Injuries to peripheral nerves can severely affect the quality of life of patients, ranging from chronic pain to persisting sensory and motor deficits and even unemployability. Autologous nerve transplants remain the prevailing gold standard treatment, although their use has several limitations, such as limited donor nerve availability, donor site morbidity, and poor functional recovery. Consequently, tissue engineering strategies seek to develop alternative treatment options, such as the clinically approved hollow nerve guidance conduits. However, use of these hollow tubes alone does not promote functional recovery, thereby highlighting the urgent need for improved treatment options that enhance peripheral nerve repair. The aim of this thesis was to investigate tissue engineering strategies that (i) advance our understanding of the influence of the lymphatic vasculature in healthy and regenerating nerves, (ii) improve blood vessel infiltration through macroscopic holes in the tubular walls of nerve guidance conduits to avoid central necrosis, and (iii) promote axon migration through longitudinally aligned, threedimensional bands of Büngner constructs via the application of mechanical strain.In the first chapter, this thesis provides novel insights into the lymphatic vascular system of peripheral nerves, thereby significantly contributing to bridging the prevailing knowledge gap of the organization and function of neural lymphatic vessels. Here, histological examinations revealed increased numbers of lymphatic vessels in autologous nerve transplants compared to healthy nerves ex vivo, while in vitro, Schwann cells showed an anti-lymphangiogenic as well as pro-apoptotic effect towards lymphatic endothelial cells. These findings indicate that lymphatic vasculature is important during peripheral nerve repair, whereas the Schwann cells’ lymph-repellent activity in vitro might explain the absence of endoneurial lymphatic vessels. Investigation of the underlying mechanisms might provide another approach to mediate nerve regeneration by enhancing lymphangiogenesis to avoid edema and inflammation.In the second chapter, a modification of hollow nerve guidance conduits by theintroduction of macroscopic holes is reported, which allowed blood vessel migration through the tubular holes and anastomosis of vasculature from the surrounding tissue with blood vessels that formed in the lumen of the conduit. Sufficient vascularization of the regenerating nerve tissue in the lumen is particularly important to avoid necrosis in the center, and althoughenhanced vascularization inside macroporous tubes induced superior axon regeneration compared to unmodified tubes in a short rat sciatic nerve defect, functional recovery was not improved. Nevertheless, these results highlight the importance of enhancing intraluminal vascularization, and further optimization of these conduits will pave the way for improved nerve grafts in the future.In the third chapter, the importance of the incorporation of cellular intraluminal fillers is addressed, since the major limiting factor for successful regeneration is the formation of bands of Büngner, which provide guidance for regenerating axons. This thesis for the first time reports that mechanical stimulation of Schwann cells embedded in a three-dimensional hydrogel matrix induces cellular alignment along the axis of strain, resembling the native bands of Büngner. Mechanical stimulation was applied by the custom-made “MagneTissue” bioreactor, which is well-established in our research group for the generation of muscle-like tissue constructs, and we recently showed that skeletal muscle hypertrophy is induced by applying cyclic tensile stress via this system. The experiences from the muscle studies laid the basis to set up and evaluate the effects of mechanical stimulation on Schwann cells. The generatedartificial bands of Büngner promoted axon migration over several millimeters in vitro, and mechanical stimulation moreover induced the expression of a pronounced repair Schwann cell phenotype. Therefore, this thesis contributes to the establishment of potential novel intraluminal fillers for nerve guidance conduits for clinical application.In summary, this thesis contributes to the establishment of improved peripheral nerve repair strategies by introducing novel insights into the role of the blood and lymphatic vascular system during nerve regeneration in vivo, and by reporting the formation of tissue-engineered bands of Büngner in vitro by mechanical stimulation. Thereby, this thesis paves the way for the establishment of advanced nerve grafts to promote functional recovery of peripheral nerve lesions.
en
Additional information:
Abweichender Titel nach Übersetzung der Verfasserin/des Verfassers
