Differential activation of astrocytes and microglia after spinal cord injury in the fetal rat

Background: As the immature spinal cord was nerve growth permissive, we examined glial reactions that influence regeneration of the spinal cord in a fetal rat spinal cord injury model. Methods: Three, 7, 21, and 35 days after intrauterine surgery, offsprings were killed and the thoracic and lumbar spinal cords were carefully removed from the spinal column and then cut into 10 mu m longitudinal sections. These sections were stained with hematoxylin-eosin, anti-glial fibrillary acidic protein antibody (GFAP) as a marker of astrocytes, and anti-complement CR3 antibody (OX-42) as a marker of microglia. A cordotomy model in a young adult rat was utilized as a control. Results: In the present study, collagen fibers and scar formation were seen in the severed spinal cords of mature rats, but scar formation was not seen in the fetal rat cordotomy group, regardless of spinal continuity. In the control group, biological activity of GFAP-positive cells increased over time. In the fetal rat cordotomy model, activity elevated slightly immediately after cordotomy, and disappeared shortly thereafter. In the control group, OX-42-positive macrophage-like cells proliferated over time. However, in the fetal rat cordotomy model, OX-42- positive macrophage-like cells were recognized on postoperative days 3 and 7, and then disappeared. At 5 mm from the cordotomy site, reactive microglia were recognized in the white matter of control group spinal cords, but these microglia were not recognized in the fetal rat cordotomy model. Conclusions: In the present study, collagen fibers and scar formation were seen in the severed spinal cords of adult rats, but scar formation was not seen in the fetal rat cordotomy group. Lack of inflammation and scar formation thus appear advantageous for regeneration of the fetal spinal cord. Between fetal and mature rats, chronological changes in the immunohistochemical reactions of astrocytes and microglia following cordotomy were compared, and the results confirmed many differences. The results of the present study suggest that the presence of activated glial cells around damaged central nervous tissue and the quick disappearance of these cells after injury are important for the repair of damaged central nervous system tissue, and that the role of glial cells in nerve regeneration can change depending on the level of maturity of glial cells or surrounding cells, site of injury, or the state of tissue around the injury.