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Many studies have shown a role of retinoid signalling in neurite outgrowth in vitro, and that the retinoic acid receptor (RAR) beta2 is critical for this process. We show here that RARbeta2 is expressed predominantly in dorsal root ganglia (DRG) neuronal subtypes that express neurofilament (NF) 200 and calcitonin gene-related peptide (CGRP), and that these neurons extend neurites in response to RA. We demonstrate that retinoid signalling has a role in neurite outgrowth in vivo, by showing that in a peripheral nerve crush model there is less neurite outgrowth from RARbeta null DRG compared to wild-type. We identify sonic hedgehog (Shh) as a downstream target of the RARbeta2 signalling pathway as it is expressed in the injured DRG of wild-type but not RARbeta null mice. This regulation is direct as when RARbeta2 is overexpressed in adult motoneurons Shh is induced in them. Finally we show that Shh alone cannot induce neurite outgrowth but potentiates RARbeta2 signalling in this process.

Fibronectin mats have proved to be successful guidance substrates for the outgrowth of neurites in models of neural damage. The overall aim of this investigation was to develop a tissue engineering conduit for surgical nerve reconstruction. In this study, fibronectin tubes were developed to provide global guidance for neural regrowth across a short gap. A viscous solution of concentrated plasma fibronectin was obtained from by-products of the human plasma fractionation process. The tube was formed by winding this material around an 18G hypodermic needle and allowing it to dry. The structure and physical properties of the tube were investigated. The spiral-wound material on the needles fused to form a durable tube, whose lumen remained patent after rehydration/swelling in buffer (swelling and stability were characterized). In vivo testing showed growth of axons through the tube 6 weeks post surgery in a rat sciatic nerve injury model. In conclusion, a tube has been developed from fibronectin which can be used to bridge gaps in damaged nerves. This could form the basis for the construction of a surgically implantable device for the tissue engineered repair of peripheral nerve injury.