The structure of the neurofilament cytoskeleton in the squid giant axon and synapse

Stereo views of thick sections (< 0.5 micron), or freeze etch replicas taken from selected regions of the squid giant axon and giant synapse, using an electron energy loss spectroscopic ultrastructural method, revealed a meshwork of anastomosing neurofilaments intersecting at a wide range of angles. This type of architectural organisation differs significantly from the ladder-like construction of the mammalian neurofilament cytoskeleton, in which longitudinally oriented core filaments are interconnected by lateral crossbridges. The deviant organization of squid neurofilaments is consistent with recent evidence that squid neurofilament proteins more closely resemble nuclear lamin rather than mammalian neurofilament proteins. A proximo-distal gradient of increasing width of the neurofilament meshes along the giant axon correlated well with the previously described gradient of increasing neurofilament phosphorylation. In these thick sections the presynaptic terminal of the giant synapse exhibited a mature neurofilament cytoskeleton that extended to the active zones without detectable signs of degradation. The observations are discussed in the context of current hypotheses concerned with the function of phosphorylation of neurofilaments, and with the steady state maintenance of the cytoskeleton in the squid axon and presynaptic terminal.