The major formed elements of axons of the ventral nerve cord of the crayfish are longitudinally oriented microtubules, filamentous matrix material, and mitochondria. Lanthanum staining delineates both microtubules and filamentous material to great advantage, and it is demonstrated that filamentous elements continuous with matrix material are associated with the surfaces of microtubules. Microtubules are interconnected by filamentous elements and connexions also exist between microtubules and the plasma membrane to form a 3-dimensional latticework. This latticework is hypothesized to play a role in translocation of materials down the axon: the filamentous material probably provides the gelatinous integrity of the axoplasm and may provide the motive force for axonal transport, with the microtubules serving as attachment sites and guideways extending the length of the axon. In pieces of nerve cord treated with cytochalasin B, the filamentous material of the axoplasm is structurally unchanged. Treatment of pieces of nerve cord with hyaluronidase, however, induces the formation of large tubular elements (macrotubules), 40-50 nm in diameter, with which lateral filamentous material is also associated.

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