Regeneration in the nervous system is a multistep process of complex organization, both in time and space, of intercellular and cell-substrate interactions; a number of families of growth factors play important roles in this process [6, 8, 11]. These include nerve growth factor (NGF). Exogenous supply of this compound in conditions of axotomy leads to reductions in central chromatolysis, death of damaged neurons, reductions in mediator contents, and retraction of presynaptic terminals in sympathetic ganglia, to reductions in the numbers of high-affinity membrane receptors and substance P production in spinal ganglion cells, to reductions in somatofugal atrophy of fibers of the dorsal roots of spinal nerves, to stimulation of protein synthesis and phosphorylation, and to sprouting of collateral fibers [2-5, 9, 13]. The distal segment of the cut sciatic nerve shows sharp increases in NGF content resulting from increased production, as well as increases in low-affinity receptors and Schwann cells [7]. The signal for these changes is provided by interleukin-1, released by activated macrophages in the trauma zone [11]. Adsorption of NGF to the surfaces of these cells produces conditions promoting positive chemotaxis in growing processes, along with increases in their length and level of myelinization [10].We elected to study the effects of NGF on the recovery of fibers in crushed sciatic nerves in rats. MATERIALS AND METHODSStudies were carried out using 45 mongrel male rats; under hexenal (100-150 mg/kg) anesthesia, the sciatic nerve was clamped at the level of the middle third of the femur under sterile conditions. One group of animals (n = 23) received daily i.m. NGF (10-15 ~g/g) from 3 to 14 days after section, before collection of material for investigation; another group (n = 17) received the same volume of isotonic saline. Intact animals served as an additional control group (n = 5), and were given isotonic saline. Nerves from these animals and from operated rats (n = 18) were examined by electron microscopy, using standard methods [1], or were used in electrophysiological studies in a thermostatted (37~ humidity chamber, using standard apparatus. Sections of nerves of 35-40 mm in length were placed on two pairs of suspended platinum electrodes with an interelectrode distance of 2-3 mm with the damaged zone between them. The proximal electrode pair was used for stimulation, and the distal pair for recording. Measurements were made of the thresholds, latent periods, amplitudes, and conduction rates of action potentials using single supramaximal (up to 1 V) square-wave impulses lasting 0.1 msec, at a frequency of 1 Hz. RESULTS AND DISCUSSIONClamping significantly altered the ultrastructure of the sciatic nerve distal to the trauma site. By the third day in control rats given isotonic saline, axial cylinders were disrupted at a distance of 5 mm from the damage zone, myelin coatings were swollen and broken into fibrils, and there were increases in the numbers of collagen fibers oriented in different directions; th...