Evaluation of collateral sprouting after end-to-side nerve coaptation using a fluorescent double-labeling technique

The mechanism of end-to-side neurorrhaphy is believed to be by collateral sprouting, although evidence for this is lacking. This study validates whether axonal sprouting originates from the donor intact nerve by collateral sprouting with the use of a fluorescent double-labeling technique. End-to-side neurorrhaphy was performed on adult female Sprague-Dawley rats. Eight and 12 months postoperatively, animals were injected with true blue and diamidino yellow into the tibialis anterior and/or gastrocnemius muscles and were transcardially perfused with fixative after 7 days of retrograde transport. The lumbar enlargement and the dorsal root ganglia from L3 to L6 were harvested and serial sectioning and fluorescent microscopy were performed. No double-labeling neurons were observed in control animals, but a group of neurons that were greenish or yellowish in color were seen with single labeling. Double-labeling neurons, however, were seen in animals treated with end-to-side neurorrhaphy whether or not perineurotomy was performed. These results demonstrate that one parent nerve fiber can emanate another axon by collateral sprouting following end-to-side neurorrhaphy. We hypothesize that the causes of collateral sprouting might result from "switching signals" and/or "switching factors."