Migration of olfactory ensheathing cells grafted into adult rat spinal cord

Abstract

Olfactory ensheathing cells (OECs) are non-myelinating glial cells that provide ensheathment for axons of the olfactory nerve in vivo. OECs have been shown to facilitate the regeneration of CNS axons, to assemble a myelin sheath around demyelinated axons, and it has been suggested OECs migrate very well within the microenvironment of the injured CNS. However, there has been no direct test of their migratory ability in vivo. The aims of this study were to determine whether: 1) OECs can be induced to migrate towards an ethidium bromide (EtBr)-induced focal (~1 mm long) demyelination of the spinal cord white matter; 2) OECs migrate away from a focal demyelination either into normal CNS tissue or towards a second demyelinated lesion; 3) microglial reactivity is required for the generation of the migratory signal(s) inducing OECs to migrate towards a focal demyelination; 4) OECs grafted into the subarachnoid space surrounding the spinal cord will migrate into the neuropil in the absence of demyelination. To achieve these aims, we developed an in vivo model for studying the migratory ability of OECs within the adult rat spinal cord. A small focal EtBr-induced demyelination of the dorsal funiculus (unilaterally) of the spinal cord was made at variable distances from the site of a DiI-labelled OEC graft. The major findings were: i) the strength of the migratory signal(s) inducing OECs to migrate increased as the demyelinated lesion was located closer to the grafting site; ii) the OEC migration towards a distal demyelinated lesion was greatly enhanced when the cells were grafted directly into a second demyelinated lesion; iii) the cell migration occurred along a migratory path containing many reactive astrocytes and microglia; iv) the migration of OECs was significantly reduced when the microglial reactivity was dampened using minocycline; and v) OECs survived grafting into cerebrospinal fluid (i.e. subarachnoid space) and migrated into the neuropil of the brain and spinal cord. The major conclusions are that OECs can respond to migratory signal(s) arising as a result of a focal EtBr-induced demyelination and that microglia are one potential source of these migratory signal(s).