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The tube concept in nerve repair.

The use of tubes to bridge experimental defects in nerve continuity was introduced 20 years ago when “mesothelial tubes” were used as conduits for axonal growth (1 1,12). The hypothesis was that various types of neurotrophic factors supporting axonal growth were synthesized in injured nerves and that such factors might accumulate and form a physiological environment within a tube system. As an experimental tool, the tube concept proved very valuable for studies of environmental factors supporting nerve regeneration, and it was quickly found that silicone tubes served this purpose in a most appropriate way (13). When a 10-mm gap in a rat sciatic nerve was bridged by a silicone tube, within 4 weeks the gap was successfully bridged by a spontaneously formed nerve structure of more or less normal appearance (7,13). The nature and quality of the nerve structure bridging the gap were directly related to the length of the gap, and a critical limit of 10-15 mm could be defined beyond which no or very inferior regeneration occurred in the rat sciatic nerve (7,9). The experimental tube model was found ideal for a detailed analysis of biological processes occurring in the gap between the proximal and distal segments of a severed nerve. Inside the tube, a fibrin cloth is formed first between the nerve ends. The cloth becomes invaded by capillaries and cellular elements from the proximal and the distal segments. Axons migrate into the matrix from the proximal nerve segment, and laminin as well as fibronectin can be demonstrated in the matrix at an early stage (2,3,7). Neurotrophic factors addressing motor, sensory, and sympathetic neurons accumulate in the chamber; at least two types of neurotrophic activities have been defined: NGF (nerve growth factor) and CNTF (ciliary neurotrophic factor) (2). There is a major peak in occurrence of neurotrophic factors occurring as quickly as 3-6 h after nerve injury (2).