Axial and radial filamentous components of the neurofilamentous network

Summary

The three-dimensional structure of the neurofilamentous network of the giant axon of the squid has been investigated by stereo electron microscopy and optical image analysis. The neurofilamentous network with its structural associations intact was partially purified by means of extraction of extruded axoplasm in a physiological buffer. The authors employed a “double-grid mounting technique” for critical point drying of axoplasm which provides high contrast preparations having great depth suitable for the analysis of spatial relations. There is a striking improvement in the perception of the continuity and three-dimensionality of the network, particularly in thin areas produced by teasing apart the specimen prior to mounting. Sidearms and cross-bridges are readily identifiable.

In 1-μm thick preparations of the extracted axoplasm the neurofilamentous network is composed of two structural entities: (i) long 10-nm wide filaments approximately parallel with the long axis of the extracted axoplasmic cylinder (axial), (ii) and short, finer filaments projecting from them as sidearms or crossbridges (radial). Optical analysis of micrographs of extracted axoplasm indicates that the radial filamentous components of the neurofilamentous network are distributed predominantly in the range of 32° to 67° with respect to the long axis of the axial filaments. We tentatively assign the 220,000 mol wt peptide observed by SDS-PAGE in this preparation to the radial filaments and the 68,000 mol wt peptide to the axial filaments.