane, forming associations with cytoskeletal bundles (yellow), mitochondria (brown) and endoplasmic reticulum (green). The extracellular side of the anchor site of the cytoskeleton with the plasma membrane had triplets of glial cell fingers. Scale bar = 500 nm. <b>D</b> After 20 Hz stimulation, vesicles were apposed to the membrane near the anchor sites of the cytoskeleton. Mitochondria had maintained their proximity to the vesicle clusters and endoplasmic reticulum. The presence of multivesicular bodies suggests vesicle degradation upon endocytosis. Scale bar = 500 nm.