The expression of cell adhesion molecules on the growth cones of chick cutaneous and muscle sensory neurons

In the developing chick hindlimb, axons that will project along a given peripheral nerve sort out together, as they traverse the plexus region at the base of the limb, and become segregated from axons that will project along other peripheral nerves. This sorting out may involve, at least in part, the differential expression of various cell adhesion molecules (CAMs). To begin to explore this possibility, we have compared the relative levels of immunofluorescent labeling on the growth cones of two populations of sensory neurons whose axons become segregated from one another: cutaneous and muscle sensory neurons. We took a tissue culture approach, since this allowed us to readily visualize the immunofluorescent labeling of individual growth cones, and identified the two cell types by previous retrograde labeling with DiI. Two dorsal root ganglion explants, one containing DiI-labeled cutaneous neurons and the other containing DiI-labeled muscle sensory neurons, from opposite sides of the same embryo, were plated together in each culture dish. For all the CAMs were studied (NCAM, polysialylated NCAM, G4/L1, axonin-1, SC1/DM-GRASP/BEN, and N-cadherin), the intensity of immunofluorescent labeling typically was fairly uniform on the growth cone, it filopodia, and the portion of the neurite just proximal to the growth cone. Only one CAM, axonin-1, exhibited labeling that was especially intense at sites of interneuronal contact. Quantification of labeling intensities using image analysis showed that cutaneous and muscle sensory growth cones did not consistently differ from one another in their levels of expression of G4/L1 or of axonin-1. The latter finding stands in contrast to recent reports claiming that axonin-1 is not expressed on muscle sensory neurons. Each of the other CAMs (NCAM, polysialylated NCAM, SC1/DM-GRASP/BEN, and N-cadherin) also showed considerable overlap in the distribution of labeling intensities between the two populations, but overall, expression levels on muscle sensory growth cones were greater than on cutaneous growth cones. How the differential expression of some CAMs could potentially contribute to the way that cutaneous and muscle sensory growth cones become segregated from one another and the implications of these results for sensory neuron specification are discussed.