The accuracy of reinnervation in peripheral nerves following second degree injuries, which do not disrupt the longitudinal continuity of the endoneurial sheaths, has been studied in rats. The sciatic nerve or lumbar spinal nerves (that is the extraspinal nerves before their fusion in the sciatic plexus) were crushed with fine watchmakers' forceps in neonatal and adult rats. In addition, the lumbar spinal nerves were frozen in a group of 5 adult rats. After allowing reinnervation to occur for 5 to 9 weeks, the motoneurons whose axons ran in the plantar nerve were labelled retrogradely with horseradish peroxidase. Their positions in the grey matter of the lumbar spinal cord were recorded and compared with those labelled from the contralateral unoperated plantar nerve. Very few errors of projection occurred after a crush lesion of the adult sciatic nerve but all the other lesions produced significant numbers of errors. The order, starting with the preparations with fewest errors was as follows (numbers in brackets = percentage of neurons misplaced): sciatic crush in adult (3%), sciatic crush in neonate (23%), spinal nerve freeze in adult (23%), spinal nerve crush in adult (35%), and spinal nerve crush in neonate (72%). It seems that a significant number of axonal growth cones cross endoneurial sheaths after crush or cryoinjuries. Explanations for the difference in observed reinnervation accuracy between young and old rats and between lesions in peripheral nerves and spinal nerves are discussed. The first is that axons in peripheral nerves in older rats have a less penetrable endoneurial membrane encasing them. The second is that the amount of misrouting is the same at all lesion sites but is much less easily detectable after sciatic lesions than spinal nerve lesions. This is because axons are organized in a 'musculotopic' manner in peripheral nerves and exchange of axon positions will occur largely between axons destined for the same peripheral target. In contrast, exchange of positions of axons in spinal nerves will lead to more overt errors because at this site axons destined for particular muscles do not lie side by side but are intermingled with axons innervating other peripheral targets.

The positions of internal intercostal motoneurons within their motor pool were studied, following reinnervation of the intercostal muscles by their original nerves. Six to 9 weeks after proximal nerve section in 10-d-old and adult rats, 0.1 microliter injections of wheat germ agglutinin (WGA)-HRP were made in the distal part of the reinnervated internal intercostal muscle. The corresponding region of the contralateral control muscle was also injected. The positions of the retrogradely labeled motoneurons were mapped in 100 microns transverse sections of thoracic spinal cord that had been stained for HRP according to the method of Mesulam (1982). In normal rats, motoneurons innervating distal muscle fibers are found largely in the more dorsal part of the internal intercostal motoneuron pool (Hardman and Brown, 1985). In adult rats, regenerated motor axons did not show any selectivity; distal muscle fibers were innervated by motoneurons whose cell bodies were distributed throughout the internal intercostal pool. However, in rats operated on at 10 d of age, distal intercostal muscle fibers were reinnervated by motoneurons that were distributed mainly in the dorsal part of the motor pool. These results support the view that positional signals may be of importance in organizing the distribution of axon terminals within muscles during development.