MICROGLIA IN HUMAN RETINA - A HETEROGENEOUS POPULATION WITH DISTINCT ONTOGENIES

Microglia of the adult human retina are a heterogeneous population of cells, some having characteristics of dendritic antigen presenting cel ls (DC) and others resembling macrophages, or MPS cells. Studies of th e development of microglial distributions in human retina suggest that cells bearing macrophage markers are ontogenetically distinct from mi croglia that do not. Quantitative studies indicate that macrophage ant igen immunoreactive microglia are a subpopulation CD45- and MHC-immuno reactive microglia. While CD45 and MHC-I and -II immunoreactive microg lia are seen in the retina prior to the arrival of the vasculature, si gnificant numbers of macrophage-positive microglia only arrive along w ith the vascular precursors, at about 14 to 15 weeks of gestation. Mic roglia appear to enter the retina from the ciliary margin prior to vas cularization but from both the optic disc and ciliary margin, postvasc ularization. Macrophage antigen positive microglia enter the retina ma inly via the optic nerve head. It is argued that macrophage-antigen po sitive microglia become established in the retina as vessel associated (perivascular and paravascular) microglia and that the MHC-positive, but macrophage-antigen negative microglia (representing DC), become es tablished as the parenchymal, ramified microglia of adult retina.