Recovery from peripheral nerve injuries is rarely complete. In a previous SFN abstract, we demonstrated the use of an injectable nerve regeneration conduit that allows direct manipulation of the regenerative microenvironment. Administration of a broad spectrum nitric oxide synthesis inhibitor (L-NAME) increased the number of regenerating myelinated fibers through the conduit following sciatic nerve transection in rats. In the present work, we address ultrastructural features of the regenerating nerve bridges that traversed an imposed 3mm gap inside the nerve conduits. Such bridges involve regrowth of axons not exposed to the products of Wallerian-like degeneration. Numbers and caliber of unmyelinated fibers in 20 sites from 5 systematically selected grids from each bridge/rat (n=8/group) were measured and size-number histograms constructed. Most unmyelinated regenerative profiles resembled those regrowing through degenerating nerve stumps: multiple closely apposed profiles sharing a common basement membran...