Patterns of change in endoneurial capillary permeability and vascular space during nerve regeneration

In frog sciatic nerve crushed and allowed to regenerate, an in situ perfusion technique was used to measure the permeability coefficient-surface area product (PS) of endoneurial capillaries to [14C]sucrose and endoneurial vascular space (V) at intervals of 3 days to 12 weeks post-crush. Additionally, the amplitude and latency of the compound action potential (CAP) of the regenerating nerve were also monitored. There was a delayed increase of both PS and V which peaked at 2-3 weeks after the crush and then declined. This is strikingly similar to the pattern seen in transected nerves. Whereas both PS and V reached near normal levels 6 weeks after transection, in regenerating nerves V continued toward normal levels but PS increased again to peak around 9 weeks post-crush. This second peak of PS coincided with a rapid increase of the CAP amplitude and a sharp decline of its latency. As is the case for transected nerves, the initial increase of PS is probably induced by breakdown products of axons or chemical signals from attendant Schwann cells and related to the clearance of debris from the endoneurium. The later increase of PS may be an endoneurial homeostatic mechanism to increase blood-nerve exchange during rapid axonal growth and remyelination.