Pioneer growth cone adhesion in vivo to boundary cells and neurons after enzymatic removal of basal lamina in grasshopper embryos.

The Ti1 pioneer neurons of embryonic grasshopper limbs extend axons between the limb epithelium and its basal lamina. Their growth cones exhibit high affinity for both limb segment boundaries and immature neurons. We have investigated the role of the basal lamina in growth cone adhesion to neurons and segment boundaries by removing the basal lamina with mild enzymatic digestion when the Ti1 growth cones are in contact with different cellular substrates. If the basal lamina is removed when the Ti1 growth cones are in contact with other neurons, the growth cones remain in contact with the neuronal somata, and the Ti1 cell bodies in contact with the neuronal somata, and the Ti1 cell bodies reposition proximally. This suggests that the basal lamina is involved in the adhesion of the Ti1 somata to the substrate but not in growth cone-neuronal adhesion. This is the first direct evidence that growth cones establish adhesive cell-cell interactions with other neurons in vivo. Enzymatic treatments that remove the basal lamina also cause embryonic limbs to elongate. If the Ti1 axons are strongly apposed to 2 segment boundaries prior to protease treatment, their somata reposition to the nearest segment boundary, yet their axons do not retract off of the segment boundaries, despite severe stretching by the enzyme-induced limb expansion. These results indicate that the affinity of the Ti1 cells for segment boundaries is due at least in part to adhesive cell-cell interactions that are resistant to proteolytic digestion and independent of the basal lamina.