By studying the response of hippocampal microglia to anterograde axonal and terminal degeneration in the dentate gyrus we have identified several subsets of microglia, including immigrant bone marrow (BM)-derived and resident microglia, and resident microglia expressing different levels of CD34, of which the majority were induced to undergo proliferation upon injury. Based on the population kinetics elucidated until now, we have evidence that a subpopulation of microglial cells may undergo repeated proliferation upon injury while some microglial cells may not proliferate at all within the investigated period. Furthermore, resident cells in the mouse may be supplemented by BM-derived cells. A picture emerges where new cells are being born, while other cells, including the newly generated and recruited cells are beginning to be cleared from the site of lesion, at least partly by apoptotic mechanisms. Increased insight into basic microglial cell biology may improve diagnostic and therapeutic possibilities for patients suffering from conditions that are currently without real treatment options.