Columns of Schwann cells extruded into the CNS induce in-growth of astrocytes to form organized new glial pathways.

Our previous work showed that stereotaxic microextrusion of columns of purified peripheral nerve-derived Schwann cells into the thalamus of syngeneic adult rats induces host axons to grow into the column and form a new fiber tract. Here we describe the time course of cellular events that lead to the formation of this new tract. At 2 h postoperation, numerous OX42-positive microglia accumulated at the graft-host interface, after which donor columns became progressively and heavily infiltrated by microglia/macrophages that took on an elongated morphology in parallel with the highly orientated processes of the donor Schwann cells. The penetration of host astrocytic processes into the Schwann cell columns was substantially slower in onset, being first detected at 4 days postoperation. This event was contemporaneous with the in-growth of host thalamic axons. Between 7 and 14 days postoperation, GFAP-positive astrocytes became fully incorporated into the transplants, where they too adopted an elongated form, orientated in parallel with the longitudinal axis of the graft. Thus, the columns became a mosaic of elongated and highly orientated donor Schwann cells intimately mingled with host microglia, astrocytes, and numerous, largely unbranched 200-kDa neurofilament-positive axons from the adjacent thalamus. Electron microscopy demonstrated that the processes of donor Schwann cells and host astrocytes within the column formed tightly packed bundles that were surrounded by a partial or complete basal lamina. Control columns, formed by extruding freeze-thaw-killed Schwann cells or purified peripheral nerve fibroblasts induced a reactive injury response by the adjacent host microglia and astrocytes, but neither host astrocytes nor neurofilament-positive axons were incorporated into the columns. A better understanding of the mechanisms that regulate the interactions between donor and host glia should facilitate improved integration of such grafts and enhance their potential for inducing tissue repair.

Spontaneous orientation of transplanted olfactory glia influences axonal regeneration.

Transplanted olfactory ensheathing cells (OECs) have previously been demonstrated to support axonal growth and myelination in the adult rat CNS. Here, the capacity of donor OECs to control the direction of axonal regeneration has been investigated following transplantation, as elongated columns, into the thalamus of adult rats. The OECs formed a 'glial bridge' which extended from the thalamus to the hippocampus. Transplanted OECs rapidly adopted a spindle-shaped morphology which was orientated along the vertical axis of the transplant. Numerous host axons grew into the transplants and followed the highly orientated OEC cell matrix across the choroid fissure. Thus, the spontaneous elongation and orientation of donor OECs may support highly directional host axonal growth across natural barriers within the CNS.

Extrusion transplantation of Schwann cells into the adult rat thalamus induces directional host axon growth.

In a previous study we found that Schwann cells microtransplanted into the central nervous system rapidly dispersed from the transplantation site and became intimately associated with host grey and white matter. We have now investigated whether this migratory behavior of the donor Schwann cells is compatible with the production of stable, continuous anatomical cell tracks and whether such tracks can induce directional host axon growth. During the gradual withdrawal of a micropipette, highly purified suspensions of cultured adult peripheral nerve Schwann cells were continuously extruded to form a vertical column of cells extending for up to 4 mm through the thalamus and across the choroid fissure into the hippocampus of adult rat hosts. The donor Schwann cells were identified by immunohistochemistry for low-affinity nerve growth factor receptor, vimentin, and Rat 401. Although donor Schwann cells migrated into the host tissues, a large number remained along the axis of the injection track to form a column which was maintained for up to 3 weeks. From 4 days, increasing numbers of parallel, unbranching host RT97-positive axons entered the Schwann cell column in alignment with the long axis of the Schwann cells in the vertical tracks. The axons did not fasciculate directly with each other, but mingled diffusely with the Schwann cells. The Schwann cell tracks were able to convey host axons out of the dorsal thalamus, across the extracellular space of the choroid fissure, and into the ventral hippocampus. Thus, Schwann cells, transplanted in the form of elongated tracks, can establish bridges across boundary membranes in the brain and carry substantial numbers of nerve fibers from one area to another.