The neurofilament infrastructure of a developing presynaptic calyx

Calyx-type synapses appear to be specifically designed to support fast, reliable, high-frequency excitatory transmission. In the chick ciliary ganglion, calyx terminals from preganglionic neurons in the midbrain form early in development on ciliary neurons. We find that labeling the calyx membranes with a lipophilic dye delivered by diffusion down the preganglionic nerve reveals a large membrane structure engulfing the postsynaptic cell by the end of embryogenesis. In contrast, labeling the calyces with a water-soluble dye by diffusion through the preganglionic nerve suggests large discontinuities in the calyx. A similar pattern of discontinuities is seen when presynaptic neurofilaments are labeled with antibodies selective for highly phosphorylated neurofilaments. The neurofilament infrastructure of the calyx first appears as a single thick bundle, which subsequently bifurcates during development and eventually generates a fine meshwork of filaments subdivided by several large neurofilament bundles encircling the postsynaptic cell body. The large bundles probably produce protruding ridges in the otherwise thin calyx cup, accounting for the disparity in staining patterns observed with membrane and cytosolic dyes. The postsynaptic membrane also undergoes restructuring during development with the appearance of large folded mats of somatic spines heavily invested with nicotinic receptors. The large presynaptic neurofilament bundles do not overlap the postsynaptic receptor clusters but do codistribute with large tracks of presynaptic microtubules. The neurofilament bundles may act as girders to provide structural support while at the same time defining conduits for microtubule-dependent transport of materials and rapid propagation of electrical signals throughout the extended calyx.