Journal of Neuroscience, Vol 8, 3190-3199, Copyright © 1988 by Society for Neuroscience
Compartmentalization of anterogradely and retrogradely transported organelles in axons and growth cones from chick optic tectum
TP Cheng and TS Reese
Laboratory of Neurobiology, IRP, NINCDS, Marine Biological Laboratory, Woods Hole, Massachusetts 02543.
Previous work suggests that organelles contacting microtubules in axons are
in fast transport. Here, we examine the distribution of organelles
contacting microtubules in growing axons and growth cones from chick optic
tectum. Five axon segments, each 10 microns long, and 4 entire growth cones
were reconstructed from serial electron micrographs of quick-frozen,
freeze-substituted chick optic tectum. Organelles contacting microtubules
in axons are evenly distributed along all microtubules. Smaller organelles,
presumably in anterograde transport, are enclosed in fascicles of
microtubules, while larger organelles in retrograde transport lie outside
the fascicles. In contrast, organelles contacting microtubules are
prevalent only in the most proximal parts of the growth cone, before the
microtubule fascicles splay out more distally. The distance between
noncontacting organelles and microtubules also becomes progressively
greater, reaching a maximum in the mid- and more distal region of the
growth cone. Contacts with microtubules of both the smaller, presumably
anterogradely transported organelles, as well as the larger, presumably
retrogradely transported organelles, abruptly become less frequent in the
proximal midregion of the growth cone. It is therefore of possible
significance in stopping and starting microtubule-based organelle transport
that microtubules change from a straight to an undulating configuration in
the midregion of the growth cone. The decrease in organelle binding to
microtubules at the demarcations between the straight and undulating
microtubule segments may depend on proteins or other local factors as well
as the splaying out of the microtubule bundles.