Fetal nerve healing: an experimental study.

An experimental study was performed to assess fetal nerve repair and regeneration both qualitatively and quantitatively. The posterior tibial nerves in one hindlimb were transected in 16 midgestational fetal lambs and in their mothers. The nerves were then repaired with epineurial sutures and allowed to progress to 2, 4, 6, and 8 weeks postinjury. Qualitative assessment was performed through standard nerve histologic staining, including Luxol fast and toluidine blue for myelin and Bielschowsky stain for axons, and quantitative assessment through nerve conduction velocity studies and morphometry to determine mean myelinated fiber diameter, total fiber number, and density. A frequency histogram of the distribution of myelinated nerve fibers according to fiber diameter also was generated. In our model, the subsequent fetal nerve response to injury was characterized by earlier degeneration than in the adult counterparts. Repair and regeneration proceeded with dense collagenous scar formation in both groups. Electrophysiologic studies showed nerve impulse conduction across the repair site only at 6 and 8 weeks postinjury in both fetus and adult. Action potential amplitudes at 6 and 8 weeks were measured at 3 to 5 percent of control nerves in both nerve types. No electrophysiologic differences in the recovery of the injured fetal and adult nerves could be identified. Morphometry revealed that fetal nerve regeneration appeared to occur at a rate equivalent to that of the adult, although by 8 weeks the total percentage of remyelinated nerves appeared more complete in the fetus than in the adult (87 versus 59 percent), suggesting that fetal nerves may have a more favorable regenerative capacity than their adult counterparts.