Journal of Neuroscience, Vol 9, 2678-2686, Copyright © 1989 by Society for Neuroscience
Removal of the basal lamina in vivo reveals growth cone-basal lamina adhesive interactions and axonal tension in grasshopper embryos
ML Condic and D Bentley
Neurobiology Group, University of California, Berkeley 94720.
The Ti1 afferent neurons are the first cells to undergo axonogenesis in
embryonic grasshopper limbs. The Ti1 growth cones migrate between the limb
epithelium and its basal lamina. We have investigated the nature of growth
conebasal lamina interactions in vivo by removing the basal lamina with
mild enzymatic digestion. Treatment with elastase, ficin, or papain removes
the basal lamina when viewed in scanning electron microscopy. Trypsin and
chymotrypsin leave the basal lamina intact. If the basal lamina is removed
after the Ti1 growth cones have extended over intrasegmental epithelium but
are not yet in contact with either differentiated segment boundaries or
neurons, the growth cones retract to the cell somata. If the basal lamina
is removed by elastase, and the Ti1 neurons are allowed to extend axons
after treatment, a second elastase digestion does not cause the axons to
retract. It is therefore unlikely that axon retraction is due to general
proteolysis. These results suggest that if Ti1 growth cones have initially
extended in the presence of an intact basal lamina, they are dependent on
the lamina to remain extended over this region of the limb. The retraction
of the Ti1 axons after removal of the basal lamina is inhibited by
cytochalasin D, suggesting that microfilament-based cytoskeletal components
underlie this event. This result indicates that the axons are under tension
in vivo. The ability of the Ti1 growth cones to resist axonal tension
suggests that adhesive interactions between the growth cones and the basal
lamina underlie normal axon outgrowth in vivo.
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