Changes in the segmental pattern of sensory neuron projections in the chick hindlimb under conditions of altered cell...

In the developing chick hindlimb, pathfinding by sensory axons is affected by their interactions with other axons entering the limb. Cell adhesion molecules (CAMs) expressed on the growing axons are likely to influence these interactions. Accordingly, to elucidate how CAMs affect sensory axon pathfinding, we injected antibodies that block the functions of NCAM, G4/L1, or N-cadherin into the hindlimb, starting at St. 25, when all motoneuron axons but only a few sensory axons had entered the plexus. In each case, the segmental pattern of projections was assessed 2-3 days later by retrogradely labeling individual peripheral nerves. With all perturbations, the gross anatomical nerve pattern developed normally; that is, some sensory axons formed cutaneous nerves while other sensory axons projected to muscles. However, the segmental pattern of sensory projections was changed when either G4/L1 or N-cadherin function was blocked in that fewer sensory axons crossed the anterior-posterior axis of the plexus. A likely reason for this effect is that anti-G4/L1 and anti-N-cadherin each decreased the amount of fasciculation and that sensory axons are less able to travel across the plexus when they are defasciculated. Anti-G4/L1 affected both cutaneous and muscle sensory projections while anti-N-cadherin affected cutaneous but not muscle sensory projections, in accord with known differences in the expression of these two CAMs on sensory and motoneuron axons. Although anti-NCAM did not appear to alter sensory projections, when polysialic acid (PSA) was enzymatically removed from NCAM, there was a marked increase in cutaneous projections from the most proximate DRG, although muscle sensory projections were unchanged. PSA removal may cause an increase in fasciculation that forces sensory axons to track along neighboring axons. Thus, without PSA, cutaneous axons project more in accord with the relative anterior-posterior positions they had as they entered the plexus. Taken together, these studies suggest that axonal fasciculation mediated by CAMs and regulated by PSA influences the ability of sensory growth cones to navigate through the plexus and project along the correct peripheral nerves.