Microglial stability and repopulation in the retina

Background/aims: Parenchymal central nervous system microglia are repopulated by bone marrow derived monocytes more slowly than any other reticuloendothelial cells. The contribution of bone marrow derived monocytes to the uninflammed retina has not been studied. The present study sought to determine repopulation of retinal microglia in uniflammed retina by bone marrow derived monocytes in bone marrow chimeric rats. Methods: Chimeric (YRX) Lewis rats were constructed by transplanting 56107 male bone marrow cells into lethally irradiated female recipient rats. The chimeras were sacrificed 8, 10, 12, 30, and 52 weeks after bone marrow transplant, and retina, brain, lung, and spleen samples were collected. DNA was extracted and quantified. Y positive infiltrating cells in the collected samples were detected by polymerase chain reaction amplification of a Y chromosome specific 104 bp fragment. Results: There was a rapid repopulation of haematopoietic tissues in the spleen ( at 8 weeks), confirming the establishment of chimerism, and to a lesser extent, of lung ( at 30 weeks). This repopulation was absent in the brain parenchyma and retina until 52 weeks after transplantation. Conclusions: These data indicate that resident microglia in the retina, much like those in the brain, are stable in number in the retinal compartment ( up to 1 year), and repopulation by bone marrow derived cells may be delayed for a year.