Axonal microtubules are stained and cross-linked by highly cationic polyethyleneimine.

Highly cationic polyethyleneimine was used as an electron microscopic tracer for anionic sites in the axoplasm of rat sciatic and optic nerve fibres. Microtubules showed a markedly increased electron density and aggregated to form large groups, mostly located in the immediate proximity of membranous axoplasmic organelles. Walls of adjacent microtubules were fused; there was also fusion of microtubules with the membranes of smooth axoplasmic reticulum and with axolemma. In contrast, neurofilaments had unaltered electron density and axoplasmic distribution. Staining and clustering of microtubules were interpreted as electrostatic binding of cationic polyethyleneimine to acidic tubulin. These findings may be relevant to the role of microtubules in fast axonal transport.