Microglia contribute to normal myelinogenesis and oligodendrocyte homeostasis during adulthood

Whereas microglia involvement in virtually all brain diseases is well accepted their role for the control of homeostasis in the central nervous system (CNS) is mainly thought to maintain neuronal function through the formation, refinement and monitoring of synapses in both the developing and adult brain. Although the prenatal origin and the neuron-centered function of cortical microglia has recently been elucidated much less is known about a distinct amoeboid microglia population formerly described as “fountain of microglia” that appear only postnatally in the myelinating regions such as corpus callosum and cerebellum. Using large-scale transcriptional profiling, fate-mapping and genetic targeting approaches we identified a unique molecular signature of this microglia subset that arose from a CNS endogenous microglia pool independent from circulating myeloid cells. Microglia depletion experiments unexpectedly revealed an essential role of postnatal microglia for the proper development and homeostasis of oligodendrocytes. Our data provide new cellular and molecular insights into the myelin-supporting function of microglia in the normal CNS.