caption: Synaptic modification by microglial cells. Microglia monitor and interact with synapses to modulate neural circuit formation and function. Microglia can phagocytose ``weak'' synapses during development and injury to modify synapse numbers (top panels). The lower panels indicate mechanisms mediating functional microglia-synapse interactions. Candidate molecules such as MHC class 1 proteins (lower left panel) and complement cascade proteins (lower center panel) have been suggested to contribute to the phagocytosis of synapses by microglia. Activated microglia can also modify functional transmission at synapses via signaling pathways involving ATP, astrocytes, and glutamate receptors as indicated (lower right panel).