Specific Aims The regeneration of the mammalian skeleton requires the action of both intrinsic and extrinsic inductive factors from multiple cell types which function in a hierarchical and temporal fashion to control skeletal progenitor cell proliferation and differentiation. Sensory nerves have been shown to be an integral part of the bone fracture repair process, driving the processes of vascularization, ossification, and mineralization of bone. In contrast to the bone repair process, regeneration, where new growth replaces both the amputated bone and surrounding soft tissue varies widely in vertebrates. In mammals, regeneration is restricted to only the distal phalangeal element. More proximal amputations result in the formation of a hypertrophic callus and failed regeneration. Significant efforts have been placed on dissecting out the distinguishing signaling pathways differentiating regenerative versus non-regenerative amputations. Beyond the desire to promote full regeneration, unraveling these processes could allow us to leverage regenerative mechanisms during repair and tissue-engineering based bone therapeutic approaches. A handful of prior studies have implicated innervation as an essential component of regeneration, however they relied on complete sciatic nerve resection, making it impossible to distinguish nerve-specific regenerative outcomes from mechanical loading-induced effects, and the relationship between innervation and regeneration remains unclear. Using transgenic mouse models and pharmacological inhibition, our preliminary results point to a severe delay in digit regeneration following inhibition of sensory nerve tropomycin receptor kinase A (TrkA). In the context of previous literature, we propose that sensory nerve TrkA signaling is necessary for proper digit regeneration. Specifically, we propose that: i) sensory nerves are recruited to the amputation site early in the healing process through the nerve growth factor (NGF)-TrkA signaling axis established in our lab, ii) sensory nerve-derived signals play an essential role in promoting blastema formation and maintaining cells in a proliferative, osteogenically primed state, and thus, iii) disruption of sensory nerve signaling through transgenic and/or pharmacological inhibition severely impairs digit bone regeneration. Specific Aim 1: Define the spatiotemporal patterning of sensory innervation and characterize the effects of sensory nerve TrkA signaling disruption during digit regeneration Hypothesis: Sensory nerve outgrowth and signaling coincides with wound closure, blastema formation and proliferation, initiating overall digit regeneration. Preliminary results using a transgenic knockin mouse model (TrkAF592A), demonstrate a substantial deficit in digit regeneration. In Aim 1 we will first conduct a comprehensive study on the temporal and spatial patterning of neurotrophin expression and sensory innervation during early and late stages of digit regeneration. Here, we wil...