Cumulative studies have found an age-related decline in neurogenesis, both in the aged adult and in the diseased brain. Because aging may contribute to microglial dysfunction and neurotoxicity, as we discussed previously in this review, one could assume that microglial dysfunction may also be involved in the downregulation of neurogenesis in the aged or diseased brain [188, 189] . Even though very few studies have focused on the effect of microglial dysfunction on neurogenesis, we can still find a clue from Zhu's study that the difference in microglia function patterns between the immature and juvenile brain might be related to a decrease in neurogenesis in the juvenile brain [190] ; however, stronger evidence from the direct comparison of microglia-associated neurogenesis between aged and young brains is needed to support this view.