Reinnervation of denervated iris by transplanted sympathetic ganglia: Effect of neuronal age

Neuronal outgrowth in vivo is aggressive postnatally, but is diminished with increasing age. This may be attributable to intrinsic features of the neuron or its interaction with other components of the developing organism. The purpose of this study was to determine if there is an age‐dependent reduction in the intrinsic ability of sympathetic neurons to initiate fiber outgrowth. Superior cervical ganglia from donor rats aged 3–4, 11–12, 27–28 and 45–46 days were removed and transplanted to the anterior chamber of the sympathectomized eye of host rats 85–89 days of age. Ganglia with host irides were removed at 3, 6 and 10 days post‐transplant and whole mounts were analysed using catecholamine histofluorescence for maximum sympathetic fiber density, length and initial rate of outgrowth. Fluorescent fibers were present in host irides of donors of all ages and at all post‐transplant times. However, maximum fiber density was less for the 3–4 day‐old donor ganglia (e.g. 43–71% of 11–46‐day‐old donor ganglia at 600 μm, 10 days post‐transplant). Maximum fiber length was also less in the youngest group (e.g. 35–49% of 11–46‐day‐old donor ganglia, 10 days post‐transplant). Further, the initial rate of outgrowth was decreased for the 3–4‐day‐old donor ganglia (128±46 μm/day for the 3–4‐day‐old ganglia vs 253±48 μm/day for the 11–12‐day‐old ganglia, 307±35 μm/day for the 27–28‐day‐old ganglia and 260±22 μm/day for the 45–46‐day‐old ganglia). We conclude that the capacity for neuronal outgrowth is limited in the early postnatal period, but rapidly attains the mature level by the second postnatal week. Therefore, diminished outgrowth in mature rats does not appear to be due to a decrease in the intrinsic ability of sympathetic nerves to initiate fiber outgrowth, but is attributable to other factors.