The microglial response to progressive hydrocephalus in a model of inherited aqueductal stenosis.

Although gliosis has been reported to be a common and persistent feature in the white matter of hydrocephalic brains, no studies have identified the cell types that characterize this response. Therefore, the present study has employed histochemical methods to evaluate microglial cells in the brains of infant rats with inherited hydrocephalus. This strain of rats acquires hydrocephalus during late fetal stages due to aqueductal stenosis. Tissue from the sensorimotor and auditory cortices of 12- and 21-day-old hydrocephalic and normal H-Tx rats was processed and stained for the lectin microglial marker Griffonia simplicifolia (GSA-IB4). During the progression of hydrocephalus, GSA-positive cells exhibited three changes: (1) Cytologically, the cell bodies were enlarged, and their processes were thicker, longer and more numerous. These changes were most notable in the gray matter. (2) The packing density of GSA-positive cells was either increased or decreased, depending on the age of the animal and the severity of hydrocephalus. (3) Localized clusters of GSA-positive cells were conspicuous in the white matter of 12-day animals with mild hydrocephalus, and in the gray matter of 21-day animals with severe hydrocephalus. These results indicate that the microglial response is initiated during intermediate stages of hydrocephalus, and is not restricted to the periventricular white matter. These changes may signal other pathophysiologic events in the hydrocephalic brain, and demonstrate that microglia constitute one important element in the gliosis that accompanies hydrocephalus.