Developmental Neuronal Elimination

Microglia, the brain’s innate immune cell type, are cells of mesodermal origin that populate the central nervous system (CNS) during early development. Their functions which are best characterized in the developing CNS are related to programmed cell death (PCD), a physiological process that massively affects neural cell lineages and contributes to brain morphogenesis and neuronal network maturation. Although relatively scarce before advanced developmental stages, microglia can remove dead cells in an effective manner due to their migratory and phagocytic behavior. Recent studies indicate that microglia do not only scavenge cell corpses, but also eliminate neural progenitors cells and trigger or induce PCD in different types of developing neurons. Conversely, microglia were also found to promote the neuronal survival by their release of trophic factors. In this chapter we shall discuss the functional involvement of microglia in the loss of neural cells during normal development and review the mechanisms and cell signalling that underlie microglial regulation of PCD and elimination of dead cells.