Microglial cells are a component of the perivascular glia limitans.

The ultrastructural relation between microglial cells and cerebral blood vessels was studied in rat brains by immune electron microscopy using antibodies against the common leukocyte antigen (Ox1), the complement receptor 3 (Ox42), and against class I and class II histocompatibility antigens (MHC antigens; Ox3, Ox6, Ox18, and I1-69). Microglial cell processes were found incorporated between the astrocytic foot processes of the glia limitans in 4-13% of cerebral microvessels. After intravenous injection of gamma-interferon, either alone or in combination with tumor necrosis factor, these microglial cell processes expressed classes I and II MHC antigens. Studies in (Lewis X DA)F1-DA bone marrow chimeras demonstrated that these cell processes belonged to resident microglia. This study suggests that microglial cells may play an important role in antigen recognition at the blood-brain barrier.