Microglia in the human nervous system during development. This
is the latest version of this eprint. Microglia are the
principal resident mononuclear phagocytes of the central nervous
system (CNS). They are representatives of the immune system
intrinsic to this organ. These cells are morphologically,
phenotypically and functionally distinct from other populations
of mononuclear phagocytes associated with the CNS, such as
perivascular macrophages, supraependymal macrophages, epiplexus
cells of the choroid plexus and meningeal macrophages. While the
origin of microglia has been the subject of controversy for many
years, the prevailing view holds that they are derived from
mesenchyme (mesodermal elements) or from circulating blood
mononuclear progenitors (monocytes) that penetrate the nervous
tissues early in development. This article will review the
location, distribution, morphology and phenotype of microglia in
the developing human CNS. Potential functional roles for
microglia are discussed in relation to dev