Simard_2004_Neuroreport_15_2305

Unlike most organs, tissue regeneration and repair are not very efficient in the CNS, which explains the severity of neurodegenerative diseases. Many have hoped that stem cells would provide an effective mean to solve this problem. Unfortunately, evidence supporting this approach remains controversial. In this review, we discuss the capacity of stem cells to generate the cells that reside in the brain. Neural stem cells are able to generate new neurons, astrocytes and oligodendrocytes, but not microglia. The latter are instead replenished by self-replication and monocyte recruitment across the blood-brain barrier. The fact that blood-derived monocytes can enter the brain and differentiate into microglial cells has many implications for neurodegenerative diseases. They are more efficient antigen-presenting cells and produce proinflammatory molecules that can be both detrimental to the brain and beneficial to recovery and repair after insults. It is therefore very important to better understand the role of these newly differentiated microglia before devising therapeutic strategies to either inhibit or improve their recruitment at diseased and injured sites.