A developmental study of epiplexus cells and supraependymal cells and their possible relationship to microglia.

Intraventricular macrophages are first seen attached to the presumptive choroid plexus in the 11 days postconception mouse. By 12 day postconception supraependymal macrophages are present as well as a few macrophages lying apparently free within the ventricles. The number of epiplexus, supraependymal and free macrophages increases rapidly and reaches a peak at 17 days postconception, then fall rapidly until 4 days post natum. A second increase in epiplexus and supraependymal cells occurs at 8 days post natum followed by another decrease up to 15 days post natum. Microglia were first observed in the brain at 13 days postconception and at 17 days postconception a microglial cell was observed apparently entering the ependyma from the ventricle. It is suggested that the large decrease in intraventricular macrophages after 17 days postconception is due to many of these cells migrating into the brain to give rise to microglia. The intraventricular macrophages themselves probably arise from within the choroid plexus, either from stromal cells or from blood-borne macrophages.