THE VIABILITY AND MYELINATING POTENTIAL OF TRANSPLANTED NERVOUS TISSUE

Abstract

This dissertation describes a series of experiments designed to study the viability and reinnervation of segments of nervous tissue transplanted from one site of the nervous system to another. Both central (optic nerve) and peripheral (tibial, peroneal) nerves were grafted to sites within the central nervous system (midbrain) and peripheral nervous system (peroneal nerve). Control nerves consisted of intact optic nerve and peripheral nerves as well as control transected nerves allowed to degenerate in situ.;The controls and grafts were examined by light and electronmicroscopy for morphological change, revascularization, reinnervation by local ingrowing axons and for remyelination of the axons. In general, the results showed that all the grafted nerves survived in their new sites. Both optic nerve and peripheral nerve could be reinnervated by new axons at sites either in the CNS or PNS. Oligodendrocytes could form their typical CNS-type myelin around ingrowing peripheral fibers, and Schwann cells could form their typical PNS-type myelin around ingrowing central fibers. In general, the peripheral nerve grafts in the CNS were reinnervated to a greater extent than the central nerve grafts, with more myelination of the ingrowing axons. In addition, the implants were connected to the adjacent CNS by astroglial projections. Indirect evidence suggests these bridges might have played a role in the guidance of axons into the peripheral nerve grafts. Schwann cells survived the process of transplantation much better than oligodendrocytes, even several weeks following transection in situ and never lost their ability to myelinate ingrowing axons. Oligodendrocytes on the other hand demonstrated a limited regenerative capacity but, when transplanted to the PNS, some did survive and proliferate CNS-type myelin. Optic nerve implants into the CNS were less successful, largely on account of poor placement in the CNS and tissue response to damage.;In conclusion, grafted CNS and PNS tissue can survive and some ingrowing axons acquire myelin sheaths. Future studies might consider mechanisms by which tissue damage and reactivity might be lessened in order to facilitate axonal regeneration.