Cytoplasmic transport in axons.

Transport along axons and dendrites ranges from the fast movement of vesicles (50-500 mm day"), which can be observed directly by light microscopy, to the slow progress of cytoskeletal proteins. The latter, which has been followed only by using radiolabelled proteins (reviewed by Lasek et al. 1985), occurs at 0-2-8-0 mm day", but this is still faster than diffusion. The differential speeds of various components exclude cytoplasmic streaming models; instead, it is thought that transport is effected by molecular motors generating force between specific components of the axoplasm. Recent work has illuminated several aspects of these processes, with implications for transport in less specialized cells. Slow transport is unidirectional and anterograde (away from the cell body) and can itself be resolved into faster and slower components. An intriguing finding is that these continue at the same rates after growth has ceased: the cytoskeleton is evidently replaced continuously throughout the life of the cell. The slowest group (SCa) consists almost exclusively of neurofilament and microtubule proteins. Lasek and colleagues, after careful observation of the moving label, concluded that tubulin and neurofilament proteins move en bloc and suggested they are transported as complete microtubules and filaments. Since the membrane of an axon apparently remains stationary after being laid down near the growth cone, the cytoskeletal components must move relative to the cell surface, perhaps by motors that engage with proteins bound to the membrane. The fast-polymerizing ends of axonal microtubules lie away from the cell body (see Heidemann et al. 1984). Since they are known to be discontinuous, it is probable that they are pre-assembled in the cell body before being transported intact along the axon. Their distal ends come near to the surface membrane and are apparently capped, at least in nematode axons (Chalfie et al. 1986). Cold-stable microtubular segments occur