CHICK WING INNERVATION .3. FORMATION OF AXON COLLATERALS IN DEVELOPING PERIPHERAL-NERVES

Axon navigation during vertebrate limb innervation has been shown to b e associated with position-dependent changes in size and complexity of the axon growth cones, and sometimes with bifurcation of terminal gro wth cones and axon branching (Hollyday and Morgan-Carr, companion pape r). We have further examined axon branching and asked whether it exten ds to the projection of collaterals to different nerves. Injections of horseradish peroxidase or Dil were made into individual peripheral ne rves in the wings of chick embryos at stages 28-35, and the trajectori es of solidly labeled axons were traced proximally from the injection site in tissue sections. During stages when the peripheral nerves were first forming in the shoulder region, collaterals of retrogradely lab eled axons were frequently observed to project into uninjected nerves proximal to the injection site. These two-nerve collaterals were forme d by a small percentage of axons in a high percentage of the embryos s tudied and could occur in both motor and sensory axons. Two-nerve coll ateral projections were observed between nerves separated along both t he proximodistal and anteroposterior axes of the limb, but they were l imited in spatial extent to nerves supplying adjacent limb regions and were never seen between nerves projecting to widely disparate regions of the limb. Collaterals were not seen at the plexus projecting to bo th dorsal and ventral pathways. The apparent frequency of two-nerve co llaterals was found to decline progressively from stage 28-29 to stage 32; no two-nerve collaterals were seen in the proximal wing at stage 33 and older. The mechanism of their elimination is presently unknown. These observations suggest that some axon branching seen during outgr owth is sufficiently divergent to result in axon collaterals which pro ject to two different peripheral nerves. Presumably, two-nerve collate rals reflect both the neuron's ability to branch and some imprecision in the axonal guidance mechanisms. Together these give rise to minor e rrors in projection which are subsequently removed. (C) 1995 Wiley-Lis s, Inc.