Temporal response of the phagocytic microglia in the axotomized rat retina: optic nerve crush vs. transection

Abstract

AbstractPurpose To analyze the appearance of phagocytic microglial cells (PMC) in rat retinas after optic nerve crush or transection.Methods Retinal ganglion cells (RGCs) from adult Sprague Dawley rats were traced with fluorogold applied to both superior colliculi. One week later, the left optic nerve was either crushed (ONC, n=32) or transected (ONT, n=32). Rats were euthanized at 2, 5, 9 and 14 days after the axotomy (n=8/group). Retinas were dissected as flatmounts and Brn3a immunodetected to identify RGCs. Retinas were photographed for each signal. Brn3a+RGCs were automatically quantified and their distribution visualized by isodensity maps. The position of each PMC found in the ganglion cell layer (identified by their transcellularly labelling with fluorogold) was dotted on the retinal photomontages. These dots were counted and graphically represented with neighbour maps.Results PMCs were observed at 2 days(43±8, ONT; 11±5, ONC). Their number increased quicker after ONT than after ONC in opposition with the loss of RGCs which was slower after ONC. The peak of PMCs was observed at 9 days after ONT and at 14 days after ONC (19,842±2534 and 19,757±1968, respectively). The appearance of PMCs and the loss of RGCs was highly correlated (r2=0.8 for both lesions). PMCs were observed across the retina and their distribution matched the retinal areas of RGC loss.Conclusion ONC induces a slower loss of RGCs than ONT, and this is followed by a slower response of microglial cells.