Development of Peripheral Innervation in the Frog Xenopus Laevis

Abstract

The skin in Xenopus laevis is innervated by two different sets of mechanosensory neurons at different times during development. Rohon Beard (RB) neurons start differentiating during gastrulation, innervate the embryonic skin and mediate sensory function during hatching. Dorsal Root Ganglion (DRG) neurons start differentiating after neural crest migration, innervate adult epidermal targets and mediate mechanosensory function during larval and adult stages and eventually replace RB neurons. The change in sensory neurons occurs during the transformation of skin, sensory structures, and behavior from their embryonic to their larval forms. We hypothesized that developmental changes in either the sensory end organs or the skin underlie the switch in mechanosensory systems. We initially tested the development of sensory innervation of the skin by marking neurites that innervate the skin and measuring their spatial and temporal changes. We also showed that sensory neurites rapidly disappear following denervation. We then switched targets by transplanting denervated skin between animals of the same stage (isochronic) and animals at different stages (heterochronic). Quantification of the percent area reinnervated by neurites from the host showed that reinnervation of all three stages of larval development following isochronic transplant, was approximately half that of controls. Reinnervation of heterochronic transplants was approximately half of the isochronic transplants. Our results show that neurites have difficulty innervating denervated skin at an age different from itself. In addition, the pattern of innervation changes during this period of development. Sensory neurons totally encircle a subset of dispersed cells in the skin. Innervation of other cells follows, but the number of encircled cells gradually decreases. Following heterochronic skin transplants, embryonic neurites encircle cells in a similar way in both embryonic and larval skin, indicating that the neurons, rather than the skin determines the pattern of innervation. However, older neurons do not show age-specific patterning, which suggests that they no longer determine the pattern of innervation. Thus, the skin targets help regulate target innervation, but the patterning of innervation, at least partially depends on the neurons.