Microglial cells in the retina of Carassius auratus: effects of optic nerve crush.

To study the morphology and distribution of the retinal microglial cells of the goldfish retina in normal conditions and after optic nerve crush, we have used the nucleoside diphosphatase (NDPase) technique, applied to whole-mounts or sections, for light and electron microscopy. In normal retinas, two populations of NDPase-positive cells were identified: compact cells associated with the retinal vessels on the vitreal surface of the retina and microglial cells in various retinal layers. The microglial cells had a bipolar or multipolar morphology. Bipolar cells were observed in the nerve fibre layer, and multipolar cells were visualised in the ganglion cell layer (GCL), inner plexiform layer (IPL), and outer plexiform layer. The highest densities of multipolar cells were observed in the IPL layer, where they adopted a regular mosaic-like arrangement in which the occasional spaces were occupied by cells of the GCL. After optic nerve crush, we observed an increase in the number of compact cells associated with the vessels and changes in NDPase activity, morphology, and distribution of the retinal microglial cells. These cells showed an increase in NDPase activity in all retinal layers from day 1 to day 15 after axotomy, and retraction of their processes from day 1 to day 7. In addition, the densities of microglial cells increased in the GCL between 2 and 15 days after axotomy, and decreased in the IPL by day 4 after axotomy. These microglial changes resemble those observed in other regenerating and nonregenerating neuronal systems and may reflect a general response of microglia directed to help the regeneration process.