Identification of microglia cell of the rat retina by light and electron microscopy

Distribution of microglia cells was studied in the normal retina of Wistar rats kept in a specific-pathogen-free condition, using a combined technique of light and electron microscopy with the weak silver carbonate staining method of del Rio-Hortega. Typical microglia cells were found mainly in the inner plexiform layer. By electron microscopy these cells were easily identified by metallic deposition in the cells, and had a small oval nucleus with a prominent chromatin pattern, which was one of the most characteristic features of the cell. Based upon these fine structures, microglia cells were identified in the nonimpregnated, ordinarily prepared retina by electron microscopy. These cells contained lysosomes, lipofuscin granules, Golgi apparatus, and long, narrow cisternae of rough-surfaced endoplasmic reticulum mainly at both sides of the perikaryon. These features were identical with those of microglia cells in the central nervous system. Microglia cells were found to be widely distributed in the inner portion of the retina, most of the cells locating closely to ganglion cells or capillary walls, and occasionally in the inner plexiform layer. The nuclear feature of the cell was also recognizable by light microscopy in 1-micron-thick Epon sections stained with toluidine blue. Serially cut thick sections of the retinal ribbon revealed that microglia cells were widely distributed from the peripapillary to the ora serrata region. From the present study, it is concluded that the microglia cell can be considered a component of the normal retina. This is the first report on the precisely described fine structures of the retinal microglia cells.