Facial-Hypoglossal End-To-Side Neurorrhaphy: Exploration of the Source of Axonal Sprouting

Background The clinical application of end-to-side (ETS) neurorrhaphy is under debate partly due to a lack of consensus on the source of axonal sprouting. Methods In this study, 24 rats were divided into three groups: sham operation, facial-hypoglossal ETS neurorrhaphy, and end-to-end (ETE) neurorrhaphy. Electrophysiological tests were employed to detect the evoked compound muscle action potentials (CMAPs) in differentsituations, and the latencies and maximal amplitudes of the CMAPs recorded were compared. Fluorescence retrograde tracing studies, hematoxylin and eosin (HE) staining, and immunohistochemical staining of growth-associated protein 43 (GAP-43) were performed. The number and the diameter of myelinated axons proximal and distal to the coaptation sites were measured. Results Twelve weeks after the surgeries, reinnervation of whisker pad muscles by hypoglossal nerves in both the ETS and ETE groups were confirmed via electrophysiological study. The maximal amplitudes of the CMAPs recorded in different situations and the quantification of myelinated axons supported the coexistence spontaneous collateral sprouting and regenerative sprouting of axons. Double-labeled neurons were found within the hypoglossal nuclear areas in the ETS neurorrhaphy group and HE staining illustrated the axons crossed the coaptation site into the facial acceptor nerve. Although immunohistochemical staining of GAP-43 revealed different timeframes between ETS and ETE neurorrhaphy groups, no significant difference on latency or diameters of the myelinated axons distal to the coaptation sites was noted between ETE and ETS groups. Conclusion Both spontaneous collateral sprouting and regenerative sprouting of axons coexisted following ETS neurorrhaphy, which represents an alternative approach to peripheral nerve reconstruction.