Microglia
The ability to view and characterize different neural cells including microglia began in 1880 when Nissl staining was developed by Franz Nissl. Franz Nissl and F. Robertson first described microglial cells during their histology experiments. The cell staining techniques in the 1880s showed that microglia are related to macrophages. The activation of microglia and formation of ramified microglial clusters was first noted by Victor Babeş while studying a rabies case in 1897. Babeş noted the cells were found in a variety of viral brain infections but did not know what the clusters of microglia he saw were. Pío del Río Hortega, a student of Santiago Ramón y Cajal, first called the cells "microglia" around 1920. He went on to characterize microglial response to brain lesions in 1927 and note the "fountains of microglia" present in the corpus callosum and other perinatal white matter areas in 1932. After many years of research Rio-Hortega became generally considered as the "Father of Microglia." For a long period of time little improvement was made in our knowledge of microglia. Then, in 1988, Hickey and Kimura showed that perivascular microglial cells are bone-marrow derived, and express high levels of MHC class II proteins used for antigen presentation. This confirmed Pio Del Rio-Hortega's postulate that microglial cells functioned similarly to macrophages by performing phagocytosis and antigen presentation.
Radiation-induced cognitive decline
Radiation therapy usually results in chronic inflammation, and in the brain this inflammatory response comes in the form of activated microglia cells. Once activated, these microglia cells start to release stress hormones and various pro-inflammatory cytokines. Some of what is released by the activated microglia cells, like the glucocorticoid stress hormone, may result in a decrease in neurogenesis. To investigate this concept, an animal study was performed by Monje et al. in order to determine the specific cytokines or stress hormones that were released by activated microglial cells that decrease neurogenesis in an irradiated hippocampus. In this study, microglia cells were exposed to bacterial lipopolysaccharide to elicit an inflammatory response, thus activating the microglia cells. These activated microglia were then co-cultured with normal hippocampal neural stem cells. Also, as a control, non-activated microglia cells were co-cultured with normal hippocampal neural stem cells. In comparing the two co-cultures, it was determined that neurogenesis in the activated microglia cell culture was 50% less than in the control. A second study was also performed to ensure that the decrease in neurogenesis was the result of released cytokines and not cell-to-cell contact of microglia and stem cells. In this study, neural stem cells were cultured on preconditioned media from activated microglia cells and a comparison was made with a neural stem cells cultured on plain media. The results of this study indicated that neurogenesis also showed a similar decrease in the preconditioned media culture versus the control.