Siglec Expression in Microglia

Microglia are a minor population of nervous system cells that originate from blood monocytes, which settle down in the brain to becomes the "macrophages" of the central nervous system.  Similar to macrophages in other tissues, they defend against infection by engulf invading pathogens and reacting to inflammation.  However, they also have trophic functions in supporting cells of the neural lineages, and may be involved in overrreacting to some inflammatory conditions.  While most peripheral tissue macrophages express multiple signaling molecules called "Siglecs", microglia usually do not not, under normal conditions.  The exception appears to be Siglec-11 and Siglec-16, which are expressed in the microglia of human but not "great ape" microglia.  This difference is associated with a human-specific gene conversion that involved the SIGLEC16 and SIGLEC11 genes.