mental disorders might be the result of more subtle dysfunction •" microglia. Long ignored by scientists, glia are turning out to be a lot more important to neura development than just as "glue" for neurons.X-rays destroy bone marrow, so no new blood cells are produced. The mouse would die with no further treatment.When bone marrow cells from another mouse are injected, they colonize the: | bone marrow and restart production of blood cells, including precursors to microglia.Some of the donor cells colonize the brain to provide new microglia. In this way, MeCP2 knockout mice can be provided with microglia from normal mice, while the neurons of the brain will still lack the MeCP2 gene.29 RESCUING MICE FROM RETT SYNDROME: (A) Rett syndrome is caused by a dysfunctional MeCP2 gene, so an animal model was created by knocking out that gene inmice. Researchers destroyed the mutant microglia by exposing the *;nockout mice to radiation, then infused bone marrow cells from normal animals. Some bone marrow cells entered the brain and differentiated into microglia. (B) Humans with Rett syndrome have smaller brains. In the animal model, providing the knockout mice with microglia from normal mice restored brain weight. (C) The normal microglia also allowed the animals to gain weight and avoid early death. Note that providing knockout mice with microglia from other knockouts was ineffective. (D) Providing the knockout mice with normal microglia also rescued their behavior. The lines represent the paths that a representativemouse from each group took while exploring an open field.