Cytoskeletal organization in the neurohypophysial axons of rats

Abstract

Cytoskeletal organization in axons of the rat neurohypophysis was examined by quick-freeze deep-etch electron micrscopy and conventional thin-section electron microscopy. In the neurohypophysial axons, neurotubules and neurofilaments were arranged almost in parallel with each other, and interlinked by cross-bridges. Because the localization of MAP1A on the axonal cytoskeleton was shown by immunoelectron microscopy, MAP1A is a component of the cross-bridges between neurotubules. The neurotubules and neurofilaments occupied their respective domains in the axon. The neurotuble:neurofilament ratio was variable axon by axon. In some axons, neurotubules overwhelmed neurofilaments in number. In other axons, neurofilaments were predominant, and neurotubules were few. The neurosecretory granules were located in association with either the neurotubule or neurofilament domain. Fine fibrils 5–10 nm in diameter were associated with the inner surface of the axonal plasmalemma. In the Herring body, a few neurotubules occurred in the marginal region while neurofilaments were rarely seen, indicating that most of the neurosecretory granules have no relationship with the neurotubules in the body. These results suggest that the cytoskeleton in neurohypophysial axons may be involved in regulation of axonal transport and storage of neurosecretory granules.