Microglial cell responses in the rabbit retina following transection of the optic nerve

The response of retinal microglial cells, which accompanies retrograde degeneration of ganglion cell axons and perikarya (induced by transection of the optic nerve), was studied in whole-mounted rabbit retinae labeled enzyme-histochemically for nucleoside diphosphatase (NDPase), which is a microglial cell marker. A few days after transection, the number of microglial cells/mm2, as well as their staining intensity, began to increase in the inner plexiform layer. The mosaic-like distribution of the star-shaped microglial cells present in the inner plexiform layer of a normal rabbit retina was preserved during ganglion cell degeneration. As in the normal retina, processes of individual cells never overlapped with those of neighboring cells in the inner plexiform layer because individual cells in the "degenerating" retina acquired shorter processes, i.e., the cells occupied a smaller territory compared to the normal retina. In the nerve fiber layer the number and staining intensity of NDPase-labeled microglial cell processes (most of which are aligned in parallel with degenerating ganglion cell axons) transiently increased and returned to normal values by 5 months post-transection. Microglial cells that are not detectably NDPase labeled in the outer plexiform layer of a normal rabbit retina acquire intense staining a few days after the nerve is cut. The functional significance of the increased NDPase activity in the plasma membrane of microglial cells during degeneration remains to be determined.