The effects of target tissues on the outgrowth of chick cutaneous and muscle sensory neurons.

In some developing systems, growing axons are attracted to their target site by a diffusible molecule released by the target tissue. In the chick hindlimb, this mechanism could explain how axons, after having reached the plexus region, grow to muscle or to skin. To begin to test this possibility for limb-innervating sensory neurons, we cocultured dorsal root ganglion explants and potential target tissues in three-dimensional collagen gels. In particular, we wanted to know if target tissues, at early stages of development, specifically attract axons of only the appropriate type of sensory neuron. To identify each type of sensory neuron, we used DiI to retrogradely label either cutaneous or muscle sensory neurons in the embryo, prior to culturing. The results showed that dermal and muscle explants could each enhance the outgrowth of both cutaneous and muscle sensory axons. In contrast, the epidermis and the connective tissue associated with developing muscle appeared to inhibit the outgrowth of both cutaneous and muscle sensory axons. These results suggest that, in the embryo, the dermis and muscle cells both release diffusible factors that cause sensory axons to diverge from the plexus, extend toward the sources of these factors, and thereby form discrete peripheral nerves. The inhibitory effects of epidermis and muscle-associated connective tissue may serve to limit the growth of sensory axons to the structures, i.e., dermis and muscle cells, that ultimately receive sensory innervation. However, since for each of the difference types of limb tissue, the responses of cutaneous and muscle sensory neurons were always similar to one another, sensory axons must not be responding to target-derived factors when they decide whether to grow to skin or to muscle.