Mechanical modeling of tissue response during early stage entubulated peripheral nerve regeneration

Abstract

After severe peripheral nerve injury, such as transection, regeneration has been observed over variable gap lengths with the use of the nerve stump entubulation method. Following entubulation, exudate from transected nerve stumps coagulates into a fibrin matrix within the entubulated space by the end of the first week. Over the following four weeks, two distinct groups of cells migrate into the fibrin-filled gap. The first group consists of fibroblasts which differentiate into the myofibroblast phenotype, forming concentric, force-applying layers collectively known as the contractile capsule around the periphery of newly forming tissue. The second group of cells includes undifferentiated fibroblasts, Schwann cells, and endothelial cells which migrate into the fibrin core preceding axon elongation and synthesis of new nerve tissue. The pressure cuff hypothesis states that the forces applied by myofibroblasts in the contractile capsule deform new tissue and disrupt regeneration. Previous efforts to model the pressure cuff hypothesis have described the release of exudate and the effect of forces applied by cells on the deformation of the regenerate. However, the effects of the conduit chosen for entubulation and the kinetics of early processes have not yet been analyzed. This work aims to further describe the pressure cuff hypothesis, emphasizing activity that occurs within the first two weeks post-transection. A new mechanical model is presented to describe the mechanical attenuation of myofibroblast forces by a nerve conduit, and a novel kinetic model is presented to describe the effects of fibrin clot formation, nerve tissue formation, and contractile capsule formation on the wound healing outcome. The results suggest that the presence of a nerve conduit acts to attenuate the load applied to coagulated fibrin inside the gap during the early stages (< 2 weeks post-injury) of wound healing and that the deformation of fibrin is highly dependent on the growth rate of the capsule and the antagonistic activity between cells that synthesize new nerve tissue and cells that form the contractile capsule. Ex vivo evidence is presented to support the hypothesis that the properties of nerve conduits can affect capsule growth and the migration of Schwann cells and axons into the gap. In the context of the mechanical model, these effects result in altered mechanical states of fibrin that may affect downstream regenerative processes. Additionally, an in vitro model that can be used to further study the effects of biomaterial properties on capsule formation is proposed.