Invasion of microglia/macrophages and granulocytes into the Mauthner axon myelin sheath following spinal cord injury of the adult goldfish, Carassius auratus

Rapid activation of resident glia occurs after spinal cord injury. Somewhat later, innate and adaptive immune responses occur with the invasion of peripheral immune cells into the wound site. The activation of resident and peripheral immune cells has been postulated to play harmful as well as beneficial roles in the regenerative process. Mauthner cells, large identifiable neurons located in the hindbrain of most fish and amphibians, provided the opportunity to study the morphological relationship between reactive cells and Mauthner axons (M‐axons) severed by spinal cord crush or by selective axotomy. After crossing in the hindbrain, the M‐axons of adult goldfish, Carassius auratus, extend the length of the spinal cord. Following injury, the M‐axon undergoes retrograde degeneration within its myelin sheath creating an axon‐free zone (proximal dieback zone). Reactive cells invade the wound site, enter the axon‐free dieback zone and are observed in the vicinity of the retracted M‐axon tip as early as 3 hr postinjury. Transmission electron microscopy allowed the detection of microglia/macrophages and granulocytes, some of which appear to be neutrophil‐like, at each of these locations. We believe that this is the first report of the invasion of such cells within the myelin sheath of an identifiable axon in the vertebrate central nervous system (CNS). We speculate that microglia/macrophages and granulocytes that are attracted within a few hours to the damaged M‐axon are part of an inflammatory response that allows phagocytosis of debris and plays a role in the regenerative process. Our results provide the baseline from which to utilize immunohistochemical and genetic approaches to elucidate the role of non‐neuronal cells in the regenerative process of a single axon in the vertebrate CNS. Ultrastructural features of microglia/macrophages and neutrophil‐like granulocytes within the M‐axon sheath 24 h after vagal‐level selective axotomy. (a) Light micrograph of a 1 μm thick plastic cross section of an M‐axon stained with toluidine blue. The arrowhead and double arrows mark neutrophil‐like granulocytes while the asterisk marks a microglia/macrophage cell within the M‐axon myelin sheath seen as a dark ring around the periphery of the photomicrograph. (b) A transmission electron micrograph with markers that identify the three cells seen in (a).