Axotomy or compression is required for axonal sprouting following end-to-side neurorrhaphy

Abstract

End-to-side (ETS) nerve repair remains an area of intense scrutiny for peripheral nerve surgeonscientists. In this technique, the transected end of an injured nerve, representing the "recipient" is sutured to the side of an uninjured "donor" nerve. Some work suggests that the recipient limb is repopulated with regenerating collateral axonal sprouts from the donor nerve that go on to form functional synapses. Significant, unresolved questions include whether the donor nerve needs to be injured to facilitate regeneration, and whether a single donor neuron is capable of projecting additional axons capable of differentially innervating disparate targets. We serially imaged living transgenic mice (n=66) expressing spectral variants of GFP in various neuronal subsets after undergoing previously described atraumatic, compressive, or epineurotomy forms of ETS repair (n=22 per group). To evaluate the source, and target innervation of these regenerating axons, nerve morphometry and retrograde labeling was further supplemented by confocal microscopy as well as Western blot analysis. Either compression or epineurotomy with inevitable axotomy were required to facilitate axonal regeneration into the recipient limb. Progressively more injurious models were associated with improved recipient nerve reinnervation (epineurotomy: 184±57.6 myelinated axons; compression: 78.9±13.8; atraumatic: 0), increased Schwann cell proliferation (epineurotomy: 72.2% increase; compression: 39% increase) and cAMP response-element binding protein expression at the expense of a net deficit in donor axon counts distal to the repair. These differences were manifest by 150 days, at which point quantitative evidence for pruning was obtained. We conclude that ETS repair relies upon injury to the donor nerve.

Functional Recovery following an End to Side Neurorrhaphy of the Accessory Nerve to the Suprascapular Nerve: Case Report

“…Several authors have concluded that axonal damage to the donor nerve is required to induce motor collateral sprouting but may not be required to induce sensory sprouting in end-to-side neurorrhaphy [1,12,28,29]. In a recent review by Dvali et al, the authors concluded that the degree of motor axonal sprouting following end-to-side repairs depends upon the degree of axonal damage, while sensory sprouting was less dependent on the presence of injury [8].…”

“…In a similar review by Rovak et al, the authors also concluded that collateral sprouting depended not only upon axonal damage but also was highly dependent on the extent of injury, with more collateral sprouting associated with increased damage [26]. While several authors have reported excellent functional results with facial nerve reinnervation using an end-to-side repair using the hypoglossal nerve as a donor [2,3,9,27,31,32], the most definitive experimental study on end-to-side repair was recently reported by Hayashi et al using Thy1-GFP mice that express green fluorescent protein in their motor axons [12]. This study utilized the femoral nerve as a donor and a transected sciatic nerve as the recipient.…”

“…Autografting may be necessary when dissection cannot expose sufficient nerve length to permit a tension-free coaptation. We have recently investigated a definitive end-to-side repair mouse model, in which axons fluoresce and can be accurately traced to the motor neuron pool [12]. We have shown that collateral motor sprouting will not occur without injury, while collateral sensory sprouting will occur without injury.…”

Influence of Breaching the Connective Sheaths of the Donor Nerve on Its Myelinated Sensory Axons and on Their Sprouting into the End-to-Side Coapted Nerve in the Rat

“…46,47 Accordingly, a recent study in transgenic mice demonstrated faint YEP labeling in the recipient nerve in two of six non-injurious end-to-side coaptations in mThy1-YEP mice in which all peripheral axons are labeled, whereas no GFP-labeled axons entered the recipient nerve after end-to-side repair in mThy1-GFP mice in which *10% of motor, but no sensory, axons are labeled in peripheral nerve. 36 How sprouting axons penetrate the connective tissue layers of a donor nerve is not known. As shown earlier, the invasion of Schwann cells from the recipient nerve into the epineurium of the donor nerve was a critical step for induction of axonal sprouting.…”

“…7,[29][30][31][32][33][34][35] However, recent observations in transgenic mThy1-GFP mice and mThy-YFP mice indicated that axotomy or compression of the donor nerve axons is required for reinnervation of the recipient nerve following end-to-side neurorrhaphy, and suggested that sprouting of the regenerating injured axons of the donor nerve is an important mechanism as well. 36 The connective tissue sheaths of a nerve (endoneurium, perineurium, and epineurium) contain condensed layers of collagen fibrils, perineurial cells, and basement membrane, [37][38][39] as well as repulsive axon guidance substances such as myelin-associated glycoprotein 40 and tenascin-C. 41 This may represent an important barrier to axon sprouting from the donor nerve into the recipient nerve. On the other hand, surgical breaching of the epineurium and perineurium may result in a marked axonal injury in the peripheral regions of the endoneurium.…”